Can Trees Keep Up With Climate Change?

It’s amazing to think that 15,000 years ago, as the ice sheets retreated from North America and temperatures climbed, forests might have migrated northward as fast as one kilometer per year (~0.62 miles per year). As Duke University Professor James Clark told me, “that would have been like a wave of seedlings spreading across the landscape.” It’s hard to imagine, but that’s what biologists figured had happened.

The latest research by Clark and others suggests, however, that this kind of rapid migration may not be what happened after all, which could spell trouble for trees adapting to a warming climate. Many biologists were already worried that trees would be unable to migrate easily in response to climate change because modern landscapes are so heavily fragmented by things like strip malls, housing developments and agricultural fields. Species would simply not be able to get across such a patchy landscape. But a new study is suggesting that there is another barrier.

Migration is not happening, Clark says, because seedlings can’t rapidly establish themselves in novel environments. So, even if they can get to a new location, that doesn’t mean they can grow.

Recent studies suggest that trees may not be able to migrate quickly enough to keep up with the pace of climate change. Credit: flickr/Rubin10

The study, published in Global Change Biology, was led by Clark’s student Kai Zhu, and also included National Forest biologist Christopher Woodall. It took advantage of a huge forest inventory conducted by the US Forest Service and done in a consistent way across thousands of acres of forested lands in the United States. The inventory provides both counts and size estimates of all the different kinds of adult trees and seedlings growing in each location. This allowed the researchers to look at where the seedlings were growing relative to the adults. Given the warming in the last century, they expected that if tree species are migrating, the seedlings would tend to be growing north of where their parents were.  

Having both the seedling and the adult data across so many plots was key. Most studies just look at adult trees, but it’s hard to tell what is going on because adults are so long-lived. This study, however, allowed the researchers to do a direct test of migration in response to warming. As Clark explained: “If migration is going on, seedlings would need to be out in front of the adults.” This is not what they found. Instead, they saw patterns of migration in all directions, consistent with what you would expect if the plants were not responding to climate change.

The idea that species will migrate to stay in their climate niche is central to forecasting how plants and animals will respond to climate change. (A niche is defined as the particular conditions in which an individual species can grow.) As the climate changes, the location where that species can grow also changes. And because the global temperature is increasing, we expect species to move either north or higher in elevation, where temperatures tend to be cooler.

A number of studies over the last decade have documented that some species’ ranges are doing just that, so Zhu, Clark and Woodrow’s study showing no migration comes as something of a surprise.

Clark wasn’t shocked, though. In fact, his work over the last 10 years had already been suggesting that trees can’t migrate as rapidly as biologists had thought from historical studies of periods like the expansion after the last ice age. For trees to do that, it would mean that the seedlings showing up in new areas would need to be sort of supercompetitors, able to out-compete all the other seedlings, including those of species that were already there.

In other work from Clark’s lab, they tested this idea and found that seeds imported from more southerly locations were not able to outcompete the locals, even in the locations with the warmest temperatures. Some of this may have to do with things like soil type. For instance, in the North Carolina forests where Clark and his students do much of their work, the soil types in the Piedmont are different than the Coastal Plains. Species coming from the Plains may not be adapted to the Piedmont, so even though the climate is suitable, the soil may not be.

This kind of complexity is one reason why it's incredibly hard to predict how species are going to respond to global climate change. Biologists like myself can make models and predictions, but ultimately we have to do tests in nature to see if those predictions are right. When we get new data like this, it helps us to revise our models. This research suggests that migration — one of the main ways species can cope with climate change — may not be occurring as widely or as rapidly as we thought.