Geoscience Reference
In-Depth Information
frees the nutrient for use by trees, plants, and microbes.
the microbes in decaying wood also immobilize phos-
phorus and calcium. considering that annual nitrogen
inputs from fixation and precipitation are low in Wyo-
ming coniferous forests, as are decomposition rates, the
nutrient-enriched decaying logs appear to be an impor-
tant source of nitrogen for sustained site productivity. 24
Silvicultural practices that remove excessive amounts
of wood may be detrimental to long-term site produc-
tivity, as discussed later in this chapter. notably, only
about 4 percent of a forest ecosystem's nitrogen is in
living biomass. Most of it, about 90 percent, is in soil
organic matter (see fig. 12.2), with about 6 percent in
aboveground litter (including decomposing wood). Soil
organic matter (including associated bacteria and fungi)
is mineralized very slowly; therefore, only a small pro-
portion of the nitrogen in that compartment is made
available each year.
Limiting nutrients like nitrogen are retained tena-
ciously once they are in the coniferous forest biomass,
both above- and belowground. 25 one of the most
remarkable adaptations of many plants is the ability to
withdraw some nutrients from senescing leaves back
into stems and twigs prior to leaf fall. this process,
known as nutrient resorption, conserves as much as
half the nitrogen that might have otherwise been lost.
in this way, the plants avoid the inevitable competition
with other organisms that would occur if the nutrients
were returned to the soil. More nutrient resorption was
found to occur in lodgepole pine than in engelmann
spruce and subalpine fir, which surely contributes to the
ability of the pine to tolerate less-fertile soils. in gen-
eral, the nitrogen concentration of forest floor litter in
coniferous forests is low (less than 1 percent), a factor
that limits the metabolism of bacteria and fungi and
thereby slows decomposition. Depending on site condi-
tions, complete mineralization of the leaves of lodge-
pole pine is a slow process, requiring 12-22 years. Boles,
branches, and woody roots decompose slowly as well,
with a 12-inch diameter tree bole requiring about 100
years for complete mineralization . 26
canopies, with important consequences for the abun-
dance and diversity of the organisms making up the
community. Disturbances also alter nutrient cycling
and energy and water flows—effects that are both
immediate and long term. the following sections first
focus on the effects of fire and insects, disturbances
that have affected Wyoming's forest ecosystems for
millennia, and then compare such effects to those of
timber harvesting—a more recent and different kind
of disturbance.
Fire and Insects
the effect of fire on ecosystem processes depends on
the amount of heat released and the amount of vegeta-
tion burned, characteristics that are highly variable.
At one end of the spectrum are low-intensity surface
fires, which historically were most common in foothill
woodlands dominated by ponderosa pine and Douglas-
fir. Such fires burn primarily litter, small trees, and
understory plants. Most canopy trees survive the fire,
and most forbs, grasses, and shrubs quickly regrow by
sprouting, depending on rooting depth and fire inten-
sity. the leaf area of the forest as a whole remains about
the same as before the fire, and only a portion of the
litter is consumed. consequently, low-intensity surface
fires bring about only small changes in plant growth,
streamflow, and nutrient cycling, though such fires are
important for maintaining an open stand structure in
ponderosa pine and Douglas-fir forests (see chapter 11).
At the other end of the spectrum are high-intensity
crown fires, which are typical in lodgepole pine and
spruce-fir forests. these fires kill almost all above-
ground plant parts and greatly reduce the amount of
leaf area (fig. 12.8). the immediate effects are dramatic.
Plant growth stops until understory plants produce
new sprouts, and evapotranspiration is greatly reduced,
because most of the leaf area has been burned. conse-
quently, a much larger portion of the annual precipita-
tion leaves the watershed as streamflow.27 . 27 c ommonly,
flood peaks are higher than before the fire, depending
on such factors as weather conditions during the snow-
melt period, as discussed previously. Also, mineral soil is
exposed when the forest floor is burned, thereby increas-
ing the potential for erosion . 28 the actual amount of
erosion depends on several factors, including slope, fire
Effects of Disturbances on Ecosystem Processes
As discussed in chapter 11, both natural and human-
caused disturbances thin or temporarily remove forest
 
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