Category Archives: Forests

Extinction debts catch up quickly

Chiew Larn Reservoir islands

John T. Curtis, a 20th century ecologist, drafted a simple, four-panel map that appeared in a volume of research presciently titled Man’s Role in Changing the Face of the Earth. It’s an important map for a number of reasons, not least of which was the impact it had on my life. This is how I described it in my first non-introductory post here at Per Square Mile:

It is a simple map, or rather series of maps. Four panels, four dates—from left to right: 1831, 1882, 1902, and 1950. In each successive panel, the dark swaths of ink that represented forest cover in Cadiz Township, Wisconsin, grew successively smaller and more fragmented.

Curtis's map

It’s a powerful image, one that drives home just how much we have affected this world. But like many images, there’s a lot that’s both implied and unknown. One of the unknowns of Curtis’s map was how life was faring in those small flecks floating in a sea of farm fields.

Now, we may be a step closer to understanding how dire that situation really is. A team lead by David Bickford, a professor at the National University of Singapore, recently wrapped up a 25-year study of forest patches turned into islands by the filling of the Chiew Larn Reservoir in Thailand in 1986 and 1987. The reservoir flooded nearly 64 square miles (165 square kilometers), isolating more than 100 patches of species-rich tropical forest. What had been hilltops were transformed into islands. Five to seven years after the flooding, the research team surveyed small mammal populations on 12 of the islands and 16 of the islands 25 to 26 years after.¹ None of the islands had signs of human impact.

Bickford and his team discovered that species vanished from the islands at an astonishing rate. Nearly all of the native small mammals were gone on the smaller islands (under 24 acres or 10 ha) in just five years, while on larger islands (24-138 acres or 10-56 ha), they were nearly extinct after 25 years. Their findings jibes with a message conservation biologists have been sharing for some time—the smaller the island, the fewer the species, and the longer the time since isolation from the mainland, the fewer species.

There are a number of possible reasons why small mammals disappeared from these islands, none of which are very heartening. The researchers point out that invasive Malaysian field rats were a problem on the islands, likely outcompeting or outright killing the native species. By the 25-year time point, “all islands were dominated by the invasive rodent and if not already in ecological meltdown, were well on their way to becoming Rattus monocultures,” Bickford and his team note.

But there are other possible reasons, too. Small habitat patches may not be large enough to sustain a viable population. When ranges are compressed, populations face a number of hardships, from increased competition for resources to inbreeding and intrapopulation strife that can raise stress, increase conflict, and lower breeding rates.

This study isn’t just about isolated islands in a remote corner of Thailand. It’s also about the flecks of land we cordon off every time we fell a forest, plow a field, or plat a subdivision. We’re creating small islands of habitat surrounded by seas of human dominance. Certainly some animals and plants can move between those islands, but not all do and not all at rates needed to sustain remnant populations. Some animals may be better than others at navigating human oceans, but even they may be doomed, unable to withstand competition or predation from introduced species. If we are to minimizing the impact we have on the environment—whether those be cities, farms, or even oil fields—we can’t just plan the land we’ll occupy, we have to plan the land we won’t.

  1. I would have liked to see a control transect on the mainland to see how the islands’ biodiversity compares, but they didn’t do that for whatever reason. (Perhaps they couldn’t find an area that wasn’t affected by humans.)

Image courtesy of Antony Lynam


Gibson L., Lynam A.J., Bradshaw C.J.A., He F., Bickford D.P., Woodruff D.S., Bumrungsri S. & Laurance W.F. (2013). Near-Complete Extinction of Native Small Mammal Fauna 25 Years After Forest Fragmentation, Science, 341 (6153) 1508-1510. DOI:

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More reasons to stop putting trees on skyscrapers

Bosco Verticale

Robert Krulwich, disagreeing with me:

Two residential towers, dense with trees, will have their official opening later this year in downtown Milan, Italy, near the Porta Garibaldi railroad station. (The image is not a photograph, but an architect’s rendering. The towers are built and the trees are going in right now.) I love this. I think these towers are gorgeous. Milan is a very polluted town; these trees will cleanse the air, pumping out oxygen and greening the cityscape. I think cities one day could look like mountain vistas; I’m enthralled.

But I am not Tim De Chant, tree lover, blogger, critic, who says this won’t work. All these trees, he thinks, are about to be dead. He recently posted an essay on his Per Square Mile blog, aimed at architects. He called it, “Can we please stop drawing trees on top of skyscrapers?” He thinks builders know squat about trees. I hope he’s wrong.

I know I seem like Buzz Killington to a lot of architects—and non-architects, Krulwich included—but that wasn’t my point…entirely. To me, trees atop buildings have become an architectural crutch, a way to make your building feel sustainable without necessarily being so. And that’s a charitable assessment. Here’s how I really feel—trees on skyscrapers are a distraction from rampant development and deforestation. They’re trees for the rich and no one else. They’re the soma in architecture’s brave new world of “sustainable” development.

In reality, trees on skyscrapers will likely be anything but sustainable. Structures built to support trees need to be over-engineered compared with their abiotic equivalents—trees are heavy, so is dirt (multiply so when wet), and so are watering systems required to keep them alive. If those trees are to have a chance on these windy precipices, their planters had better be deep, which further compounds problems raised in the previous sentence. A skyscraper that’s built to support trees will require more concrete, more steel, more of anything structural. That’s a lot of carbon, not to mention other resources, spent simply hoisting vegetation dozens of stories up, probably more than will ever be recouped in the trees’ lifetimes.

Bosco Verticale, the oft, and often only, cited example of a tower to be built with trees on top, is expected to cost $85 million. Stefano Boeri, the architect, estimates adding trees to the design pushed up construction costs about 5 percent. (No word on maintenance costs.) Whether that’s true or not, we’ll have to take his word for it. If we do, that means they will spend $4.25 million to put 2.5 acres—one hectare—of forest onto the side of a building.

Now, let’s say we take that money and resuscitate the region’s natural habitat.¹ Average costs run about $500 per acre for reforestation in U.S. national forests, with a top end of about $2,000 per acre. Let’s assume the worst. That means that with $4.25 million, you could restore 2,125 acres, or about 860 hectares, of forest. That’s 860 times more forest than is plastered on the side of Bosco Verticale. At the least. If restoration costs come in at the low end, about $200 per acre, it could be as high as 8,600 times more.

Then there’s the ecological value of each. Bosco Verticale will be home to a few birds (most of which will live in the city regardless) and some invertebrates, but not much else. It’ll also require massive human inputs—water, fertilizer, tending, and replacement. I covered the first of those three in my previous essay, so I’ll just elaborate on the last point here, replacement. Let’s say trees on a skyscraper will live for an average of 20 years—a generous assumption given that more than 50 percent of street trees, which are exposed to more benign conditions, die after just 10 years—what will we have gained? A skyscraper that needs an overhaul every 20 years.

A real forest, on the other hand, can replace itself. It can also support hundreds, even thousands of species, even in the middle of the city. A survey of the 315-hectare Central Park, for example, found over 800 species. Near Milan, at Parco Regionale di Montevecchia e della Valle del Curone, there nearly 1,000 known species on it’s 2,350 hectares. Biodiversity is just one measure. These forests also purify water, maintain nutrient cycles, and don’t require much in the way of maintenance (if any).

Here’s an alternate plan: Instead of planting trees on buildings, let’s focus on preserving and restoring places that already have, or desperately need, trees. Boeri and I agree on the importance of the latter. Bosco Verticale is the first stage of Boeri’s larger plan, one that includes preservation and restoration of existing land.² Bravo. It’s clear that Boeri understands the big picture, that to make a truly sustainable city, you have to incorporate ecosystem function on a broad scale.

We still disagree on the value of trees on skyscrapers: he, and Krulwich, see them as an inspiration; I see them as a distraction and potential liability—what if the Bosco Verticale becomes a brown eyesore, turning people off to his larger vision? I’d love it if Bosco Verticale and other proposed arboreal skyscrapers were sustainable and successful.³ Who wouldn’t want to live in a city full of tree towers? But I just can’t make a case for it. Plant physiology tells me that the trees, if they do survive, will require constant and costly maintenance throughout their short, brutal lives. Finance tells me that the money required to afforest a building would be more effectively used for restoration and preservation. And my gut tells me there are more equitable ways to give people trees, not just to those who can afford it.

  1. You could also plant street trees or reserve more land for parks, both laudable and equitable uses.
  2. Among the proposals is a greenbelt around the city. They’ll be great parks, but won’t do much to contain the city.
  3. Really, Robert, I do!


Roman, Lara. 2006. Trends in street tree survival, Philadelphia, PA. Master’s thesis.

Gorte, Ross W. 2009. U.S. Tree Planting for Carbon Sequestration. Congressional Research Service R40562.

Illustration of Bosco Verticale.

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Can we please stop drawing trees on top of skyscrapers?

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Can we please stop drawing trees on top of skyscrapers?

Editt Tower

Just a couple of years ago, if you wanted to make something look trendier, you put a bird on it. Birds were everywhere. I’m not sure if Twitter was what started all the flutter, but it got so bad that Portlandia performed a skit named, you guessed it, “Put a Bird On It“.¹

It turns out architects have been doing the same thing, just with trees. Want to make a skyscraper look trendy and sustainable? Put a tree on it. Or better yet, dozens. Many high-concept skyscraper proposals are festooned with trees. On the rooftop, on terraces, in nooks and crannies, on absurdly large balconies. Basically anywhere horizontal and high off the ground. Now, I should be saying architects are drawing dozens, because I have yet to see one of these “green” skyscrapers in real life. (There’s one notable exception—BioMilano, which isn’t quite done yet.) If—and it’s a big if—any of these buildings ever get built, odds are they’ll be stripped of their foliage quicker than a developer can say “return on investment”. It’s just not realistic. I get it why architects draw them on their buildings. Really, I do. But can we please stop?

There are plenty of scientific reasons why skyscrapers don’t—and probably won’t—have trees, at least not to the heights which many architects propose. Life sucks up there. For you, for me, for trees, and just about everything else except peregrine falcons. It’s hot, cold, windy, the rain lashes at you, and the snow and sleet pelt you at high velocity. Life for city trees is hard enough on the ground. I can’t imagine what it’s like at 500 feet, where nearly every climate variable is more extreme than at street level.

Wind is perhaps the most formidable force trees face at that elevation. Ever seen trees on the top of a mountain? Their trunks bow away from the prevailing winds. That may be the most visible effect, but it’s not the most challenging. Wind also interrupts the thin layer of air between a leaf and the atmosphere, known as the boundary layer. The boundary layer is tiny by human standards—it operates on a scale small enough that normally slippery gas particles behave like viscous fluids.

For plants, the boundary layer serves to control evapotranspiration, or the loss of gas and water through the tiny pores on a leaf’s underside, known as stomata. In calm conditions, a comfortably thick boundary layer can exist on a perfectly smooth leaf. But plants that live in hot or windy places often have adaptations to deal with the harsh conditions, including tiny hairs on their leaves which expand each leaf’s surface area and thus its boundary layer. Still, plants in these environments aren’t usually tall and graceful. In other words, not the tall trees we see in architectural drawings.

Next let’s add extreme heat and cold to the mix. Extreme cold, well, we all know what that does. It can kill a plant, turning the water inside its cells into lethal, crystalline knives. At the other end, hot conditions post a different set of challenges. To cool off, plants can “sweat” by opening their stomata to release water vapor, at least as long as there’s water available. But even then, plants reach a limit. At certain temperatures, which vary from plant to plant, the photosynthetic machinery inside a leaf starts to break down. Keep in mind these are temperatures on the surface of a leaf, not ambient air temperature. The surface of leaf—especially in direct sunlight, as on the unshaded side of a skyscraper—can be many degrees hotter than the air, up to 14˚ C in some species (nearly 26˚ F).

Then there are the logistical concerns. How are these trees going to be watered and fertilized? Pruned? How will they be replaced? How often will they need to be replaced? As someone who grows bonsai, I can tell you that stressed plants require constant attention. Daily monitoring, in fact, and sometimes even more frequently. It’s not easy. Growing simple green roofs is a chore, and those plants are chosen for their hardiness and low maintenance. Trees are generally not as well adapted to the wide range of conditions likely to be experienced on the side of a skyscraper.

All of this may sound a bit ridiculous coming from someone like me, an advocate for more trees in urban spaces. It probably comes from having seen one too many sketches of a verdant vertical oasis but too few of them actually built. Plus, having studied plant physiology, I know that it’s a pipe dream in many ways. Trees just weren’t made for such conditions. Now if someone want to gin up a tree that can survive on top of a skyscraper, go ahead, I guess. But I can think of far better things we should be putting our time and effort into, like preserving places that already have trees growing on them or planting more on streets that need them.

  1. “What a sad little tote bag. I know! I’ll put a bird on it.” Etc.

Illustration of Editt Tower, a proposed 26-story building in Singapore.

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Against invasive plants, underdog natives hang on

Dianella ensifolia

One of the scourges of our globalized economy is invasive species. In California, annual Mediterranean interlopers have upended the state’s once perennial grasslands. The Australian outback has been blanketed with prickly pear cacti from the American Southwest. And wattles from Down Under are a scourge in South Africa. But as widespread as invaders are, we’re only just beginning to understand how they move around the globe, establish themselves, and reshape the ecosystems they disturb.

A key unsettled debate is whether or not invasive plants change patterns of biodiversity. Some studies have found that biodiversity suffers when nonnative plants arrive and take over. Others have found the opposite, that new species add to the mix rather than deplete or homogenize it. Well, the authors of a new paper published today in Science say both answers are right. According to them, it’s all a matter of scale.

Studies on invasive plants and biodiversity can generally be classified according to scale, small and large. Small scale studies pore over tiny patches of land, generally less than 25 square meters. In those cases, researchers have generally found that plant biodiversity suffers when invasives are present. Other scientists say that, on the large end of the scale, they see no difference.

In an attempt to reconcile these consistently disparate findings, Kristin Powell of Washington University in St. Louis and her colleagues sought to bridge both scales. They set up both large (500 m2) and small (1 m2) plots in Hawaii, Florida, and Missouri, each of which has its own problematic nonnative. In Hawaii, it’s the fire tree, Morella faya; in Florida the cerulean flax lily, Dianella ensifolia; and Missouri the Amur honeysuckle, Lonicera maackii. The study subjects run the gamut from overstory tree (fire tree) to mid-story shrub (honeysuckle) to understory herb (flax lily). Each state in the study has parts that are invaded and parts that are not, a fact which Powell and her colleagues used to their advantage by surveying plots on either side.

What they found is as nuanced as you might expect from a confused and messy situation involving the natural world. On small scales, the 1 m2 plots, they found that biodiversity was, in fact, lower. In the large plots, species richness was a slight bit lower, but it was close enough to be a wash. These results essentially jibed with those found by other scientists.

If you dig a little deeper, things get more interesting. They also found that ecosystems hosting invasive plants are generally more homogenous—the patchy pastiche you would normally expect just wasn’t there. But they’re not ecological clean rooms, either. Though diversity was down, there didn’t seem to be evidence of extinctions. Native plants may have disappeared from a large number of the smaller quadrats, but they typically weren’t absent from the larger plot. The diversity was there, it was just hiding among the invaders.

That’s good news, in a way. We probably won’t lose interesting and potentially important plant species because of competition from invasive plant species. But that’s not to say the natives are free and clear, though. If their populations are suppressed too much, they could be sitting ducks for another disaster, such as a catastrophic fire or human development. They’d be one step away from being wiped off the map.

This study’s results suggest that we should reevaluate how protections like the Endangered Species Act can—or can’t—help in the case of invasives. If, as this study says, invasions seldom lead to extinctions, then protections like the ESA won’t help much against invasive species. But if invasives depress population numbers enough, native species would be vulnerable, meaning the ESA would be more relevant than ever. In those cases, we should be even more vigilant in areas overrun with invasives. On the surface, they may not look like healthy ecosystems. But lurking within are the native remnants of one. If we buy those systems enough time, they may sort themselves out.


Powell, Kristin I., Jonathan M. Chase, and Tiffany M. Knight. 2013. “Invasive Plants Have Scale-Dependent Effects on Diversity by Altering Species-Area Relationships.” Science 339: 316-318. DOI: 10.1126/science.1226817

Photo by SSKao.

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To cultivate better students, plant a few trees

Lone tree

Look at that tree up there. Do you feel smarter? You should, provided that you’re a teenager. And that tree above? Well, it’s should be right outside your high school’s cafeteria windows, or even your classrooms’. Better still if all of your school’s windows are filled with a similarly arboreal tableau.

That’s the upshot of a 2010 paper that reports on how the view out high school windows can influence students’ academic performance. That may seem limited, but this is science, where we have to take things one step at a time. This step involves high schoolers. It’s possible that such benefits extend beyond just high schoolers—after all, people who have views of nature while at work are happier at their jobs—but for now, it’s just high schoolers.

The study looked at 101 public high schools in southeastern Michigan. I presume they spanned a variety of socioeconomic conditions and geographic settings, though the author didn’t confirm that, so I can’t be certain. However, when calculating his final statistics, he did control for socioeconomic status, as measured by student participation in free and reduced lunch, ethnicity, school size, and age of the school building.

For each school, he also gathered a variety of environmental variables, including the area of the campus taken up by athletic fields, parking lots, and other landscaping, as well as the ratio of trees to shrubs to lawns in that landscaping. He measured the size of the windows in classrooms and cafeterias and noted whether students could eat lunch outside. Finally, he judged how nice those views were, how much nature they contained. It’s a somewhat subjective measure, but an important one nonetheless.

To see how those views affected students, the author considered three academic and two behavioral criteria—graduation rates, four-year college plans, percentage of students who won the Michigan merit award (based on standardized test scores), disorderly conduct, and truancy.

Lo and behold, views of nature did have an effect. Students who could see real nature outside their cafeteria windows were more likely to graduate, to plan to attend a four-year college, and to win a Michigan merit award. They were also less likely to commit a crime. The bigger the windows, the better the results in some cases, including for four-year college plans (they were higher) and criminal activity (it was lower). Classroom windows didn’t have a noticeable influence, perhaps because students spend more time staring at the teacher and the board than out the window. That’s probably a good thing.

Put simply, students performed better academically at schools with more nature around it and bigger cafeteria windows to view it. They were more likely to graduate, more likely to plan to attend college, and less likely to be miscreants. The opposite was also true. If the school was surrounded by a large parking lot or expansive athletic fields, students there were less likely to want to attend college.

Now, planting trees and installing floor-to-ceiling windows in the cafeteria isn’t suddenly going to turn a struggling school into a star performer. Socioeconomic conditions, quality of life at home, and classroom sizes all exert far more influence over student performance. But the results presented in this paper aren’t insignificant. Views of nature improved most measures by 5 to 15 percent. That’s pretty impressive.

It also suggests that we shouldn’t build schools as cheaply as possible, which often means fewer windows and more cheap cinder block.¹ Opening up the walls, and making the views worthwhile, could do wonders. A few trees here and there along with a little less mowing could boost student academic achievement, reign in delinquency, and do so inexpensively. It’s not going to solve the problems that plague many of our schools,² but planting trees is so simple that it would be foolish to ignore. What are we waiting for?

  1. I should know. My high school was one of those.
  2. See the previous paragraph for what really needs to be tackled.


Matsuoka R.H. (2010). Student performance and high school landscapes: Examining the links, Landscape and Urban Planning, 97 (4) 273-282. DOI:

Photo by Kevin.

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Lone tree

David Nowak is at it again. The tireless ecologist and his frequent collaborator Eric Greenfield have given us another comprehensive snapshot of urban forests in the United States. Nowak has published numerous studies on urban trees, one of which I’ve covered previously. But that study—which discovered that some cities are leafier than their pre-urban states—takes a longer view and paints a different picture. In this new study, Nowak and Greenfield focus on the more recent past, the 2000s, and find that 17 of the 20 cities they surveyed had significantly fewer trees than just five years earlier. Sixteen became more developed, too, with an increase in impervious surfaces like roadways, parking lots, rooftops, and sidewalks. Taken together, these findings suggest that cities across the U.S. are steadily defoliating.

In this most recent study, Nowak and Greenfield turned to a favorite tool of mine for evaluating forest cover—aerial photographs. For each city, Nowak and Greenfield found a pair of images taken three to six years apart. They scattered a minimum of 1,000 points across each city’s photo pair and manually classified them into eight categories: trees and shrubs, grass and herbaceous cover, bare soil, water, and three classes of impervious surfaces (buildings, roads, and other). They were accurate to within a few percent in every class.¹ Nowak and Greenfield also compared their subset of 20 cities to other large cities in the U.S. by sampling an additional 1,000 points in metro areas across the country. The results were largely similar, meaning what they found in the 20 cities would be indicative of nationwide trends.

With the exception of Syracuse, all cities in the study had fewer trees in the later image. Overall, existing tree cover decreased 0.9 percent per year in 18 of the 20 cities (New Orleans and Detroit were excluded from most statistics—more on why later). That may not sound like much, but like any annualized percentage, it compounds. When a small amount of trees are lost from one year to the next, the change may not be that noticeable. But over 10 years, the results can be drastic. And when you remember that trees account for just 28 percent of land area in these cities, at these rates it won’t be long before trees cover the barest sliver of our urban areas.

Nowak and Greenfield excluded two of their surveyed cities, New Orleans and Detroit, from most statistics based on their extraordinary circumstances. In New Orleans, they were interested in seeing how Hurricane Katrina changed the urban forest, and in Detroit, they were looking for signs of the emerald ash borer infestation, an invasive insect that has decimated ash trees throughout New England and the Midwest. Detroit was lucky—it emerged relatively unscathed, losing only 0.7 percent of its total tree cover, in part because vacant lots make a great home for young trees. New Orleans, on the other hand, fared poorly, losing more trees than any other city in the study, nearly 10 percent.

Those cities aside, the trends don’t bode well for urban forests. Most concerning is the conversion from trees to impervious surfaces. Losing trees to grass and bare soil may be concerning, but it’s not catastrophic. Those cover types are easy to reforest. But when land becomes covered by roads, parking lots, or buildings, it very rarely reverts to vegetation of any type. At least 71 percent of the time, the move to impervious surface was because of development, not, say, a tree dying and revealing a parking lot beneath. The majority of the time, it was the more permanent type of conversion.

Which leaves us with a question—how are we going to maintain, let alone increase, tree cover in cities where buildings, streets, and parking lots are taking over the landscape, often in irreversible ways? One is for cities to take direct action, by planting trees in existing public spaces. Lots of cities have planting programs, but in many cases they’re barely able to keep pace with deaths from disease, urban stress, and old age.

Another option is to open gaps in the concrete along streets to provide new opportunities for planting. Not only does this give the city more places to plant, it sends a good message to the neighborhood, showing that the city cares. Planting trees may also be a way of leading by example, an exercise that could be followed by the offer of free trees for neighborhood residents. It’s subtle, but potentially more effective because there’s often more land behind people’s homes than in front. Cities could also add more parkland. Trees in parks aren’t exposed to the same stresses as street trees, but parkland can be expensive to procure.

That Nowak and Greenfield’s results span the last decade makes them that much more alarming. We’ve lost 1 percent of our existing tree cover per year in that time. Where will that leave us ten years from now? We don’t know, exactly. Until we have more data from earlier years, we just won’t know for certain—these sorts of changes could be cyclical. But given trends in development and population density and the rate at which trees have been converted to impervious surfaces, I’m guessing we’re headed toward fewer trees.²

  1. Just as it’s easy for even the untrained eye to see differences in income based on relative differences in tree cover, it’s a simple task for a trained photointerpreter to tell trees from grass. It’s difficult for computer algorithms to match human photointerpreters’ accuracies, but sometimes the data set is too large for manual classification. It’s always a tradeoff. In this case, the areas covered were of manageable size, so a human touch made sense.
  2. I should point out that I’m not condemning population density. Rather, we need to think long and hard about the way we accommodate high population density. Cost can’t be the only consideration.


Nowak, D.J. & Greenfield, E.J. (2012). Tree and impervious cover change in U.S. cities, Urban Forestry & Urban Greening, 11 (1) 30. DOI: 10.1016/j.ufug.2011.11.005

Photo by Franco Folini.

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Urban trees reveal income inequality

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Income inequality, as seen from space

Last week, I wrote about how urban trees—or the lack thereof—can reveal income inequality. After writing that article, I was curious, could I actually see income inequality from space? It turned out to be easier than I expected.

Below are satellite images from Google Earth that show two neighborhoods from a selection of cities around the world. In case it isn’t obvious, the first image is the less well-off neighborhood, the second the wealthier one.

Rio de Janeiro


Rocinha, Rio de Janeiro

Zona Sul

Zona Sul, Rio de Janeiro


West Oakland

West Oakland


Piedmont, California (enclave of Oakland)


Fourth Ward

Fourth Ward, Houston

River Oaks

River Oaks, Houston



Hyde Park

Hyde Park, Chicago



Fengtai, Beijing


Chaoyang, Beijing

Boston metro area, Massachusetts

Ball Square, Somerville

Somerville, Massachusetts

West Cambridge

Cambridge, Massachusetts

Your examples

Do you have other cities or neighborhoods in mind? I’d love to hear from you. Send me an email with photos or link to your blog post. In the next couple of weeks, I’ll put together a follow up article that features your examples.

Be sure to include the names of the cities and neighborhoods you’re highlighting and if you’d like me to mention your name.


Your examples are now posted! The response to my call for examples has been unbelievable. I’ve received hundreds of messages. I have the first batch up, and as I have time, I’ll be adding many more. Keep ’em coming.

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Urban trees reveal income inequality

Street trees

Wealthy cities seem to have it all. Expansive, well-manicured parks. Fine dining. Renowned orchestras and theaters. More trees. Wait, trees? I’m afraid so.

Research published a few years ago shows a tight relationship between per capita income and forest cover. The study’s authors tallied total forest cover for 210 cities over 100,000 people in the contiguous United States using the U.S. Department of Agriculture’s natural resource inventory and satellite imagery. They also gathered economic data, including income, land prices, and disposable income.

They found that for every 1 percent increase in per capita income, demand for forest cover increased by 1.76 percent. But when income dropped by the same amount, demand decreased by 1.26 percent. That’s a pretty tight correlation. The researchers reason that wealthier cities can afford more trees, both on private and public property. The well-to-do can afford larger lots, which in turn can support more trees. On the public side, cities with larger tax bases can afford to plant and maintain more trees. Given the recent problems New York City has had with its aging trees dropping limbs on unsuspecting passers-by—and the lawsuits that result—it’s no surprise that poorer cities would keep lean tree inventories.

But what disturbs me is that the study’s authors say the demand curve they see for tree cover is more typical of demand for luxury goods than necessities. That’s too bad. It’s easy to see trees as a luxury when a city can barely keep its roads and sewers in working order, but that glosses over the many benefits urban trees provide. They shade houses in the summer, reducing cooling bills. They scrub the air of pollution, especially of the particulate variety, which in many poor neighborhoods is responsible for increased asthma rates and other health problems. They also reduce stress, which has its own health benefits. Large, established trees can even fight crime.

Fortunately, many cities understand the value trees bring to their cities. New York City is aiming to double the number of trees it has to 1 million. Chicago has planted over 600,000 in the last twenty years.¹ And London has been working to get 20,000 new trees in the ground before it hosts the Olympics.

But those cities are relatively wealthy. It’s the poorer ones that probably need trees the most but are the least able to plant and maintain them. The Arbor Day Foundation is a great resource in those cases, but like many non-profits, it is stretched too thin. Compounding the inequality is the fact that most tree planting programs are local. Urban forestry has sailed largely under the federal government’s radar. The U.S. Forest Service does have a urban and community forestry program, but is woefully underfunded, having only $900,000 to disperse in grants. Bolstering that program could help struggling cities plant the trees they need. After all, trees and the benefits they provide are more than just a luxury.

  1. Though like many of Chicago’s boasts that number was probably inflated by including replacement trees.


Zhu, P., & Zhang, Y. (2008). Demand for urban forests in United States cities Landscape and Urban Planning, 84 (3-4), 293-300 DOI: 10.1016/j.landurbplan.2007.09.005

Photo by Alex E. Proimos.

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The United States is not run by godless Communists. Neither is most of the rest of the world. In fact, the godless Communists that do remain are not all that Communist anymore. I bring that up because command and control economies can dictate what development happens where. Land conservation under such a system is technically easier, even if the actual results in Communist nations like the Soviet Union weren’t that inspiring. Land conservation in the free world is a trickier game, one played with carrots and sticks as opposed to edicts and directives. Here, money is your best friend.

Conservation organizations have focused on preserving big tracts of land, and rightfully so. Big buys are often more cost effective and easier to manage. But they’re also becoming trickier to execute in a world dominated by curving cul-du-sacs and one acre lots. If we want functioning ecosystems in these places, we need to focus on land conservation within the subdivision, not along its borders.

Luckily, the carrot seems to be working in those places. A study of subdivisions in Maryland between Washington, D.C., and Baltimore shows that developers have been incorporating more open space into their subdivisions. That’s not because they’re interested in land conservation. Part of it is a bit of command and control—Maryland’s Forest Conservation Act forces developers to conserve a modicum of forested land—but it’s also simple economics. Developers can sell lots and houses at higher prices if open space is nearby. Because proximity matters, that open space typically needs to be within the subdivision.

To developers, though, that open space is fungible. It can exist either as public parks or larger private lots—both raise prices. The Maryland study also found that minimum lot sizes—which governments typically use to preserve open space—can push developers away from shared open space toward larger lot sizes.

This poses a problem for maintaining healthy ecosystems. Like many laws, the way the Maryland Forest Conservation Act is interpreted matters. People can uphold the letter of the law—maintaining forest cover—without changing their usual habits—mowing their entire lot. The result is something that looks like a forest from above but doesn’t function like one.

In a perfect world, everyone would happily tend a few thousand square feet around their house and leave the rest to nature. But that’s not always the case. People will spend all Saturday mowing acres of grass and grumble about it afterwards. That’s because for many people owning a country manor is more alluring than owning a chunk of the great outdoors. You can fight that mentality by increasing minimum lot sizes to the point where mowing it all becomes completely unreasonable,¹ but the closer you get to a metro area, the less tenable that becomes.

There’s also no guarantee that laws dictating minimum lot sizes will remain in place. As the city creeps closer, pressure to further subdivide will mount. Open space preserved in private lots could easily disappear.

Parks, on the other hand, tend to stick around. Unlike large lots, they’re seldom subdivided. Instead, they tend to become institutions. People like their parks and are loathe to lose them—no one wants to see their neighborhood park disappear. So let’s put that to use. Instead of—or in addition to—minimum forest cover and minimum lot sizes, let’s institute minimum park sizes. Everyone will benefit. Developers will be able to sell lots at higher prices. Kids will have playgrounds. Adults will have walking paths. And because big parks often have big natural areas, ecosystems will have a better chance at surviving. It’s a solution that’s a bit more command and control than current vague regulations, but everyone will benefit. It’s also more carrot than stick. Even if you don’t particularly like carrots, it’s better than getting hit with a stick.

  1. Though there will always be exceptions—near where I grew up, one guy mowed 18 acres. He had to buy a bonafide farm tractor so it wouldn’t take him all week.

Photo by JD Hancock.


Lichtenberg, E., Tra, C., & Hardie, I. (2007). Land use regulation and the provision of open space in suburban residential subdivisions Journal of Environmental Economics and Management, 54 (2), 199-213 DOI: 10.1016/j.jeem.2007.02.001

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Red-eyed Vireo

Earlier this week I pointed out that urban areas can actually increase tree cover over time, albeit with a caveat. The two studies I cited measured tree cover and only tree cover—they made no claims about ecological function. Luckily, other studies have done just that, including one that looked at migratory bird use of greenways in urban areas.

Migratory routes are important, though most research into migratory bird decline has focused on habitat loss in their breeding and wintering grounds. That has left a large piece of the puzzle unsolved—the habitat between point A and point B. Think of it this way: If snowbirds—you know, northern (human) retirees who flock to warmer climes in the winter—started disappearing and our best solution was to look for them at their apartment in New York or their rental in Boca Raton—ignoring rest stops and motels along I-95—we’d be doing a great disservice to our older generations. Ignoring flyways is similarly foolish.

There have been studies in more recent years that aim to fill this gap, and one published in 2009 by Salina Kohut, George Hess, and Christopher Moorman picks up the trail along, well, trails. They surveyed bird species abundance and richness—how many and how varied the itinerants were—in 47 greenways in and around Raleigh, North Carolina.

Greenways are a common and convenient way for cities to conserve natural habitat. Their linear form is well suited to urban areas, and they easily double as parks or recreational trails. They also can serve as stop-over habitat for migratory birds. Kohut, Hess, and Moorman were hoping to find the right type of corridor for migrating birds, where our feathered friends can take a load off and fatten up.

It turns out that most birds were not picky and would stop at just about any greenway, regardless of vegetation, adjacent land use, or corridor width. That’s not to say all greenways were entirely equal. Overall, birds favored corridors with taller trees and lots of native shrubs teeming with fruit. And among birds that live in forest interiors far away from human development and even open fields, greenways wider than 150 meters (about 500 feet) surrounded by low-intensity development were the most popular.

None of the greenways Kohut and her colleagues studied were as good as a regular forest, though. Still, with some tweaks—including widening corridors, siting them near low-intensity development, and planting with natives—greenways can make decent stand-ins for the real thing, at least as far as migratory birds are concerned. Residential neighborhoods can even make themselves into agreeable stopover habitat by mimicking vegetation found at popular stops along the flyway.

So greenways make for good bird habitat, but let’s not forget that they’re good neighbors, too. In addition to helping migrating fauna, they boost property values, add recreational opportunities, and work well as commuting corridors for cyclists. Five benefits from one land use. Not too shabby.

Photo by qmnonic.


Kohut, S., Hess, G., & Moorman, C. (2009). Avian use of suburban greenways as stopover habitat Urban Ecosystems, 12 (4), 487-502 DOI: 10.1007/s11252-009-0099-6

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Tree City

City tree silhouette

Cities aren’t called “concrete jungles” for their leafy greenness. But perhaps it’s an inappropriate nickname. Several cities actually have more—not less—tree cover than what came before them. By way of example, take this from historian William Cronon: “There are more trees in southern Wisconsin now than at any point in the last 7,000 years.” That’s in part due to more than a century of fire suppression, but also the intense pace of urban development.

There’s ample scientific evidence to back up Cronon’s assertion. In the early 1990s, David Nowak, an urban forester with the U.S. Forest Service, found that tree cover in Oakland, California, between 1850 and 1989 rose sharply from 2 percent to 19 percent. Now, a new study by Adam Berland, a PhD student at the University of Minnesota, found a similar pattern in and around Minneapolis, Minnesota.

Oakland and Minneapolis—and many other metro areas, I suspect—were sparsely forested before urban development. As far back as 1500 BCE, what would become Oakland was regularly burned by the Coastanoan Indians to clear out the underbrush to simplify acorn gathering. What trees remained in the 1700s were logged for lumber and firewood by the missions. Then in 1848, what was left nearly vanished when gold was discovered in California. By the time Oakland incorporated in 1852, its namesake was nearly gone.

Fire likewise held forests in southern Minnesota at bay for thousands of years. Yet unlike in central California, a part of central Minnesota quickly afforested during a brief climate cooling 400 years ago. It wasn’t long lived, though—shortly after their arrival, European settlers swiftly knocked down most of the Big Woods for farming. The remaining flecks large enough to be called forests cover only 2 percent of the original area. In other words, forests near Oakland and Minneapolis had nowhere to go but up.

The arrival of dense settlement was something of a godsend for trees. Young neighborhoods and cities are often depauperate—it’s easier to build without big trees in your way—but they tend to accumulate tree cover as they age. And relative to the denuded landscape that came before Oakland and Minneapolis, those urban forests are more akin to a real jungle than a concrete one.

Urban forests are certainly an improvement from a tree’s perspective, but they’re not a panacea for habitat loss. Neither of these studies examined how those forests function ecologically. Just like 11 random people do not make a soccer team, a bunch of trees is not the ecological equivalent of a real forest. Not only is the understory substantially different in cities—houses are terrible forage for most insects and animals—but the types of trees are often radically different.

Still, these two studies should make abundantly clear that cities do function as ecosystems, albeit limited ones. And in some cases, they are more diverse and productive than what came before. This is especially true for metropolitan Minneapolis, where monocultures of wheat and corn were less diverse than the Big Woods they replaced and maybe less ecologically complex than the cities that replaced them. These two cases also underline the need for an urban ecology that doesn’t just study what systems cities create, but strives to shape those systems for greater ecological complexity and diversity.


Berland, A. (2012). Long-term urbanization effects on tree canopy cover along an urban–rural gradient Urban Ecosystems DOI: 10.1007/s11252-012-0224-9

Nowak, David J. (1993). Historical vegetation change in Oakland and its implications for urban forest management Journal of Arboriculture, 19 (5), 313-319

Photo by frozenchipmunk.

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Ghosts of ecology

Roman mosaic

If you want a glimpse of our ecological future, take a look at present-day Europe. Continuous and intensive human habitation for millennia have crafted ecosystems that not only thrive on human disturbance, they’re dependent on it. But even in places where pastoral uses have fallen by the wayside, the ghosts of past practices linger. If you have any doubt that the changes we’re making to the earth right now will be felt thousands of years from now, these two studies should wipe those away.

This post was chosen as an Editor's Selection for ResearchBlogging.orgThe first takes place in a post-apocalyptic landscape masquerading as a charming woods, the Tronçais forest. Smack in the middle of France, Tronçais is the site of a recent discovery of 106 Roman settlements. Photographs of the settlements call to mind Mayan ruins in Yucatan jungles, with trees overtaking helpless stone walls. Tronçais was not unique in this way—following the fall of the Roman Empire, many settlements reverted to forest after the 3rd and 4th centuries CE.

Ecologists studying plant diversity in the area noticed two distinct trends. First, the soil became markedly different as they sampled further from the center of the settlements. Nearly every measure of soil nutrients declined—nitrogen, phosphorous, and charcoal were all lower at further distances. Soil acidity declined, too. Second, plant diversity dropped off as sample sites moved further into the Roman hinterland, and likely a result of changes in the soil.

The researchers suspect the direct impacts of the settlement and Roman farming practices are behind the trends. High phosphorous and nitrogen levels were probably due to manuring. The abundance of charcoal is clearly from cooking fires, while soil pH was affected by two uses of lime common in the Roman empire—mortar used in building and marling, the spreading of lime and clay as a fertilizer. The combined effects of these practices fostered plant diversity after the settlements fell into ruin, the effects of which can be seen to this day.

The second study was undertaken by another group of ecologists who canvased grasslands in northern and western Estonia. While threatened today by the usual suspects—intensive agriculture and urbanization—the calcareous grasslands of Estonia have a long history of human stewardship which helped a wide variety of grasses and herbs to flourish. They were greatly expanded by the Vikings, who settled the area between 800 and 1100 CE. Knowing this history, the researchers suspected population density may have boosted floral diversity. They sampled exhaustively, recording plant species and communities in 15 quadrats at 45 sites for a total of 675 sample plots. They also drew 20 soil samples at each site. To estimate population density during the Viking Period, they used an established model that estimated settlement size and extent based on known ruins.

Soil qualities naturally had an affect on present-day plant diversity, but human population density during and shortly after the Viking Period also emerged as a significant predictor. As with the Roman study, changes to soil nutrients because of human activities are likely behind the results. But that’s not all. The researchers point out that seed dispersal 1,000 years ago also influenced present-day diversity. When the Vikings expanded the grasslands, they connected different patches that had previously been isolated, allowing previously isolated species to germinate in new areas.

These are not the first studies to reveal a shadow of human habitation in present day ecosystems—the Amazonian rainforest is littered with evidence of agriculture before European contact, for example. But these studies show the ghosts of ecology persisting for millennia, not centuries. Not only does it bolster the notion that no landscape is pristine—an idea that has been gaining traction with the ecological community—it should underscore the persistence of any human activity.


Dambrine, E., Dupouey, J., Laüt, L., Humbert, L., Thinon, M., Beaufils, T., & Richard, H. (2007). Present forest biodiversity patterns in France related to former Roman agriculture Ecology, 88 (6), 1430-1439 DOI: 10.1890/05-1314

PÄRTEL, M., HELM, A., REITALU, T., LIIRA, J., & ZOBEL, M. (2007). Grassland diversity related to the Late Iron Age human population density Journal of Ecology, 95 (3), 574-582 DOI: 10.1111/j.1365-2745.2007.01230.x

Photo by mharrsch.

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Farms giving way to subdivisions in Southeastern Wisconsin

If you were a squirrel living in Southeastern Wisconsin, you’d be pleasantly surprised by the state of things. In many places, there are as many—if not more—trees than there were 200 years ago. But that rosy image doesn’t tell the entire story. Comparing the forests that cover the cities and suburbs around Milwaukee—and likely in many places around the world—is like comparing Rome before and after the fall. It’s still Rome, but it’s not quite the same as it used to be.

Southern Wisconsin is a case study of the changes that were affecting much of the country in the 20th century. Most of the forests had been cleared in the 1800s by farmers, resulting in a landscape that little resembled what came before. The woodlots that remained were small and scattered. In one famous study, only 4.8 percent of the original forests remained by 1935. Milwaukee and its surrounding cities grew steadily in the run-up to World War II, but positively boomed thereafter. They needed room to grow, and since cleared land is easy to build on, farm after farm was subdivided.

The path from forest to front yard seems clear cut. A woods is cleared to make way for farmland, which is later subdivided into lots and sold off to make way for homes. But the reality is much more complex than that. Though a neighborhood may maintain its wooded appearance, it’s original character is gone.

In Wisconsin, subdivisions are invariably preceded by farms. Farming is a tough life. There’s not much money to be made with a small family farm, and an farmer’s property often doubles as his retirement fund. To maximize the investment, he’ll usually subdivide it for housing. It usually works out well for him, because land that’s good for growing crops is also good for building houses—it’s not too steep and most of it doesn’t need to be cleared.

That’s not to say farms are entirely devoid of trees. Most contain small woodlots and extensive fencerows that separated fields of corn, wheat, and soybeans. They’re relics of bygone forests, and in many places that’s all that’s left. Though the relationship is a bit one-sided, relic trees and farms have existed side-by-side for decades.

Maintaining that landscape during subdivision isn’t difficult. Building houses around trees is easy if you don’t take a cookie cutter approach, and houses with big trees in their yards tend to sell for more. But conservation rarely happens. That’s the conclusion of one study of Southeastern Wisconsin. It looked at the fate of extant vegetation as farms gave way to subdivisions between 1937 and 1975. Though the sum total of forested land didn’t drop as much as anticipated, very little of the original vegetation that made it through the transition. By 1975, the trees that dotted subdivisions and roadsides were almost entirely new.

That study reminds us that sum totals seldom tell an entire story. The relationship between forests, farms, and yards is complex and multidirectional. Forests are often cleared for farms, but abandoned farms can return to their forested state over time—much of New England underwent this process. However, urbanization can intervene along the way, removing the little remaining vegetation and replacing it with landscaped yards. But that’s not all the forest loss development is responsible for. Though many subdivisions are carved from land cleared previously for farms, they can be indirectly responsible for the loss of even more forests. Street and yard trees can’t offset this entirely. Similar patterns are well documented in developing nations. In Brazil, for example, expanding soy production has pushed cattle ranchers to clear land further into the frontier. It’s easy to forget these same processes are at work here in the United States.

Even when subdivisions spring fully formed from forested land—skipping the intermediate farm stage—their lots are often cleared of existing vegetation. Some of my research in graduate school documented the stark changes forest edges undergo when houses move in. In old black-and-white aerial photographs, the bare earth of cleared building sites stood out in stark contrast to the dark gray of the surrounding woodlands. Straight, sharp lines separated the two. In time, the edge bled back into the yards, but it wasn’t quite the same.

Suburban development isn’t going away anytime soon, but some of the structure and function of the old woodlands they replaced can be recovered. Homeowners can plant native trees. People can lobby their cities to plant native trees as well, rather than the whatever low-maintenance tree is in fashion among city foresters this year. The result won’t be the same as an intact woodland, but at least it will be similar.


Sharpe, D., Stearns, F., Leitner, L., & Dorney, J. (1986). Fate of natural vegetation during urban development of rural landscapes in Southeastern Wisconsin Urban Ecology, 9 (3-4), 267-287 DOI: 10.1016/0304-4009(86)90004-5

Photo by sierraromeo.

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Salvaging disturbed forests may not save biodiversity

Never buy a car with a salvage title. Anyone who has ever driven a car after a major accident can tell you why—it’s just not the same as before the crash. Though all the parts might be in the right place and the paint just as shiny as before, there’s invariably some new rattle, shake, or whistle that you can’t fix. The magic that is gone, and nothing will bring it back. Cars are a lot like primary tropical forests in that way.

Biodiversity thrives in undisturbed tropical forests. But once they have been selectively logged, burned, or leveled, what grows back in their place just isn’t as rich, vibrant, or diverse as the original, according to a new paper released online today in Nature. The meta-analysis—written by a number of authors including Bill Laurence and Tom Lovejoy, two deans of tropical conservation—synthesized 2,220 pairwise comparisons of primary and disturbed tropical forests from 138 different studies on four different continents to arrive at that one conclusion.

The dominant image of deforestation—at least from an American perspective—is the Amazon. Photographs and satellite images of logging and agricultural conversion show in graphic detail splintered tree stumps, smoking ashes, and herringbone tentacles of human influence. But while the authors found South American forests are greatly threatened by human disturbance, Asian forests are even more imperiled.

To compare results from numerous studies, the study’s authors the measured effect size of human disturbance on biodiversity. It’s a statistical technique which describes the magnitude of differences between populations. The effect size of land-use changes in Asia was more than twice that of second place South America and even larger still than those of Africa and Central America.

To give you an idea of the severity of Asia’s biodiversity threats, let’s review the guidelines on interpreting effect sizes. Generally, a small effect size is 0.2, medium is 0.5, and large is 0.8 and above. In the study, Central America checks in at 0.11, Africa at 0.34, and South America at 0.44. (A quick caveat before we continue: The African result may not be representative. The continent’s tropical forests are understudied because of continued conflict, and future disturbance rates could accelerate in the face of population growth.) Asia is far ahead of the rest of the pack, blowing them all away with an effect size of 0.95.

Asian tropical forests are more threatened by every type of human impact than tropical forests on other continents. Agricultural conversion is responsible for a large portion of biodiversity loss in the region, with plantations and selective logging operations following not far behind. Plantations are of particular concern because the crops they yield—primarily palm oil and exotic woods—are lucrative. Their profit potential draws interest not only from multinational corporations, but governments as well. These organizations have large amounts of capital and can convert vast tracts of primary forest into ecologically sterile plantations that practically print money.

Plantations also have the advantage—for governments and corporations, at least—of looking deceptively like natural forests to many people. Asia Pulp & Paper, a company with large plantation holdings throughout Southeast Asia, has been exploiting this confusion through a series of recent TV ads. The Indonesian government has been in on the ruse, too, suggesting that it may push for their plantations—many of which were carved from primary forests—to count as forest land under REDD schemes, or reduction of emissions through deforestation and forest degradation. That means the government would not only profit from the plantations’ crops, but also from international payments to purportedly offset or reduce carbon emissions.

If we have to use forest land at all, the best bet to preserve biodiversity seems to be selective logging. Though the practice still harms overall biodiversity, it does so less than other land uses. Still, the paper’s authors caution that selective logging’s ill effects may be masked by proximity to less disturbed primary forests, which may export species to depauperate tracts. If this is the case, then selectively logged areas may be running the ecological equivalent of a trade deficit with primary forests. Without some reciprocation, the two will eventually go bankrupt.

This new meta-analysis confirms what many ecologists have long suspected—that minimally disturbed primary forests are some of the best bastions of biodiversity. It puts another hole in the idea that agroforestry projects, plantations, and even selective logging can extract resources without adversely affecting ecosystems. Like a car that’s been in an accident, primary can never be the same as before. But unlike cars, we can’t go out and buy new ones.


Gibson, L., Lee, T., Koh, L., Brook, B., Gardner, T., Barlow, J., Peres, C., Bradshaw, C., Laurance, W., Lovejoy, T., & Sodhi, N. (2011). Primary forests are irreplaceable for sustaining tropical biodiversity Nature DOI: 10.1038/nature10425

Photo by WWF Deutschland.

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The importance of sentimental landscapes

Looking out from Melrose Rock

When I was packing for the move from Chicago to Cambridge, I figured the transition would be easy for two reasons, both of which are related. First, the two cities share a temperate climate. I grew up in Wisconsin and love—absolutely love—the changing seasons. For example, I’m not merely unfazed by below zero weather, I revel in it. The second reason is partially a consequence of the first—the Midwest and New England share a similar flora. Deciduous forests were the playground of my youth, where I went to escape the heat of the summer or romp through the snowy winter.

Having been a Cantabrigian for just just over two months, I can’t speak to the winters yet. But I can say something about the plants. A jaunt to Middlesex Fells over the Labor Day weekend affirmed my fondness for temperate deciduous forests. Still, I wasn’t quite at home. The Fells has a marvelous mix of deciduous oaks and evergreen pines perched on rolling hills and rocky outcrops. The whole landscape is reminiscent of the Calvin and Hobbes cartoons I devoured as a kid, but there was something missing. That something is my history with the place, or lack thereof. Research confirms it.

I wasn’t a part of the study in question—it took place almost a decade ago—but its findings confirm why I am both predisposed to liking New England’s woods and why they aren’t quite home yet. The study’s authors surveyed 328 park users in Ann Arbor, Michigan, to see whether they were attached to a particular park or just a particular setting. The study’s authors classified participants as park neighbors, visitors, volunteers, or staff, reasoning that these backgrounds would tint the lenses through which people viewed the parks.

The researchers found that neighbors who frequented a particular park were smitten by that place in particular. Perhaps the bond was formed during solitary reflective walks, or maybe weekend picnics with the family. Regardless, they liked those place in particular and didn’t find substitutes as appealing. Park volunteers and staff, however, were more inclined to treasure a park’s ecological contributions rather than sentimental ones. When shown photographs of a particular ecosystem, say a prairie, volunteers and staff were more likely to rate those shots highly regardless of their location. Volunteers and staff, who the researchers reasoned to be more ecologically knowledgeable, were also more open to restoration projects that supplanted invasive species with natives. Park neighbors and visitors tended to be happy with the landscape the way it was and generally opposed changes.

The differing perspectives of sentimental park users and ecologically principled individuals may help explain my hesitant fondness for the Massachusetts wilderness. The study seems to confirm that I straddle the line between two types of people. I have a feeling that many people are like me, especially those who recently moved. Our sentimental side aches for a favorite tree or preferred vista, but the rational ecologist in us appreciates native plant assemblages and landscapes.

People develop not just an affinity for nature, but the nature outside their window. That suggests not only that we should get outside, but also bring the outdoors closer to home, whether that be in the form of a city park or wild backyard. First-hand experiences with nature can be powerful ways to inspire people to adopt their own environmental ethic. I’m not the first to posit this theory—David Gessner does just that in his book My Green Manifesto, which I’m currently reading, as have others before him. Indeed, I can trace part of my own environmental ethic to a childhood spent in the park down the street or at the seven acres of scrubby, overgrazed woods just outside of town that my dad was rehabilitating. They are the type of landscapes I love and am fighting to preserve. Indeed, part of the reason I’m fascinated with higher density living is the potential it has to keep the wild places wild, the semi-wild places semi-wild. Calvin and Hobbes’s zany woodland adventures captured my childhood imagination because I saw in them a bit of my own al fresco self. I want future generations to have that chance, too.


Ryan, R. (2005). Exploring the Effects of Environmental Experience on Attachment to Urban Natural Areas Environment and Behavior, 37 (1), 3-42 DOI: 10.1177/0013916504264147

Photo by Paul-W.

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Spare or share? Farm practices and the future of biodiversity

Forest-farm edge in the Bolivian Amazon

Farming giveth and farming taketh away. Let’s parse that statement: Farming provides food—that much is obvious. But farming also gobbles up land that would otherwise accommodate endless biodiversity and beneficial ecosystem services. To counter the ecological harm done by farms, we have two options. One is to make farming more ecosystem friendly. Known as land sharing, this choice differs from garden variety organic farming by enmeshing cultivation with conservation rather than just minimizing detrimental impacts. The other option, land sparing, intensifies current cultivation while leaving other land as wild as possible. If you’re looking to feed people and maximize biodiversity conservation, you have to pick one.

The correct answer, at least according to a study published today in Science, is land sparing. The study’s authors examined farms and forests in southwest Ghana and northern India. They found more overall diversity of bird and tree species per square kilometer in land sparing setups—where farming is intense and reserves off limits—than in land sharing schemes—where farming and conservation occur on the same plot of land.

The ecologists involved in the study mapped out 25 one square kilometer plots in Ghana and 20 in India. The Ghanaian plots were divided almost equally among forest (8), large-scale oil palm plantations (8), and forest-farm mosaic (9). In India, they were split among five forest and 15 farm plots, five of which were low yield and ten of which were high yield. In each plot, the researchers measured average population densities of bird and tree species and binned each species into two broad categories—those that would thrive under a particular farming regime and those that would suffer. They then compared biodiversity statistics for land sparing regions (which contained both farmed and forested plots) with land sharing ones.

Unsurprisingly, all species fared worse when land was farmed. But the disheartening part—at least for those of us who dream of harmonious, ecotopian farms—was that more species were worse off on a region-wide basis under land sharing than land sparing. So although land shared between farm and forest is better for biodiversity on a single plot scale, the overall region is better off when some plots are intensively farmed and others are left alone.

In other words, sparing appears to be the least worst option. While some generalists thrive under land sharing, less mobile species with higher habitat constraints need special protection. Habitat reserves provide that, and land sparing schemes can support larger reserves. The only way land sharing excels at protecting biodiversity is when farm yields are impossibly low.

Land sharing, then, is the futon of biodiversity conservation. Just as a futon is both a middling bed and mediocre couch, land sharing is merely passable at producing food and so-so at protecting biodiversity. Neither futons nor land sharing systems excel at their dual tasks. As The Dude in The Big Lebowski would say, “This is a bummer, man.”

One drawback of land sparing is that it requires an immense amount of self-control on the part of individuals and society as a whole. Time and again we’ve challenged the inviolability of protected areas when we are—or think we are—short on resources. Conservation is hard, and plowing more land will always be the easier option. To prevent ourselves from doing that, we need to raise yields, which takes resources, training, and discipline. None of this will be easy.

Furthermore, raising yields sustainably, which the authors endorse, is going to be difficult. There are certainly some easy places to start—yields in much of Africa are dishearteningly low. But the world has embraced fossil fuel-driven, industrial agriculture for a reason—it’s the easiest way to squeeze more food from the land. If non-fossil fuel farming were the easiest option, we would have done that by now. Land sharing, on the other hand, trades low yields for closeness to nature. Locally this may be more sustainable, but is there enough land to feed 10 billion people that way? Probably not.

The choice between land sparing and land sharing is just one of many we will face as the planet’s resources stretch thin. While I’m quietly rooting for integrated, ecologically friendly approaches, there seems to be growing evidence that intensively exploiting a limited footprint may be the better option. If that’s true, the Romantic in me hopes we don’t lose our connection with nature in the process.

Ben Phalan, Malvika Onial, Andrew Balmford, & Rhys E. Green (2011). Reconciling Food Production and Biodiversity Conservation: Land Sharing and Land Sparing Compared Science, 333 (6047), 1289-1291 : 10.1126/science.1208742

Photo by Sam Beebe / Ecotrust.

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Swedes move to the city, but don’t leave the forest behind

Swedish forest

If there’s one thing that comes to mind when you think of Sweden besides Ikea and meatballs, it’s probably forests. They cover nearly 70 percent of the country. As a result, Swedes have a very close relationship with their forests, though the nature of it has changed in the last few decades.

Swedish forests have traditionally been accessible to all, regardless of ownership. People could walk into any old forest to gather firewood or pick berries. Berries in particular hold a special place in Swedish hearts. People still take weekend walks to pick lingonberries and blueberries. In fact, the Swedes love their berries so much that one paper from 1980 asked the question if picking, in conjunction with timber harvesting, was endangering berry supplies for future generations! But berries aren’t their only passion—hunting, fishing, and hiking are also high on the list.

Since transitioning from an rural society to an urbanized one between 1930 and 1970, Sweden’s forests have also undergone significant changes. As people no longer required a woodlot for self-sufficiency, industrial logging intensified. Timber companies reduced time between cutting from hundreds of years to 60 or 80. They also favored fast growing pines and spruces at the expense of old growth and deciduous species. After large clearcutting operations, companies replaced mixed age, mixed species stands with monocultures. With rising environmental awareness, practices changed in the 1990s, and clearcut sizes decreased while deciduous tree populations grew.

The Swedes’ deep devotion to the forest is hard to shake, and despite moving to the city, Swedes remain attached to their time outdoors. Still, the way they used the forest changed. Two surveys, one in 1977 and the other in 1997, illustrate this nicely. By 1997, more people frequently visited—more than three times a week, perhaps to retain some connection to nature. Yet at the same time, berry picking wasn’t very popular, except for people in the older age groups. That’s not to say the Swedes lost their taste for berries—the average respondent picked 4.5 liters in 1997. But it’s apparent that berry picking is a pastime that’s slowly being lost in the bustle of city living.

The authors suspect the shift is because older generations still retain personal bonds to the countryside. While older Swedes may not have lived in the country themselves, perhaps they had relatives who did. Younger people were born in the city, and do not share the same attachment to specific locales. For them, any forest will suffice.

Indeed, the study also found evidence of this, albeit in a roundabout way through the distances people had to travel to get to a forest, which both rose and fell. The proportion of very short visits and very long visits dropped, while medium length visits (3-4 km) rose. The authors postulate a few reasons for this dichotomous change. Most people currently live further from the forest than their predecessors, reducing the number of very short distance trips. At the same time, these people may not have an affinity for one particular forest over another, and so do not travel as far to undwind. The rise in medium length visits merges these effects—increased physical and emotional distance from the forest.

As an American, I’m struck by two things: The level of access Swedes have to their forests, and their commitment to them. Both are captured by a simple fact. Just a few miles from central Stockholm, a city of 850,000, large forests remain.


Axelsson, A. (2001). Retrospective gap analysis in a Swedish boreal forest landscape using historical data Forest Ecology and Management, 147 (2-3), 109-122 DOI: 10.1016/S0378-1127(00)00470-9

Convention on Biodiversity. 2011. Country Profile – Sweden.

Linder, P., & Östlund, L. (1998). Structural changes in three mid-boreal Swedish forest landscapes, 1885–1996 Biological Conservation, 85 (1-2), 9-19 DOI: 10.1016/S0006-3207(97)00168-7

Lindhagen, A. (2000). Forest recreation in 1977 and 1997 in Sweden: changes in public preferences and behaviour Forestry, 73 (2), 143-153 DOI: 10.1093/forestry/73.2.143

Kardell, Lars (1980). Forest mushrooms and berries—an endangered resource? Ambio, 9 (5), 241-247

Photo by Kjell Eson.

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Green planet, clean water

Drinking water

In a world of finite resources, fresh water stands next in line to cause shortage, misery, and conflict. Only about 2.5 percent of the world’s water is fresh, and most of that is locked up in ice sheets and glaciers—much of which is melting into the salty ocean thanks to climate change. That tiny sliver that’s left has to support billions of people. And a small portion of that sliver—surface water in rivers, lakes and streams—supports at least half of the world’s population.

Keeping that water clean is paramount for public health. Ever since John Snow discovered the source of the Broad Street cholera outbreak in 1854, we’ve been hard at work keeping our water clean. But all that treatment costs money, and often the best way to ensure safe drinking water is to not sully it in the first place.

One of the best and most cost effective ways to ensure clean drinking water is to leave native vegetation within the watershed in tact. Each additional 10 percent of a watershed that remains forested reduces water treatment costs by 20 percent. In the United States, where two-thirds of the population gets its drinking water from surface sources, that ratio can save cities a lot of money. Seattle, Portland, and Boston have taken this to heart, conserving land specifically because it is cheaper than building new treatment plants or expanding old ones.

Fortunately for many U.S. cities, the typical American drinking water watershed is still nearly 80 percent vegetated. There is bad news, though: Not all watersheds are so fortunate—some have as little as 17 percent native vegetation—and half have only 3 percent of their area set aside for conservation. Threats to native vegetation are greatest in the populous Ohio River Valley and Southeastern states, and the amount of conserved land in much of that area is startlingly low, too. The median Ohio River Valley watershed has only 1.8 percent of its land set aside, while the median Southeast watershed has a mere 0.2 percent.

Plants do more than just keep water clean—they also make it more usable. Vegetation has a calming effect on water, slowing its progress down hillsides, allowing it to seep into the ground and emerge slowly and steadily into lakes and streams. Without plants to intervene, water surges into streams and rivers, quickly filling reservoirs to capacity and leaving dam operators with little choice but to open the sluice gates, wasting the water. (These impromptu floods can be beneficial to downstream riparian vegetation in some cases, but too much force can be torrentially detrimental.) Since people’s water use tends to be steady day-to-day, dams which capture water more slowly can make more efficient use of it.

The importance of vegetation in securing a clean and reliable supply of drinking water cannot be overstated. Safe drinking water is extremely difficult to come by in much of the world. Places like Ethiopia and Haiti have chronic shortages of safe drinking water. Not coincidentally, many of those same countries have lost nearly all of their forests. While the loss of native vegetation is not the only cause of these countries’ water woes, it plays an important role. Fresh water is an already scarce commodity. It’s important to keep it as potable as possible.


Ernst, Caryn, Richard Gullick, and Kirk Nixon. 2004. Protecting the source: conserving forests to protect water. Opflow 30:1–7. (available online)

Wickham, J., Wade, T., & Riitters, K. (2011). An environmental assessment of United States drinking water watersheds Landscape Ecology DOI: 10.1007/s10980-011-9591-5

Photo by Cyron.

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Animals seek calm seas in oceans of human influence

Highfields, looking over the valley

Habitat loss can be like death by a thousand cuts for ecosystems. Each conversion to farmland, housing, or pasture, when taken on its own, may seem a small, inconsequential nick on the surface of a vast planet. But together, and over decades and centuries, these cuts add up, leaving only tiny remnants of the original scattered across the landscape. The result can be devastating for animals and plants which depend on the original ecosystem. Yet unlike the grisly metaphor drawn from an ancient Chinese torture, the wounds of habitat loss are not entirely fatal. Farms and subdivisions may not supplant virgin forest or grasslands, but they need not be inhospitable wastelands, either.

This post was chosen as an Editor's Selection for ResearchBlogging.orgThe field of ecology typically focuses on the leftovers of habitat conversion—the bits and pieces that somehow evaded plows and bulldozers. But just as no island is truly separated from the rest of the world, habitat remnants remain connected by the fields and yards that constitute the interstitial spaces. Known as “the matrix,” such spaces are important to the continued vitality of habitat fragments. In fact, according to new research out of Australia, the qualities of the matrix may matter more to the survival of native animal species than characteristics of the remnants themselves.

The matrix in the study are primarily cattle stations prevalent in the Toowoomba Regional Shire in southern Queensland. The area has been grazed heavily since the early 1900s, and pastoral uses are still the predominant land use, though urban development has been increasing in recent years. To quantify the amount of human influence within the matrix, the researchers measured a host of variables, mostly pertaining to roads and ranging from length to width to driving surface and even road kill statistics. They also closely studied the forest patches within the matrix and surveyed their resident mammal populations.

Many previous studies have focused on remnants’ size, shape, and geographic relationship to one another. This study did that, too, but also investigated the matrix itself. Of all possible landscape characteristics, the intensity of human development and number of tall trees within the matrix exerted more influence than any other.

To see how the matrix plays a role in this case, imagine you’re part of a group of people who are stranded on one of those islands, and your island doesn’t have everything you need to survive. Other islands do, though, so you’d need to set sail from time to time, just as the animals in the forest remnants may have to cross a pasture or pass through a neighborhood. The island metaphor is often used to describe how habitat remnants and their inhabitants interact with each other. Typically, people wax lyrical about the size, shape, and distance separating the islands. In this case, however, its not just the span of open water between the islands that matters, but the qualities of that water.

Neither the sea nor the matrix are absolute barriers, but they aren’t entirely hospitable, either.

Now let’s say you had a choice of islands on which to be stranded. Which would you pick? Quite obviously you’d prefer an island close to the mainland, but let’s say that’s out of the question. In lieu of that, you’d probably pick one that’s surrounded by calm seas so that exploring nearby islands would be infinitely easier. Some shallow waters between islands might be nice, too, to anchor your boat when you needed to take a rest. And if I were you, I’d also go for one with a bit of elevation to avoid high tides or tsunamis.

If those three wishes were granted, you’d probably have a good chance of living to a ripe old age. Conveniently, those wishes mirror what mammals in the Toowoomba study seemed to prefer, too. The researchers found fewer mammals traversing heavily developed areas—analogous to rough seas—and more where tall trees provided cover from predators—calm seas. The animals also favored a matrix with more nooks to roost or hide as they travelled—similar to shallow water for anchorage. Finally, the best remnants were those where human activities didn’t encroach too often—akin to higher elevation islands that guard against tides and tsunamis.

As a stranded soul, you wouldn’t have much influence over the ocean. But as stewards of the matrix, we decide its flora, its structure, and its ease of passage for animals. The biggest difference we can make is in reducing our developed footprint. The changes don’t have to be as drastic as ripping up subdivisions or farms. They could be as simple as carving roads only where they are most needed or avoiding areas where animals frequently tread. And failing that, plant trees.


Brady, M., McAlpine, C., Miller, C., Possingham, H., & Baxter, G. (2009). Habitat attributes of landscape mosaics along a gradient of matrix development intensity: matrix management matters Landscape Ecology, 24 (7), 879-891 DOI: 10.1007/s10980-009-9372-6

Brady, M., McAlpine, C., Possingham, H., Miller, C., & Baxter, G. (2011). Matrix is important for mammals in landscapes with small amounts of native forest habitat Landscape Ecology DOI: 10.1007/s10980-011-9602-6

Photo by Shaun Johnston.

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Can we feed the world and save its forests?

corn pile

Nine billion is the number that will define the 21st century. That’s the number of people expected to live on this planet by 2045. But 9 billion mouths are a lot to feed, and each of them will hopefully have more than enough to eat. Achieving both goals—feeding 9 billion and feeding them properly—will be a herculean task. It’s also one that could eliminate the world’s great remaining forests, taking with them ecosystem services like carbon storage, reliable rainfall, biodiversity, and the magnificence of miles upon miles of wilderness.

So what if someone said it might be possible to ramp up food production while expanding forested land? Sounds like a perpetual motion machine, right? It smacks of the impossible. But that’s just what Eric Lambin and Patrick Meyfroidt are proposing in their paper which appears in the Proceedings of the National Academy of Science. They think that boosting agricultural intensity can not only increase agricultural output, but also reverse deforestation. In theory, doing more with less is a great way to solve the world’s problems, but these theories usually come with a catch. And Lambin and Meyfroidt’s catch is a big one—the radical globalization of the world’s food markets. It’s an intriguing proposal with some hard data to back it up, but the authors are also quick to brush aside the potential problems.

Globalization, Lambin and Meyfroidt claim, can focus farming where it is most productive. In their vision, different regions will specialize in different crops, eking the most out of every acre of cultivated land. Those crops will then be whisked across borders to where they are needed. It’s the exact opposite of every locally-harvested, free-range, organically-grown mantra that’s proposed to save the world.

The authors outline four facets of globalization that could help or harm the cause. The first, displacement, could go either way. Displacement essentially moves crop production and timber harvesting from one place to another. As rich nations seek to protect their forests, for example, they must import their timber from elsewhere. The same goes with food stuffs. In Switzerland, for instance, the land required to grow its imported food is one and a half times more than the country’s currently cultivated land. But displacement may not be all bad, they say. For every 20 hectares of forest protected in North America and Europe, for example, only about one hectare of primary forest is logged in Russia or the tropics.

The second proviso they cite is rebound. As a new technology becomes cheaper, demand for it will increase as it becomes cost effective for more industries. Cheap gasoline, for example, has led to a proliferation of gas-powered devices (think leaf blowers, lawn mowers, and so on). More efficient agriculture, Lambin and Meyfroidt state, could also be more lucrative agriculture, driving an expansion of cropland rather than a reduction. They point to soybeans in Brazil and oil palm in Indonesia and Malaysia as examples. But they also counter that agricultural intensification since 1961 has reduced the amount of land that otherwise would have been needed to feed the world’s people.

The third caveat of globalization is cascade effects, whereby one crop displaces another, exploiting previously uncultivated land. Biofuels are a case in point. As some farmers use their land to cash in on the craze, other land is put under the plow to produce food stuffs.

Remittances are the last side-effect of globalization, and one that seems to be a net positive. Foreign workers often send money back to family members still in their home country, and the added influx of cash reduces the need for farmland in that country. With supplemented incomes, people can afford to purchase more food as opposed to growing all of it. Since subsistence farming is not very intensive, people’s remittance-assisted diets rely on less land.

Lambin and Meyfroidt offer four examples of countries in which agricultural output has risen concomitant with population and the amount of forested land—China, Costa Rica, El Salvador, and Brazil. Each case appears to be fairly unique, though, and is not enough to convince me that globalization and intensification are the best solution. China, for example, has turned to Africa to help feed its 1.3 billion people, locking up over 28,000 square miles of farmland and counting. Costa Rica’s success has been dependent on foreign groups purchasing land for conservation (a sort of highfalutin remittance), while El Salvador relies on small-scale remittances. Furthermore, statistics on Vietnam’s and China’s forests have both benefitted from plantations, which some experts have called “ecological deserts” and poor substitutes for the real thing. Furthermore, Lambin and Meyfroidt admit that many other countries in similar circumstances have not seen an uptick in the amount of forested land.

The authors also gloss over a major pitfall of globalization—pollution. Currently, shipping releases 1.12 billion metric tons of CO₂ per year into the atmosphere, or more than Germany, the world’s sixth largest emitter. Transporting food all around the world will only drive that number up. With climate change threatening to upend farming as we know it, pumping more CO₂ into the atmosphere may not be the best idea. That’s not to say the proposal is worthless—nine billion people are an awful lot to feed, after all—but there are some big questions that need to be answered before it should be seriously considered.


Lambin, E., & Meyfroidt, P. (2011). Inaugural Article: Global land use change, economic globalization, and the looming land scarcity Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1100480108

Photo by ConanTheLibrarian.

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