Slow recovery has been observed in Rocky Mountain forests where large, severe fires coupled with subsequent drought conditions have greatly altered the ecosystem. Researchers found that such extreme conditions impede the growth and establishment of vulnerable, new post-fire seedlings.
Lodgepole pines, a dominant species of the Rocky Mountains, has seed cones that are actually opened by fire, making them especially well-adapted to raging fire conditions. While most forests of the Rocky Mountains are well-adapted to fires, little is known about how trees will adapt to changing climate, such as drought.
"Fires that are followed by warm, dry conditions offer us a window into the future," explained lead author Brian Harvey, a former University of Wisconsin-Madison graduate student. "From all the best available data and modeling, and expectations about future climate, these are the kinds of fires and post-fire climates that we're going to see more of in the future."
The recent study also revealed that forest recovery has been negatively impacted by increased distances between burned areas and the sources of seeds that typically replenish trees lost to fire.
"Fires that are followed by drought - which we are very likely going to see more of with climate change - really do set a new context in which these forests are not recovering as quickly," added Harvey, who is now a postdoctoral Smith Fellow at the University of Colorado Boulder. "It's a double whammy because even if seeds can get to a burned patch, they still need to survive once they get there. That may be much harder to do in a warmer, drier climate."
Researchers examined nearly 200 subalpine forest plots in and around Yellowstone National Park and Glacier National Park in Wyoming and Montana. These dense forests, located just downslope from the alpine tundra, were subjected to severe wildfires between 1994 and 2003. Researchers measured tree species, numbers, sizes and ages of post-fire tree seedlings found in each plot, as well as the distance between each plot and the nearest tree seed sources.
To get an idea of long-term impacts, researchers investigated the drought severity experienced in the three years following forest fires. With greater post-fire drought severity, researchers found a staggering decrease in the rate of post-fire tree seedling establishment.
"By going into areas that had burned at least 10 years ago - enough time for many post-fire tree seedlings to establish - we were able to characterize how these forests will likely look for some time in the future," Harvey said.
For instance, subalpine tree species, such as Engelmann spruce and subalpine fir, were more negatively affected by drought after fire than species that grow at warmer, drier low elevations.
Wildfires often leave behind a mosaic of burned and unburned trees and, based on their findings, researchers urge that any islands of unburned tree should remain in tact, as they are valuable seed sources for forest recovery.
"Trees grow slowly, and we can't just wait 100 years to get enough opportunities to study forest recovery from fire," said Monica Turner, a professor of ecology and Vilas research professor of zoology at the University of Wisconsin-Madison. "We want to tease this apart so we can anticipate what the ecosystem services - like carbon storage and wildlife habitat - will be in the future, what the landscape will look like, what it means for people's ability to recreate and where their communities are. There are a lot of human dimensions that come from the character and distribution of these forests."
Their study was recently published in the journal Global Ecology and Biogeography.
