Climate change poses a unique set of threats to the world’s forests. Forests are vital for ecosystems, water and nutrient cycles, and carbon management, so dying trees are a worrying prospect. And increased temperatures and droughts certainly have the potential to kill trees.

But a paper in this week’s PNAS suggests that the increased CO 2 and humidity that will accompany climate change may go some way toward offsetting the risks to forests—and identifies which forests are likely to fare better and worse.

Feed the tree

What kills a tree? As with humans, the options are endless, but climate change creates some specific risks. Decreasing annual rainfall in certain regions is an obvious problem, but it’s also about when the rain falls, not just how much of it does. Drought in the growing season is a bigger problem than at other times of year. Higher temperatures also mean that trees lose more water from their leaves. On the other hand, higher humidity and more CO 2 in the air allow trees to operate more efficiently. That might allow them to compensate for drought and heat.

Yanlan Liu, a researcher at Duke University, led a team in modeling these different effects. The goal was to work out how they’re likely to affect the world’s forests under a range of different climate change scenarios. They estimated the impact on 13 forests from around the world, in both temperate and tropical regions. Their sample included broadleaf forests, like the deciduous forests in the eastern US and the always-green jungles of the Amazon. They also looked at needleleaf forests made up of conifers, like the evergreen forests of Canada.

For each of these regions, climate models provide a range of projections for future changes in things like temperature and rainfall. Liu and her colleagues combined this data with models of how trees use resources like water and CO 2 under different conditions. The result was an estimate of how different scenarios are likely to affect forest death.

Problems for pines

First, the researchers focused just on rainfall (and other types of precipitation) and temperature. Based on a global average temperature increase of around 1.4 degrees Celsius by around 2065, evergreen needleleaf forests would be hit incredibly hard. The combination of drought and heat in this particular climate change scenario would increase the absolute risk of needleleaf forest mortality from just under 3 percent to 10 percent—a relative risk increase of more than 200 percent. Deciduous broadleaf forests and jungles would see a smaller (but still substantial) increase in risk.

But once Liu and her colleagues incorporated a rise in humidity and CO 2 levels into the model, things looked different. For evergreen needleleaf forests, things still didn’t look great: the estimated relative mortality risk would increase by 101.1 percent. Jungles would face an increase in relative mortality risk of 19.6 percent—and deciduous broadleaf forests might even do a bit better, with their relative mortality risk dropping by around 18.3 percent.

Because there’s so much uncertainty built into the models, the researchers produced a wide range of possible outcomes. But the broad details are similar. In almost every climate change scenario, from the relatively mild to the truly catastrophic, evergreen needleleaf forests come out with a higher mortality risk, even after factoring in CO 2 and humidity.

Jungles look like they’re in for a bad time with more extreme climate change, but their mortality risk is substantially mitigated after factoring in CO 2 and humidity. Deciduous broadleaf forests end up largely unaffected by milder climate change and see a lower mortality risk in the more extreme scenarios.

This model might seem comforting—at least as far as oak trees are concerned—but it’s far from the full picture. “The estimated risk should be interpreted with care,” the authors write. These climate-related factors are not the only things that kill trees—climate change is also likely to change things like forest fire frequency, disease, and insect prevalence.

Our understanding of mortality risk in trees, as well as external factors like these, “is still fraught with uncertainties,” they write. But even if it’s not the full picture, this model is likely to help conservation and ecosystem specialists to account for climate change more accurately.

PNAS, 2017. DOI: 10.1073/pnas.1704811114 (About DOIs).