America’s forests have a hard time in store for them. Climate change is increasing temperatures and decreasing humidity levels across the country, not a winning combination for trees.
Researchers at UC Santa Barbara and the University of Utah are looking to find out what our sylvan ecosystem might do in the near future. The authors combined mathematical models and data collected by the US Forest Service and plant physiologists to understand the vulnerability of forests to drought.
Their findings suggest that, while most forests have the potential to adapt to warmer, drier conditions, they cannot adapt quickly enough to avoid future stress. The study, published in Biology of Climate Changeserves as a basis for future forest research, as well as a guide for conservation and management.
“We are concerned to see that forests are not changing rapidly to avoid increased water stress due to climate change,” said first author Greg Quetin, an assistant scientist at project of the UCSB Department of Geography. “But there is hope, because most forests in the continental US have enough functional diversity to increase their drought tolerance by changing species composition.”
There are several ways that forests can adapt to dry conditions. Individual trees can change their activity, physiology and gene expression to the new conditions they face. Drought-tolerant species already present in the ecosystem may also become more dominant. The composition of the forest can also change, with hardier species migrating while more susceptible species die out. Evolution can also change species through natural selection, although the effect may be small over the next century for long-lived organisms.
Quetin and his co-authors examined whether the characteristics and species already present in the country’s forests are sufficient for them to acclimate to future climate change without widespread die-offs. Most of the data comes from the Forest Inventory and Analysis program, a comprehensive database operated by the US Forest Service on the state of the nation’s forests that has been standardized since the year 2000.
This database includes forest inventory plots documenting the location, species, size, density and health of trees, as well as tree growth, mortality and harvest. Quetin and his colleagues also used data from the Xylem Functional Traits Database, where measurements of tree physiology and hydraulic characteristics are compiled, cross-referencing this database with the Forest Inventory.
Ultimately, the team developed a model that simulated the forest’s response to increased water stress. The model predicts photosynthesis (or CO2 in), respiration and growth (CO2 out), as well as plant stress. They also include an optimization technique to see how changes in leaf area can mediate stress caused by changing environmental conditions.
“All the data so far suggests that leaf area is the single biggest lever that individual trees can throw to manage water stress,” said co-author Lee Anderegg, an assistant professor in the Department of Ecology, Evolution, and Marine Biology. Forests in drier areas have smaller canopies, while forests in wetter climates reach thick foliage.
Researchers have found that many of America’s forests have the capacity to adapt. The model revealed that 88% of the forests in the continental US have the characteristics and species diversity to adapt to climate change, and they are already starting to. However, most did not adapt as quickly as the model predicted would be necessary to avoid increased water stress and subsequent mortality.
“It’s about us not seeing the necessary shifts that our model predicts should happen,” said co-author Anna Trugman, an assistant professor in the Department of Geography. “But I think there’s still room for hope.” For example, biodiversity shows its ability to buffer the impact of climate change on a particular forest.
“Trees are slow-moving, as we know,” Trugman continued. “I saw the run of the Ents in ‘The Lord of the Rings.'”
“They’re still holding Entmoot at the moment,” Anderegg added.
The higher concentration of carbon dioxide introduced a confounding factor in the team’s calculations. Plants lose water through the same pores they use to take in carbon dioxide. So if there is more CO2 in the atmosphere, plants can reduce the size of these pores and still get the carbon they need for photosynthesis. This reduces the amount of water that comes out of their leaves.
But the atmosphere is drier in a warm climate, Anderegg explained, so the leaves lose more water. It is a complex system with many uncertainties and compensatory factors, which require nuanced models to disentangle. And the energy involved in transporting this water is far from negligible, as the authors of a previous paper discovered.
The team is now collecting their own data on changes in tree physiology following climate-driven fires in Sequoia National Park, trying to empirically prove how much trees can adjust their physiology. . The authors also investigated whether trees can avoid future water stress entirely through changes in their leaves, and whether maximizing carbon gain or avoiding stress is more limited.
The forest has started to change. Sparse canopies become more common as the atmosphere becomes drier. Woodlands may also have a different mix of species than before. These factors also affect forest carbon storage. Forests currently absorb about 30% of anthropogenic emissions, but the group recently found that this is likely to decrease under climate change.
Management strategies that encourage forests to adapt will be critical. “We need to think about these forests not as static things – which should exist as they are now – but as healthy things that need to change to sustain the climate,” Anderegg said. Facilitating gradual change helps prevent sudden, catastrophic changes, such as wildfires and die-offs, that harm forests, wildlife and people living nearby.
Resource managers can begin planting areas with more drought-tolerant species and conducting prescribed burns to promote healthy forests. But above all, we need to mitigate climate change, the authors say.
Our future depends on society’s emission trajectory. Climate adaptation is not easier than climate mitigation, Quetin said. And less climate change means less adaptation is needed.
GR Quetin et al, Observed rates of forest behavior do not follow hydraulic stress from climate change, Biology of Climate Change (2023). DOI: 10.1111/gcb.16847
Provided by the University of California – Santa Barbara
Citation: Forests can adapt to climate change, but not quickly enough (2023, July 10) retrieved 10 July 2023 from https://phys.org/news/2023-07-forest-climate-quickly.html
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