The Forest Ecosystem - Mountains and Plains: The Ecology of Wyoming Landscapes - page 207

more snow into the downwind forest, where the snow
melts more slowly and higher evapotranspiration may
occur.
in general, abrupt changes between forests of dif-
ferent ages, or between forests and other kinds of veg-
etation (such as meadows), have significant effects on
snow distribution and melting. Weather and vegeta-
tion are all subject to rapid change (see chapter 11), and
these changes affect the amount of streamflow, evapo-
transpiration, and sublimation.

potentially altering the chemical composition of water
downstream.
inputs or additions of nutrients occur as the result of
dryfall (dust), wetfall (rain and snow), rock weathering,
animal immigration, surface or subsurface run-on, and
nitrogen fixation (see fig. 12.2). Losses occur with soil
erosion, leaching, animal emigration, and, for nitro-
gen, loss to the atmosphere. the conversion of nitrate
back into atmospheric nitrogen is a microbial process,
known as denitrification. Research in lodgepole pine
forests reveals that nitrogen and sometimes potassium
and phosphorus accumulate in undisturbed forests over
time, even during years of heavy snowpack and large
volumes of water outflow.22 22 Uptake by plants probably
is especially effective for nutrient retention when the
plants are growing rapidly, incorporating additional
nutrients into new tissues. 23
Dead wood is a common component of almost all
forests and plays a key role in many ecosystem processes,
including nutrient cycling and carbon sequestration. As
fig. 12.7 shows, the nitrogen content of freshly decay-
ing wood increases considerably over 5 years or more,
probably because of nitrogen uptake by the fungi and
bacteria that are using the carbohydrates in the wood
as their source of energy. the additional nitrogen most
likely comes from two sources, namely, atmospheric
deposition and transport by fungal filaments connect-
ing the log to the soil. Subsequently, nitrogen content in
the wood declines as decomposition continues, which

Nutrient Cycling in Forest Landscapes

Unlike water and energy, nutrients cycle in an area for
prolonged periods. Some are lost from the ecosystem,
but they can also be replaced. the nutrient that has
received the greatest attention in Rocky Mountain conif-
erous forests is nitrogen, primarily because this element
often limits rates of plant growth and decomposition,
nitrogen is the most abundant element in the atmo-
sphere (78 percent), where it occurs as n 2 —a form that
normally must be converted to inorganic ammonium
or nitrate before it can be taken up by plant roots. cer-
tain microbes living in the root nodules of some plant
species, such as lupine, or microbes that are free-living
in the soil, are capable of making this conversion—
known as nitrogen fixation. Some fixation also occurs
with lightning. Ammonium and nitrate are released
during the process of decomposition as well. notably,
some plants take up nitrogen from the soil in organic
forms, namely, amino acids . 21 in one form or another,
agricultural and industrial technologies have greatly
increased the amount of nitrogen available to eco-
systems, which is considered a significant environmen-
tal problem on earth as a whole (see chapter 14).
the potential for nutrient leaching from the soil is
of interest in mountain landscapes, because so much
of the water washes through the soil during the 3- to
6-week snowmelt period. that nutrients are retained
during this spring flush is remarkable , considering that
a large portion of litter decomposition occurs during
the winter just before the spring thaw. essentially,
nitrogen-limited plants and microbes scavenge the
nutrients almost as soon as they become available,
thereby immobilizing them. Such immobilization
can be disrupted by disturbances, as discussed below,

Fig. 12.7. the percentage of original nitrogen remaining in
decomposing wood in lodgepole pine forests increases before
it begins to decline; the wood slowly releases this limiting
element for the growth of other plants and microorganisms.
Adapted from Fahey (1983).

180

Nitrogen

140

100

Dry mass

60

20

0

12 24 36 48 60 72 84 96

TIME (months)