Warmer and drier climate conditions in western U.S. forests are making it harder for trees to regrow after wildfires, according to a recent study undertaken by dozens of researchers from around the country.
“We can’t expect that forests will recover following wildfires the way they have in the past,” said Kimberley Davis, lead author of the study, published in the Proceedings of the National Academy of Sciences of the United States of America. The research considered the aftereffects of 334 wildfires that occurred between 1984 and 2018, and how well eight species of conifer trees regenerated after those burns.
Davis and her coauthors found that these severe burns often wiped out the seed sources needed to regenerate conifer forests, and even when seeds were available, young trees struggled to survive in landscapes that were becoming hotter and drier.
The study, however, did offer a ray of hope. If forest managers take steps to reduce wildfire severity over the next 2 decades, they may negate, at least partially, these climate-related losses in tree regeneration—although reducing greenhouse gases to slow climate change is the real answer to saving such forests in the long term.
Climate Change Plus Fire
Davis and her coauthors noted that forests around the world are on the cusp of significant and irreversible shifts in ecology, such as woody shrubs replacing conifers. Losing many forests in such an ecological transformation, they wrote, would have catastrophic effects on carbon storage, water quality, biodiversity, and culturally important resources such as California’s redwood forests.
The authors used the term “interval squeeze,” first coined by Australian scientists in 2015, to describe how climate is limiting postfire tree regeneration while severe fires are increasing. The Australian researchers found that when trees and other woody species became stressed by warmer temperatures, they took longer to produce seeds. At the same time, the risk of high-severity fires meant there was less opportunity for those mature trees to drop their seeds before they were killed by an extremely hot wildfire.
“So while we’re needing more time for a forest to recover because of climate, we’re actually having less time because we’re having increased fire frequency,” Davis said, adding that more data are needed to show whether conifers in the United States are mirroring Australian trees and producing fewer seeds as the climate warms. “There have been some studies that look at seed production in our western conifer forests,” she said, “but it’s really variable and hard to figure out.”
Tree Recruitment Trending Down
To conduct their analysis, Davis and her coauthors used a data set of postfire conifer regeneration from 10,230 field plots. They created models of postfire recruitment probability as a function of seed availability, fire severity, average climate (warmer, drier or cooler, wetter) from 1981 to 2000, and postfire climate for the first 5 years after a fire. The model allowed them to make future projections and compare the relative impacts of changes in climate and fire severity on conifer regeneration across the West. The results showed that successful postfire tree recruitment is trending downward, from 95% between 1981 and 2000 to 74% by 2031–2050.
The model and its projections showed that a lack of surviving mature seed-supplying trees and postfire climate conditions that were warmer and drier—indicated by a higher average water deficit—hindered recruitment. Stressed seedlings have a harder time growing and surviving. “A single hot, dry year…within the first 5 years following a fire significantly reduced the probability of postfire regeneration for most species,” the researchers noted.
Window of Opportunity
The models also predicted that in the short term, decreases in fire severity would have a positive effect on tree recruitment, including seed availability.
That result indicated that there is a “window of opportunity,” over the next 2 decades, in which forest managers could use interventions such as controlled burns and tree thinning to reduce the likelihood of high-severity fires, allowing for forest regeneration rather than ecological transformation.
Time is of the essence, the authors noted, especially for the southern Rocky Mountains, where “the suitable climate window for regeneration is rapidly closing.”
Holly Barnard, a forest hydrologist at the University of Colorado Boulder who was not involved with the study, wrote in an email that the study showed “the tremendous insight we can gain in science through large-scale collaboration and data sharing.” Barnard studies how changes in climate and land use affect forest ecosystems and water resources.
“This study can be a guiding tool to land managers who are interested in preserving our western forests for carbon sequestration, wildlife habitat, and clean water for downstream communities,” she said.
—Nancy Averett (@nancyaverett), Science Writer