ABOVE: Ponderosa pine and Douglas-fir forest in the Boise National Forest, Idaho, 22 years after it burned in the 1994 Idaho City Complex Fire

IMAGE COURTESY OF KIMBERLEY T. DAVIS

Soil, temperature, and humidity conditions driven by climate change have made it more difficult for Douglas fire and ponderosa pine seedlings to establish themselves after a forest fire, researchers reported yesterday (March 11) in PNAS. At some locations in the western US, a “critical climate threshold” has already been surpassed over the past 20 years, meaning forests may not return after wildfires.

“Maybe in areas where there are really abundant seed sources, there could be some trees, but it is becoming really hard to get these trees back due to climate change,” coauthor Kim Davis, a postdoc at the University of Montana, tells CNN.

Davis and her colleagues analyzed tree rings sampled from nearly 3,000 trees in the Rockies and California from 1988–2015 to figure out when the trees had established themselves. When comparing regeneration after wildfires to annual climate conditions at their study sites, they found certain thresholds for summer humidity and temperature (too high) and soil moisture (too low) beyond which it became difficult for new trees to grow after a fire.

“Across the study region, seasonal to annual climate conditions from the early 1990s through 2015 have crossed these climate thresholds at the majority of sites, indicating conditions that are increasingly unsuitable for tree regeneration, particularly for ponderosa pine,” the authors write in their report.

The consequence is that when adult trees die in forest fires, seedlings struggle to replace them, and forests could rapidly give way to other types of vegetation. The effects are both ecological and economic, as both species of tree are important to the timber industry.