PORTLAND, Ore. October 16, 2008. 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
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.”
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.
To read the article online, visit http://www.nrcresearchpress.com/doi/full/10.1139/X08-136
The PNW Research Station is headquartered in Portland, Oregon.
It has 11 laboratories and centers located in Alaska, Oregon,
and Washington and about 500 employees.