The Forest Ecosystem - Mountains and Plains: The Ecology of Wyoming Landscapes - page 202

(such as the red squirrel and red-backed vole) commonly
graze on mushrooms, truffles, and puffballs, the repro-
ductive structures of fungi that decompose the detritus,
and birds commonly prey on these small mammals and
various invertebrate detritivores. Moreover, evidence
suggests that these animals are important in dispersing
the spores of fungi, which in turn are important not
only for decomposition but—in the case of mycorrhizal
fungi—also for the establishment and growth of new
plant seedlings, including those of all Rocky Mountain
trees . 4
the importance of mycorrhizal fungi is suggested
by estimates that up to 15 percent of the net primary
productivity in a coniferous forest goes to the main-
tenance of these fungi on roots . 5 All studies thus far
indicate that, rather than being parasites, mycorrhizal
fungi develop a mutually beneficial association with
their hosts. 6 t he fungi derive energy from the plants in
the form of carbohydrates, while the fungal filaments,
known as hyphae, extend beyond the roots and enhance
water and nutrient uptake. the decaying biomass of the
fine roots and associated fungi may contribute more to
nutrient availability than do decaying leaves, twigs, and
branches.
on average, only 2 percent or less of the energy fixed
by plants during photosynthesis flows through animals
in terrestrial ecosystems, whether forests, shrublands, or
grasslands, yet animals often influence the ecosystem
in important ways. An example is the planting of white-
bark and limber pine seeds by clark's nutcracker (see
chapters 10 and 11). the populations of different her-
bivores and carnivores continually fluctuate, with the
result that their influences are greatly amplified when
they are abundant, such as during outbreaks of bark
beetles. A less well-known example occurs when the
populations of porcupines are high. Porcupines feed on
tree bark, resulting in reduced flow of carbohydrates to
the roots, which lowers plant growth in the forest as a
whole . 7 e cosystems are dynamic, with all components
changing over time, as does the amount of energy flow-
ing through the detrital, grazing, and fire pathways.

3

Live trees
Forest floor litter
Dead trees and
downed wood

2

1

0

-1

-2

0 40 80 120 160 200

Stand Age (years)

Fig. 12.3. Rates of biomass increment or decline in live trees,
forest floor litter, and dead trees and downed wood change
as forests grow to maturity and become “old-growth” forests.
older forests have more dead trees and downed wood, but
tree growth slows as forests age (see also fig. 12.1). Dead trees
and downed wood, sometimes referred to as snags and coarse
woody debris, decline soon after fire, as decomposition rates
of this material are relatively high when water and nutrients
are more readily available for the decomposers. the forest
floor litter increases initially after a fire—which commonly
burns much of the pre-fire forest floor—but then remains
about the same, as litterfall becomes approximately equal to
the rate of litter decay. the patterns illustrated will be some-
what different for forests growing on different soils, at differ-
ent elevations, or in different topographic positions. these
graphs are based on data collected from several lodgepole pine
forests at an elevation of about 9,500 feet in the Medicine Bow
Mountains. Rates of change are expressed as metric tons per
hectare per year; see fig. 12.1 for unit conversions. Adapted
from Pearson et al. (1987); see also Smith and Resh (1999) and
Kashian et al. (2013).

decomposition occurs under snow during the winter, as
some of the decomposers are able to function at the soil/
snow interface. 3 B ecause detritus accumulates over time
and is flammable when dry, fire is another important
pathway for energy flow—a decomposition process that,
like bacteria and fungi, releases the inorganic nutrients
required for new plant growth (fig. 12.4).
the combined biomass of forest floor litter, fungi,
and bacteria is far greater than the total biomass of
vertebrates and invertebrates. not surprisingly, various
animals have evolved to depend on detritus for a signifi-
cant part of their energy. For example, small mammals

Hydrology of Forest Landscapes

Mountain landscapes are hydrologically distinctive
from the surrounding lowlands, because they receive