Forests and Water: A State of the Art Review for Colorado

• Year: 2003
• Title: Forests and Water: A State-of-the-Art Review for Colorado
• Author: Colorado State University
• Description: Colorado State University 2003
• Publications - Doc Type: Other

and water quality after wildfires (e.g., Tiedeman et al.,
1978; Robichaud et al., 2000), but much less is known
about the effects of fires on channel morphology and
aquatic habitats. The effects of fires on water quality
are highly variable, as the type and magnitude of change
depends on the size and severity of the fire, site char-
acteristics such as slope and soil type, vegetation type,
amount and type of precipitation, and the rate and type
of regrowth (Friedrich, 1951). Because there are so many
different processes operating at different time scales, it
is difficult to compare results from different watersheds
(Krammes, 1960). The seemingly disparate results can
only be resolved by understanding how each fire affects
the underlying processes of runoff, erosion, and biogeo-
chemical cycling.
By consuming organic matter, both controlled and un-
controlled fires cause some nutrient loss. These losses
are due to nutrient volatilization, particularly in the case
of nitrogen, and nutrient leaching. On the other hand,
most fires alter the soil environment in ways that lead to
increased microbial activity. The conversion of organic
matter to ash and the increase in microbial activity usual-
ly results in a short-term increase in nitrogen availability
and movement. Depending on the monitoring frequency
and site conditions, an ammonium pulse may occur
before the more commonly observed nitrate pulse. The
short-lived increase in nitrogen availability may help
stimulate any surviving vegetation or the establishment
and regrowth of new vegetation.
Soil erosion rates may increase after fires, but the magni-
tude and persistence of an increase is highly variable and
dependent in large part on fire severity (Tiedemann et al.,
1979; Robichaud and Waldrop, 1994; Benavides -Solo-
rio, 2003). Controlled or prescribed fires typically are
designed to remove fuels or selected vegetation classes,
or modify a vegetation type. Controlled burns usually do
not consume all the protective duff or litter layer over
large areas, while high severity wildfires can expose
the mineral soil over relatively large areas. Post -fire in-
creases in erosion can result from changes in a number
of processes, and this can make it difficult to generalize
or predict the effects of a given fire. Higher erosion rates
after fires can result from: (1) the decrease in litter and
vegetative cover; (2) changes in soil properties, including
the loss of organic matter, formation of a water repellent
layer at or below the soil surface, and sealing of the soil
surface; (3) increased rill erosion due to the increase in
overland flow; (4) channel incision due to the increase in
runoff and decrease in surface roughness; and (5) mass
movements. Soil erosion can reduce the amount of soil
nutrients on site, but more nutrients are usually lost by
28
burning the vegetation unless post -fire soil erosion rates
are very high.
Some studies in the Colorado Front Range have found
very little erosion after wildfires (Delp, 1968; Meyers,
1968), while erosion rates after the 1996 Buffalo Creek
fire increased by several orders of magnitude (Moody
and Martin, 2001). Morris and Moses (1987) measured
large increases in surface erosion after several fires in the
Colorado Front Range, while Cannon et al. (1997) docu-
mented an increase in mass movements after the Storm
King fire near Grand Junction. A series of detailed stud-
ies on the June 2000 Bobcat fire just west of Fort Collins
are showing that sites burned at high severity produce
much more sediment than sites burned at moderate or
low severity. Rainfall simulations on 1 m2 plots indicate
that sites burned at high severity produce 10 -30 times
as much sediment as unburned sites, and sites burned at
moderate severity produced 2 -6 times as much sediment
as unburned and low severity sites (Benavides - Solorio
and MacDonald, 2001; Benavides - Solorio, 2003). Moni-
toring of sediment production at the hillslope scale from
the Bobcat and several other fires confirm the large dif-
ferences in sediment production with fire severity (Bena-
vides- Solorio, 2003).
Data from the Buffalo Creek, Bobcat, and other fires
indicate that at least 80% of the annual erosion is driven
by summer convective storms with rainfall intensities of
10 mm (0.4 inches) per hour (Moody and Martin, 2001;
Benavides - Solorio, 2003; Kunze, 2003). In the case of
the Buffalo Creek and Bobcat fires there were unusu-
ally large convective rainfall events in the first one or
two summers after burning, but similar storms did not
occur after the Hayman fire. It has been suggested that
large burned areas may increase the likelihood of large
convective storms, and this could be an important topic
for further study.
Another important issue is the rate at which sites recover
following burning. Plot -scale studies on the Buffalo
Creek fire indicated that erosion rates on burned hill -
slopes were not significantly different from unburned
hillslopes by the fourth year after burning. Data from
rainfall simulations and sediment fences confirm that
runoff and erosion rates six years after the 1994 Hour-
glass fire were not substantially different from the rates
observed from unburned sites and areas that had been
burned at low severity (Benavides - Solorio, 2003). How-
ever, at the larger scale channel incision and the deposi-
tion of sediment in downstream areas may be much more
persistent. Hence the recovery of hillslope -scale pro-
cesses may be relatively rapid due to vegetative regrowth