For decades, scientists and resource managers have known that wildfires affect forest soils, evidenced, in part, by the erosion that often occurs after a fire kills vegetation and disrupts soil structure. But, the lack of detailed knowledge of forest soils before they are burned by wildfire has hampered efforts to understand fire’s effects on soil fertility and forest ecology.
A new study led by the Pacific Northwest (PNW) Research Station addresses
this critical information gap and represents the first direct evidence of
the toll wildfire can take on forest soil layers. It draws on data from the
2002 Biscuit Fire, which scorched some 500,000 acres in southwest Oregon,
including half of a pre-existing study’s experimental plots, which had been
studied extensively before the fire. The result was a serendipitous and
unprecedented opportunity to directly examine how wildfire changes soil by
sampling soils before and after a wildfire. The study appears in the
November issue of the Canadian Journal of Forest Research.
“Losing our experiment in the fire was hard, but the opportunity to better
understand fire as a dominant ecosystem process has been very exciting,”
said Bernard Bormann, a research forest ecologist with PNW Research Station
and the study’s lead investigator. “This study, covering over 300 acres,
provided nearly 400 soil sampling points as well as extensive tree and
understory plots to use in our analysis.”
Bormann—along with study co-author and Western Washington University
professor Peter Homann and colleagues from the PNW Research Station and
Oregon State University— conducted chemical analyses on soil samples
collected before and after the fire. They found that the combustion of the
organic layer at the soil’s surface, including woody debris, caused
intense, 1,300 °F-plus temperatures, which, in turn, displaced considerable
amounts of carbon and nitrogen from the underlying mineral soil layer and
left mostly ash behind. What was more surprising to the researchers was how
these organic materials may have been lost. Some carbon and nitrogen were
lost as gases—consisting mostly of carbon dioxide, nitrogen dioxide, and
water vapor—and some in an inch of fine mineral-soil particles, which
disappeared and left behind a crust of rocks.
“Altogether, we documented losses of more than 10 tons per acre of carbon
and between 450 to 620 pounds per acre of nitrogen,” Bormann said. “The
loss of topsoil and combustion of organic materials together led to losses
that are higher than most previous estimates.”
The loss of topsoil and carbon from soil can negatively affect a range of
processes, Bormann said, including nutrient retention and water
infiltration. In the absence of special nitrogen-fixing plants, which are
capable of converting atmospheric nitrogen into nitrogen compounds for
growth, losses of nitrogen in the order of what he and his colleagues
documented would require at least a century to be reversed.
Equally disconcerting is the role these released organic materials might
have on the atmosphere, especially in the face of a warming climate. The
burning of soil by wildfire may contribute to global warming, in the short
term, by releasing carbon as a greenhouse gas and, in the long term, by
reducing soil productivity through losses of organic matter and nutrients.
With less productive soils, Bormann said, a forest will not grow as quickly
nor reabsorb as much carbon as before a burn—a process critical to
mitigating the accumulation of atmospheric carbon, which traps heat in the
atmosphere and can, thus, raise temperatures.
“Our findings suggest that forest managers should carefully consider the
effects of wildfire on soils when planning to reduce fuels, suppress future
fires, and help trees and habitat recover after fire,” Bormann said.
