U.S. Forests Struggling to Adapt Fast Enough to Climate Change, Study Finds

Rising sea levels, accelerated coastal erosion, severe flooding and drought, rapid melting of Arctic sea ice and more frequent and intense wildfires are all symptoms of climate change that are changing our planet’s landscape. In some places, these changes are happening too fast for plants and animals to keep up.

A recent study by researchers at University of California, Berkeley (UC Berkeley), and the United States Forest Service have uncovered warning signs that forests in the Western U.S. are struggling to adapt to the rapidly changing climate.

“If you’re concerned about forest health then one thing you want to observe is whether the rate at which forest composition changes is roughly equivalent to the rate at which the climate changes,” said lead author of the study Kyle Rosenblad, a Ph.D. student in the Department of Integrative Biology at UC Berkeley, in a press release from the Rausser College of Natural Resources. “We found that climate change is outpacing the ability of forests and tree biomass to keep up, which is a potential concern for forest managers.”

The study, “Climate change, tree demography, and thermophilization in western U.S. forests,” was published in the journal Proceedings of the National Academy of Sciences.

According to the study, in the past ten years, forests in the West have become more and more dominated by trees better equipped to survive climate conditions that are drier and hotter — a process called “thermophilization.”

The precise factors affecting the thermophilization process can differ in individual forests, but on the whole, the changing composition of forests is failing to keep up with climate change.

“In many ecosystems, water is often limited, and higher temperatures can cause trees to lose water more quickly. When a tree’s access to water gets dangerously low — in other words, the tree becomes highly drought-stressed — its vascular system (akin to our veins and arteries) is under so much tension that air bubbles can develop and destroy sections of the vascular system,” Rosenblad told EcoWatch in an email. “Some trees protect themselves against these dangers by building special vascular structures that make it more difficult for bubbles to form and spread. Trees’ leaves can also wilt under drought stress, and some trees defend against this danger by closing the pores on their leaves to minimize evaporation, or by stockpiling solutes in their leaf cells so it becomes physically more difficult for water to leave.”

Each decade, scientists from the Forest Service conduct a “tree census” by measuring plots of trees and collecting data all over the U.S. This survey gives the scientists an idea of the condition and health of the millions of acres of forest across the country. It also allows them to track temporal changes.

“Our study included the roughly 100 tree species that live in the western half of the continental United States. Common examples include quaking aspen, ponderosa pine, and Douglas fir,” Rosenblad told EcoWatch. “Every tree species has its own geographic range, and one of the trends reflected in our study is that species have tended to struggle most with climate change in the warmest, driest parts of their geographic ranges, where just a little bit of warming might be enough to push them beyond their physiological limits. Quaking aspen is a good example of a species struggling heavily with climate change, although other factors like disease play a role as well.”

The research team from UC Berkeley looked at almost 45,000 subplots of forest that had been measured in the most recent tree census, the press release said. Their analysis found that the temperature in Western forests had increased an average of 0.57 degrees Fahrenheit from 2011 to 2020.

During this same time period, forest thermophilization rates were 10 times slower than the mean temperature increase.

According to the findings of the study, the most dramatic changes in forest composition occurred in plots that were on hillsides facing north, those that experienced the most extreme warming and drought and those that were subjected to insect attacks.

“[W]hen heat causes trees to become drought stressed, the trees also become more susceptible to insects like bark beetles, which can decimate large swaths of forest in a short time. Some trees can defend themselves by pushing sap into the holes formed by the beetles before the beetles can lay eggs,” Rosenblad said. “Lastly, high heat and dry vegetation can make wildfires burn hotter and spread more widely, thus making tree death more likely. Some trees defend themselves against wildfire by developing thick bark and frequent self-pruning — i.e., dropping of low branches that are no longer growing well and might otherwise help a fire climb to the canopy. Other trees embrace wildfire as a new growth opportunity, either by resprouting from a stump after the fire, or by producing seeds that germinate post-fire.”

Senior author of the study David Ackerly, who is dean of Rausser College of Natural Resources and a professor in the departments of Integrative Biology and Environmental Science, Policy, and Management, said trees that are able to tolerate the warming temperatures are not replacing those that can’t at a rapid enough pace to keep forests healthy.

“This research shows that tree species with low temperature tolerances are dying, and new trees are not growing fast enough to compensate for these changes,” Ackerly said in the press release. “These trends are an early warning that changes in the forest are lagging behind the pace of climate change, which may make them more vulnerable to warmer and drier conditions in the future.”

Rosenblad said that if the West continues to experience extreme heat and drought, the divide between the transition of forests and their changing climate will become more pronounced. This will result in successive cycles of tree death that interfere with essential ecosystem functions and allow carbon that had been stored in the trees and soil to be released back into the atmosphere.

“Healthy forests provide human society with many important services, like food, clean drinking water, wood, wildlife habitat, recreational opportunities, spiritual and mental health benefits, and — ironically — defense against climate change. Not only do healthy forests provide shade from the sun, they also pull carbon dioxide out of the atmosphere and lock the carbon into the forest ecosystem, where it cannot contribute to the greenhouse effect until it very slowly cycles back into the atmosphere again. If we cannot maintain healthy forests, then all of these benefits will be compromised,” Rosenblad told EcoWatch.

In some areas of the country, the effects may be so pronounced that landscapes change entirely.

“In some parts of the western U.S., it’s likely that forests are going to disappear and be replaced by other ecosystems like grassland or shrubland. In these places, like some low-elevation parts of California’s Sierra Nevada mountains, the best thing to do is start preparing to have fundamentally different relationships with our local ecosystems,” Rosenblad said. “In other places, healthy forests might persist, but they would likely be populated by different types of trees than those that live there today. However, one of the key findings from our study is that tree species are not filling in new parts of the map quickly enough to keep up with climate change. We may be able to help hasten their arrival through carefully planned and monitored tree planting projects. Some work like this has already begun in some parts of the country, and it is an active area of research.”

Rosenblad told EcoWatch that building sustainable forests in the face of climate change should involve looking to Indigenous peoples and their forest management practices for guidance.

“We can also help forests respond to climate change and other environmental threats by restoring land rights and sovereignty to Indigenous people. Most importantly, restoring Indigenous sovereignty is a moral obligation, but an additional benefit is that Indigenous people have built relationships with their local ecosystems over thousands of years and often have a wealth of knowledge about how to interact with these ecosystems sustainably,” Rosenblad said. “For example, before European colonization, many Indigenous groups practiced carefully planned cultural fire, which brought numerous benefits like increased food production and thinning of thick vegetation that could otherwise have fueled dangerous high-severity megafires. When European colonists forced Indigenous people to stop practicing cultural fire, vegetation density increased, and high-severity megafires became increasingly frequent and destructive.”