The forests you see today are not what you will see in the future. That's the overarching finding from a new study on the resilience of Rocky Mountain forests, led by Colorado State University scientists. University of Montana fire ecology Professor Philip Higuera is a coauthor of the study.
Researchers analyzed data from nearly 1,500 sites in five states - Colorado, Wyoming, Washington, Idaho and Montana - and measured more than 63,000 seedlings growing in 52 areas burned by wildfires during the past three decades. They wanted to understand if and how changing climate over the past several decades impacted post-fire tree regeneration, a key indicator of forest resilience.
They found significant decreases in tree regeneration following 21st century wildfires, a period that was markedly hotter and drier than the late 20th century. The research team said that with a warming climate, forests are losing their resilience to wildfires.
"While Rocky Mountain forests typically recover after wildfires, conditions are becoming increasingly stressful for tree seedlings to establish and survive," Higuera said. "Seedlings are more sensitive to warm, dry conditions than mature trees, so if the right conditions don't exist within a few years following a wildfire, tree seedlings may not establish."
Historically, the look of forests changes over time. But the research team said it will take much longer for sites to re-establish forests after a wildfire, if they return at all. In the warmest, driest forests, researchers found evidence suggesting a lack of tree regeneration after wildfires, which hasn't typically been the case.
One of the big surprises for the team was seeing the pattern of reduced tree regeneration emerge consistently across all the sites compiled in the study.
"We expect variability in how long forests take to recover after wildfires," Higuera said, "but the decrease in tree regeneration between the late 20th and early 21st century was pretty striking, and it's consistent with what we expect to see as climate becomes warmer and drier."
In one-third of the areas studied, researchers found that forests are not coming back at all. This suggests that even restoration efforts, such as planting new trees, might not be successful. There are other areas that could support certain tree species, but don't have any regeneration currently as ideal locations for managers to consider planting seedlings. When doing so, managers may want to explore planting species that are adapted to the current and future climate, not the climate of the past several decades.
The research team includes scientists from University of Idaho, The Nature Conservancy, UM, University of Washington, University of Colorado-Boulder, Tall Timbers Research Station and Land Conservancy, and the Washington State Department of Natural Resources.
"Evidence for declining forest resilience to wildfires under climate change" was published on Dec. 12 in the journal Ecology Letters and can be found online at http://onlinelibrary.wiley.com/resolve/doi?DOI=10.1111/ele.12889.