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Recovery of Rocky Mountain forests altered by drought after fire

Recovery of Rocky Mountain forests altered by drought after fire

A changing climate is altering the ability of Rocky Mountain forests to recover from wildfire, according to a new study published in the journal Global Ecology and Biogeography.

When warm, dry conditions lead to drought in the years following fires, it impedes the growth and establishment of vulnerable new post-fire seedlings. The study also shows that forest recovery has been negatively affected by increased distances between burned areas and the sources of seeds that typically replace trees lost to fire.

Fires that are followed by warm, dry conditions offer us a window into the future, says Brian Harvey, lead author of the study and a former University of Wisconsin-Madison graduate student in the laboratory of Monica Turner, E.P. Odum Professor of Ecology and Vilas Research Professor of Zoology. Harvey is now a postdoctoral Smith Fellow at the University of Colorado Boulder.

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, he says.

The new data positions researchers to better understand how forests could change in coming decades and may yield valuable information for the development of robust simulation models. Turner says the dataset -- which is the first to comprehensively demonstrate the impact of drought on forest recovery in the context of a changing climate -- provides unambiguous evidence that the climate conditions following fires are really going to matter.

The Turner lab's interest in how forest recovery after fire may change in the future began in the summer of 2000, when large wildfires raged across the southern portion of Yellowstone National Park in Wyoming. Thousands of acres of forest burned.

Turner had previously studied the massive 1988 Yellowstone Fires and in the summers following the 2000 Glade Fire, she and collaborators visited the forest to collect as much data as possible. They were surprised to see that tree density in the years after the 2000 fire was 10 times lower than comparable forests recovering from the fires of 1988.

A plausible explanation The year following the Glade Fire was unusually hot and dry, with just 30 percent of normal summer precipitation. But one fire followed by a dry summer could not provide the data to rigorously test the theory.

He and the research team visited 184 sites where 11 wildfires burned between 1994 and 2003. The work was exhilarating, but also grueling, he says, as most sites were well off trail in the backcountry of some of America's wildest places and the research team was on its hands and knees examining nearly 10,000 individual tree seedlings.

The researchers also turned to existing climate records to assess the drought severity of each location in the three years immediately following each fire and examined the distance from each plot to the nearest source of seeds -- other still-living trees typically outside the burn patch.

They found that overall, fewer post-fire tree seedlings established in years when fire was followed by severe drought and when seed sources were farther away, compared with cooler, wetter years and when burned areas were closer to seed sources.

One exception, however, was the lodgepole pine, which was less impacted by drought or seed source distance. However, these and the other species from lower elevations have not moved quickly enough into higher-elevation burned areas to replace the more sensitive subalpine tree species.

Trees grow slowly, and we can't just wait 100 years to get enough opportunities to study forest recovery from fire, says Turner.

With the study data, Turner and her collaborators will try to create models to predict future change.

Recovery of Rocky Mountain forests altered by drought after fire