Global Warming Turns Rainforest Leaves into Junk Food

Like those breakfast cereals that look healthy on the box but have even more sugar inside than Cocoa Puffs, some rainforest trees engage in false advertising. It’s not their fault—it’s ours. Climate change has made their leaves less nutritious than they used to be. And the animals who live off of those trees don’t exactly have another store to shop at.

Experiments in labs and greenhouses have given scientists mixed answers about what happens to plant tissues in a changing climate. So primate ecologist Jessica Rothman of Hunter College and her coauthors set out to get some facts from a real-world setting. To do this, they combined several decades’ worth of data from the rainforests of Uganda.

One team member, Duke University’s Thomas Struhsaker, had kicked things off way back in 1979. He’d sawed off mature leaves from various species of tree in Uganda’s Kibale National Park and let them dry in the sun. Later, researchers had analyzed the chemical makeup of those leaves. In 2007, Struhsaker returned to the park with Rothman and other researchers and retraced his steps. (Luckily, his three-decades-younger self had left a map of what he did.)

Over the next three years, the researchers gathered leaf samples from the rainforest trees. They studied 10 different tree species and sampled them at least once each year, so that a one-year fluke would be less likely to skew their results. As before, they dried the leaves in the hot equatorial sun, then crushed them and brought them back to the lab for analysis.

While they were at it, the scientists also recreated similar work that one of them, Colin Chapman of McGill University, had done in the mid-1990s. This meant gathering younger leaves from eight tree species that the local red colobus monkeys like to munch on.

The scientists analyzed all these leaves and discovered a clear pattern. Everywhere they looked, foliage had become less nutritious. All but one of the 10 tree species first studied in the 1970s now had more fiber and less protein making up its mature leaves. The young leaves from trees studied in the 1990s had also increased in fiber and decreased in protein content.

And these changes weren’t small. In the two tree species whose young leaves are most often eaten by red colobus monkeys, the ratio of protein to fiber had decreased by more than 30 percent. Overall, “Our results suggest that nutritional quality of foliage has declined…over the last 15–30 years,” the authors write.

They aren’t sure which factors in the environment are causing the change in leaf chemistry. Kibale has become substantially hotter and wetter over the last century. Earlier studies found that tropical trees receiving extra rainfall end up with lower nitrogen concentrations; nitrogen is a building block of protein. In forests that are facing drought, the results might be different.

Leaf-eating monkeys seem to choose the trees with the best ratio of protein to fiber in their leaves. If leaves are becoming less nutritious, these forests might not be able to support as many monkeys. But there’s some good news, Rothman writes. Colobus monkeys in Kibale haven’t started disappearing—not yet, at least. They may be making up for their junk food diets by eating extra leaves or switching to different preferred trees. Or there might be a lag time before we can see the effect on these animals.

Even if local monkeys are managing the changes for now, these tropical trees feed a host of other mammals, reptiles, birds, and insects. All of them are on a new food regimen they didn’t ask for, which could affect the health of the whole ecosystem. Maybe we should ship them some Cocoa Puffs.

CORRECTION: An earlier version of this post misstated Colin Chapman’s affiliation.

Image: by Marc Veraart (via Flickr)