Warmer and drier conditions limit forest recovery from wildfires

Hotter and drier climate conditions in the forests of the western United States are making trees less likely to regenerate after wildfires, according to new research published in the Proceedings of the National Academy of Sciences.

Importantly, the research also found that ecologically based forest management can partially offset climate-induced reductions in tree regeneration by limiting tree death from fire, 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 challenge of climate change

Forests are adapted to different types of fire across the West, but warmer and drier conditions in recent years have increased the way fires burn, killing more trees. All of this may 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 limits tree establishment after wildfires because hot temperatures and dry conditions can kill seedlings,” says the study’s lead author, Kim Davis, who completed the study at the University of Montana, now a research ecologist for the Forest Service at the Missoula Fire Sciences Laboratory.

The research examined how the severity of a fire – the number of trees it kills – combined with the climatic 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 the regeneration of eight large tree conifer species after 334 wildfires across the West, using information from more than 10,000 field plots collected by more than 50 research teams .

Researchers found that tree regeneration after wildfires has decreased due to warmer and drier conditions over the past two decades and this trend is expected to accelerate in the future. For example, from 1981 to 2000, climatic conditions were suitable for tree regeneration following wildfires in 95% of the areas studied, but this is expected to decrease to three-quarters of the West by 2050 under future climate scenarios. .

Forests were most vulnerable in drier regions of the Southwest and California, while forests in the wetter and cooler regions of the northern Rocky Mountains and Pacific Northwest are expected to support conifer regeneration in the near future.

“The impacts of climate change and wildfires vary across the West, and the large scope of this study allowed us to highlight where these changes are most concentrated and occurring first,” says Philip Higuera, co-author and professor of fire ecology at the Center. University of Montana.

Ecological forest management can reduce risk

The study also found that ecological forest management in fragile 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 considered unlikely unless a future fire burns at a lower intensity because these fires kill fewer trees that produce seeds needed for forest recovery .

“We know from previous research that forest thinning and controlled burns in overgrown dry forests effectively reduce fire intensity and subsequent tree death,” says study co-author Marcos Robles, chief scientist for The Nature Conservancy in Arizona. “Land managers can’t do much about drought and climate change in the short term, but they can reduce the area of ​​forests at risk of serious wildfires by accelerating ecologically based forest management”

Previous research published by The Nature Conservancy shows that ecological forest management in a major restoration project in Arizona would not only reduce tree mortality in the event of a wildfire, but would also provide additional co-benefits, including significant reductions in tree mortality associated with with drought, and an increase in carbon. storage, stream flow and tree growth.

“But the clock is ticking; it is urgent that we apply these treatments in our forests now, lest we lose them all,” says Robles.

Even in cooler high-elevation forests in the Rocky Mountains and Pacific Northwest that have experienced tree-killing wildfires in the past, the researchers also found that climate change is making it less likely that trees will regenerate after wild fire. In these forests, there may be an added urgency to planting trees after a wildfire, when given the opportunity for trees to be warmer and drier than before, trees can be established under climatic conditions.

The good news is that the federal government has allocated approximately $3 billion to fund ecological forest management and reforestation efforts over 50 million acres over the next 10 years across the West. This research provides much-needed information that will inform these efforts, especially given the pace of change occurring in Western forests.