Trees are not a panacea for climate change – two new studies of tree life and death in a warming world show why

When people talk about ways to slow climate change, They often mention treesAnd for good reason. Forest take a A large amount The global warming carbon dioxide that people put into the atmosphere when they burn fossil fuels. But will trees maintain this pace as global temperatures rise? with companies Increase investment in forests as compensationby saying that it eliminates persistent greenhouse gas emissions, is a multibillion-dollar question.

Results of two studies published in Science and Ecology Letters on May 12, 2022 – One focused on growthThe some at death It raises new questions about the extent to which the world depends on forests to store increasing amounts of carbon in the future of warming. Ecologist William Andereggwho was involved in both studies, explains why.

What does new research tell us about trees and their ability to store carbon?

The future of forests is on edge, with a tug of war between two very important forces: the benefits trees get from increased carbon dioxide levels and the stresses they face from climate, such as heat, drought, fire, pests and pests. Pathogens.

Climate pressures are increasing faster as the planet warms more than scientists expected. It was a vision massive forest fires And Forests dying due to drought Much sooner than anyone expected. When those trees die, that carbon returns to the atmosphere. We also see evidence of the benefits trees derive from rising carbon dioxide levels in a warming world. May be more limited What people realize.

This tells us that it is probably not a good idea to rely on forests for a carbon sink on a large scale during the 21st century, especially if societies are not doing so. reduce its emissions.

A forest technician chainsaw a fallen tree in a forest of dead and dying pines whose needles have turned brown.
Drought made trees more vulnerable to fire and beetle attacks.
Philip Diederich / Getty Images

Trees and forests do all kinds of other amazing things – they clean the air and water, and they provide economic value in terms of timber, tourism, and pollination. So, understanding how they grow is important for many reasons.

There is an argument that with more carbon dioxide in the atmosphere, trees will simply grow and trap that carbon away. What did your study find?

There are two main things that affect the growth of a tree: Photosynthesiswhich is the way trees convert sunlight and carbon dioxide into food, and cell division process and expansion.

There has been a long-running debate about the biggest driver of tree growth.

A good metaphor here is a two-horse chariot. The cart moving on the road is the tree growing, and there are two horses attached, but we don’t know which one is actually doing the work of pulling the cart. One horse is photosynthesis. This makes a lot of sense – it’s the source of all the carbon for building cells. But we know there is another horse – in order to grow more wood, trees must grow layers of cells, and cells must expand and divide. This cell growth process is very sensitive to climatic changes and tends to stop when conditions are dry.

Map of drought in the United States showing the western half of the country under drought conditions
Large parts of the western United States have faced severe drought conditions for years. About half of the neighboring United States was in a drought in May 2022.
Drought Monitor / UNL / NOAA / USDA

People assume that photosynthesis is the dominant process almost everywhere. But we found Stronger evidence That these drought-sensitive cellular processes actually do more to drive or limit growth.

We used tree ring data from thousands of trees across the United States, Europe and Photosynthesis measurements from constellations in nearby forests To check if tree growth and photosynthesis are related over time. If they had followed the same pattern, more or less in the same years, that would have suggested that photosynthesis is the horse pulling the cart. Instead, we found no association.

This suggests that drought, not the amount of carbon dioxide in the air, may have the biggest impact on how quickly trees will grow in the future. ready Seeing more frequent and severe dehydration in many areas.

What did you learn about the risk of trees dying in the future?

In another study, we found that reducing global greenhouse gas emissions can have an impact Great effect to avoid damage For forests from forest fires, drought and insects.

We used years of satellite observations, climate data, and Network From about 450,000 tree plots across the United States where every tree is monitored for climate stress and survival. Using this historical data, we built statistical models for the risks that American trees face from wildfires, insects, and climate stress, primarily associated with drought. We then looked at what might happen under future climate scenarios, with high carbon emissions, medium emissions and low emissions. You can explore the results on file interactive map.

The images above, from interactive maps, show how risks to forests from wildfires (orange), droughts (pink), and insects (blue) increase over time in a future averaged emissions scenario. Move the slider to see the comparison between 2020 and 2090.

The big picture: As the planet warms, a wildfire has caught up Increases risk Significantly over the course of the current century, especially in the western United States in a medium-emissions scenario, the risk of wildfires is expected to increase fourfold. Drought and insect risks increase by about 50% to 80%.

What does this mean for the use of carbon offsets?

Taken together, these studies suggest that the benefits of carbon dioxide for growth will not be as great as people think, and that the risks of climate stress, especially wildfires, droughts and insects, will be much greater than people expect.

This has huge implications for forest use Carbon offset.

So far, carbon offsetting protocols and markets have not wrestled with this updated scientific understanding of The dangers faced by forests from climate change. This tells us that climate policymakers and offset developers need to be very careful about how they rely on forest offsets to deliver benefits.

The most optimistic message is that our actions in the next decade matter. If we can rein in the speed of climate change and take a path with lower emissions, it will lead to a significant amount of reduced risks and increased benefits. This isn’t a “raise our hands and panic” situation – it’s our chance to take steps to ensure that forests remain resilient and sustainable into the future.

What we do with our emissions and efforts to slow climate change is very important to the future of forests.