Warmer and drier climate conditions in western U.S. forests are making it less likely that trees can regenerate after wildfires, according to new research published in the Proceedings of the National Academy of Sciences.
Importantly, the research also finds that ecologically based forest management can partially offset climate-driven declines in tree regeneration by limiting fire-caused tree death, but only if action is taken quickly. This study provides timely information to optimize new state and federal initiatives to increase the pace of ecologically based forest management across millions of acres of Western forests.
The Climate Change Challenge
Forests are adapted to different types of fire across the West, but hotter and drier conditions in recent decades have intensified the way fires burn, resulting in more trees being killed. All of this can result in fewer seeds available for forests to regenerate after wildfires. Even when seeds are available, a warming climate is increasingly limiting the chances that seedlings can establish and grow.
“Climate change is limiting tree establishment after wildfires because seedlings can be killed by hot and dry conditions. A problem that is especially acute in the southwestern US,” said Matthew Hurteau, a co-author and professor at The University of New Mexico.
The research examined how the severity of a fire – the number of trees it kills – in combination with the climate conditions after the fire affected the chances of tree seedlings regenerating to establish a new forest. It is the most extensive study to date, assessing regeneration of eight major tree conifer species after 334 wildfires across the West, using information from over 10,000 field plots collected by more than 50 research teams.
Researchers found that warmer, drier conditions over the past four decades have led to a decline in tree regeneration after wildfires and this trend is expected to accelerate in the future. For example, from 1981-2000, 95 percent of the areas studied had climate conditions suitable for tree regeneration after wildfires, but this is projected to decrease to only three-quarters of the West by 2050 under future climate scenarios.
The most vulnerable forests were in drier regions in the Southwest and California, while forests in the wetter and cooler regions of the northern Rocky Mountains and Pacific Northwest are still expected to support conifer regeneration in the near-term future.
“This is exactly why it is critical that we work to keep New Mexico’s forests from burning down,” said Hurteau.
Ecological Forest Management Can Reduce Risk
The study also found that ecological forest management in vulnerable dry forests could offset climate-driven changes by reducing the number of trees killed in wildfires. Specifically, in almost half of the study region, regeneration after wildfires is projected to be likely only if future fires burn at lower severities because fewer trees that produce seeds needed for forest recovery are killed in these fires.
“We know that in our dry forest types in New Mexico, we need to work to actively restore surface fire. Not only is fire restoration important for protecting our communities, but it is one of the tools we have to maintain our forests,” said Hurteau. “Given the challenges seedlings face in establishing after wildfire, efforts like the New Mexico Reforestation Center are central for restoring our severely burned forests.”
Even in cooler high-elevation forests in the Rocky Mountains and Pacific Northwest that experienced large tree-killing wildfires in the past, the researchers also found that climate change is making it less likely for trees to regenerate after wildfire. In these forests, planting trees after wildfire may take on increased urgency, given a shortening window of opportunity for trees to establish in climate conditions that are warmer and drier than in the past.
The good news is that the federal government allocated around $3 billion to fund ecological forest management and reforestation efforts across 50 million acres in the next 10 years throughout the West. This research provides much-needed information that can inform these efforts, particularly given the rapid pace of change our Western forests are experiencing.