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Black Bears Can Help Fruit Trees Escape Climate Change

ByConor GearinNOVA NextNOVA Next

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Trees can flee from climate change—with a little help from a burly friend.

Humans, we hope, can retreat from the effects of global warming. But trees—though they have limbs—can’t uproot themselves to find a new home. If a tree species has adapted to a particular temperature range, it needs a way to stay in that range or risk extinction.

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Escaping the heat by climbing up a mountain is one solution. Mountains act as shelters from climate change. Even a small increase in elevation gets you to much cooler temperatures than traveling many miles towards one of the Earth’s two poles. Of course, plants are pretty limited as far as transportation goes. While flowers like dandelions can launch their light seeds far and wide, trees that have heavy fruit don’t drop their seeds far from the branch. Yet they need a seed-spreading strategy that lets them keep up with climate change.

Black bear
The Asiatic black bear could lend a paw to cherry trees.

Thankfully, fruit trees have never been alone. They have relationships with animals going back millennia. These animals eat their fruit or get seeds stuck in their fur and then spread the seeds into new habitats. And there’s now evidence that these same relationships that have stood the test of time could also help trees scramble up mountains to keep pace with global warming.

Over three years, a team of researchers in Japan took a look at cherry seeds in the dung of Asiatic black bears. Cherry trees are sensitive to higher temperatures and would need to travel up the slopes to avoid the increase in average temperatures projected for this century. By analyzing the oxygen atoms in the seeds, the researchers were able to figure out the elevation of the seed’s mother tree. This let them calculate how far uphill the seed had traveled. They found that the bears carried the seeds an average of 300 meters up the slope—enough to offset about 1-2° C of average temperature increase. That happens to be within the 2015 Paris Agreement’s target range for limiting climate change —but the seeds are moving at a fast enough rate to escape more pessimistic warming forecasts even if that goal isn’t met.

The plant-animal partnership seems to work like a charm. Here’s Sarah Laskow reporting for Atlas Obscura:

Not only were the bears moving the seeds far enough that they had a chance of surviving, they were moving them in the right direction. What was going on?

The scientists think that the cherry trees are benefitting from the time at which they produce fruit, in the spring. In that season, bears are usually heading up the mountains, grazing as they go. Since temperatures higher up are cooler, those plants produce fruit, aka bear food, later in the season. The bears, following the fruit, just happen to be going in the direction that’s most beneficial for the cherry trees.

Here’s the thing: not all fruit trees might be as lucky as the cherry. Animals might drag autumn fruits lower into the valley, exactly the wrong direction for escaping the rising temperatures.

Still, this study shows the need to protect the animals that trees depend on to disperse their seeds, said Lee Hannah, a senior research fellow at Conservation International. “Without these dispersers, our forests are in trouble,” he said. “Many forests in Asia may have difficulty as the climate changes, since large omnivores like bears have been hunted out in so many places.”

Fortunately, the plant world has a wide variety of strategies for spreading seeds, many of which could help them survive global warming, Hannah said. It’s encouraging that scientists have shown that the cherry tree has a solution that works.