Our
quantitative-pit soil sampling across 2-ha grids, before and after the fire allows us to determine fire effects
quantitatively
at the stand scale. Some soils were
greatly affected by the fire,
and soil effects appear
related to stand conditions before the fire,
as well as temperatures
during the fire. Stands with less
mortality appear to have less
soil effects (for example, the amount of surface rock is positively related to average tree mortality in
stands).
The most affected soil appears to have lost
its entire organic horizon,
all of the top mineral horizon (A), as well as over 10%
of the upper B
horizon. More than 5 kg/m2 of soil (organic
and fine-mineral components)
are now missing, with associated changes
in particle-size distribution (for example, many rocks at the surface), bulk density, charcoal content, and many other
factors.
Nitrogen associated with these losses and changes in
remaining
soil add up to about
400 kg/ha. Combined with vegetative
losses (not yet quantified) we
expect that up to 18 years of typical N uptake in vegetation was lost. Losses of other elements know to volatilize at lower temperature (S, P, K) have yet to be
quantified.
Taken together, changes in soil organic
matter, bulk density, particle
size, and nutrient content are likely to impact forest productivity for some time to come. Tracking new growth against that observed before the fire, and that in unburned
treatments will reveal
direct measures of wildfire on productivity.
Of particular interest
will be to follow the nitrogen-fixing plants that may or
may not come to
dominate burned stands. The LTEP
program is considering growth
plots of uniform seedlings to evaluate fires of different intensities.
Unlike background changes in vegetation, soils appear relatively unchanged in unburned
stands. Thus, observed changes are easily attributable to the Biscuit
Fire.