The Forest Ecosystem - Mountains and Plains: The Ecology of Wyoming Landscapes - page 211

100

High density

50

0

-50

Low density

-100

0 50 100 150 200 250

Years since fire

Fig. 12.10. Long-term trends in plant growth and carbon
dynamics in lodgepole pine forests developing after high-
severity fire in Yellowstone national Park. nearly all growth is
accounted for by trees, but the trends are based on data for all
plants in forests after increasing time since the last fire. net
ecosystem production (neP) is the difference between carbon
dioxide removed from the atmosphere by photosynthesis
and that returned to the atmosphere by the respiration of all
living organisms, including plants and decomposers. neP
is initially negative—that is, respiration exceeds photosyn-
thesis—because of high rates of respiration by microbes and

other organisms at a time when, after the fire, water and nu-
trients are more readily available for decomposition and the
biomass of photosynthesizing plants is still low. initial post-
fire tree density has a long-lasting influence on neP, because
denser young forests have a higher rate of plant growth (see
fig. 12.9). Sparse young forests eventually become more dense,
reaching their maximum rates of neP several decades later
than the forests that initially have a higher density. Both for-
est types eventually converge on similar low rates of neP. neP
units are grams of carbon per square meter per year. Adapted
from Kashian et al. (2006).

leaf area, which could, potentially, lead to changes in
watershed hydrology . 40 For example, following the
death of more than half the trees in a spruce-fir forest
in the Medicine Bow Mountains of southeastern Wyo-
ming, evapotranspiration declined and soil moisture
increased. 41 that was predictable. However, a study by
Joel Biederman and his associates, working in southern
Wyoming and northern colorado, found no increase
in outflow after beetle-caused mortality. 42 t he explana-
tion appears to be that, although evapotranspiration
was reduced by the beetle attack in the pine forest, the
amount of sublimation from the snow increased under
the more open forest canopy. thus, no additional water
was available for streamflow.
Bark beetle outbreaks also affect nutrient cycling.
the magnitude of this effect is small compared with
the impacts of a high-intensity fire, but dead needles
falling from the canopy bring additional nutrients to
the soil surface, and dead trees no longer take up nutri-
ents from the soil, at least during the first few years after
an outbreak. 43 not surprisingly for an ecosystem where
nitrogen is a limiting factor, the surviving trees and
understory plants sequester many of the nutrients that
become available by decomposition. the net effect on

the ecosystem is a short-term increase in useable nitro-
gen in the soil and increased concentrations of nitrogen
in the leaves of surviving plant s. 44
overall, bark beetle outbreaks lead to some inter-
esting changes in hydrology and nutrient cycling, but
changes in the quantity and quality of streamwater
are still not well known. 45 Additional research is also
needed to determine whether the overall effect of the
outbreaks is to cause forests to become a source or sink
of carbon—an important consideration in modern for-
est management . 46 B eetle outbreaks may have cascad-
ing effects on forest ecosystems, 47 j ust as wolves appear
to have had in the Greater Yellowstone ecosystem (see
chapter 15).

Timber Harvesting Compared with Fires

changes in plant growth rates, hydrology, nutrient cy-
cling, and the abundance and distribution of different
plants and animals can result after timber harvesting,
just as they do after other disturbances. the biggest
distinction is whether the forest was clearcut or selec-
tively harvested. Both clearcutting and crown fires kill
all or most trees, and it is tempting to conclude that