Principles of terrestrial ecosystem ecology.pdf

(Schulze et al. 1977).The early spring growth of
spring ephemeral herbs in deciduous forests
also has relatively little influence on nitrogen
cycling because most nitrogen turnover occurs
in midseason. Phenological specialization is an
area in which species effects on ecosystem
processes could be important, but these effects
have been well documented primarily in agricultural
ecosystems.
Species Effects on Climate
Species effects on microclimate influence
ecosystem processes most strongly in extreme
environments. This occurs because ecosystem
processes are particularly sensitive to climate in
extreme environments (Wilson and Agnew
1992, Hobbie 1995). Boreal mosses, for
example, form thick mats that insulate the soil
from warm summer air temperatures. The
resulting low soil temperature retards decomposition,
contributing to the slow rates of
nutrient cycling that characterize these ecosystems
(Van Cleve et al. 1991). Some mosses
such as Sphagnum effectively retain water, as
well as insulating the soil, leading to cold anaerobic
soils that reduce decomposition rate and
favor peat accumulation. The accumulation of
nitrogen and phosphorus in undecomposed
peat reduces growth of vascular plants. The
shading of soil by plants is an important factor
governing soil microclimate in hot environments.
Establishment of many desert cactuses,
for example, occurs primarily beneath the
shade of “nurse plants.”
Species effects on water and energy
exchange can affect regional climate. Species
differences in albedo or the partitioning
between sensible and latent heat fluxes can
Albedo (%)
20 60
0
Sensible heat
(% of Rn )
Species Effects on Ecosystem Processes 271
have strong effects on local and regional
climate. The lower transpiration rate of pasture
grasses compared to deep-rooted tropical trees,
for example, could lead to a significantly
warmer, drier climate following widespread
tropical deforestation because of the lower
evapotranspiration and greater sensible heat
flux of pastures (see Chapter 2). Changes in
vegetation caused by overgrazing could alter
regional climate. In the Middle East, for
example, overgrazing reduced the cover of
plant biomass. Model simulations suggest that
the resulting increase in albedo reduced the
total energy absorbed, the amount of sensible
heat released to the atmosphere, and consequently
the amount of convective uplift of the
overlying air. Less moisture was therefore
advected from the Mediterranean Sea,
resulting in less precipitation and reinforcing
the vegetation changes (Charney et al. 1977).
These vegetation-induced climate feedbacks
could have contributed to the desertification of
the Fertile Crescent. Vegetation changes associated
with fire in the boreal forest can have a
cooling effect on climate. Late-successional
conifers, which dominate the landscape in the
absence of fire, have a low albedo and stomatal
conductance and therefore transfer large
amounts of sensible heat to the atmosphere.
Postfire deciduous forests, in contrast, absorb
less energy, due to their high albedo, and transmit
more of this energy to the atmosphere as
latent rather than sensible heat, resulting in less
immediate warming of the atmosphere and
more moisture available to support precipitation
(Chapin et al. 2000a) (Fig. 12.6). If these
vegetation changes were widespread, they
could have a negative feedback to high-latitude
warming and reduce the probability of fire.This
Evapotranspiration
(% of Rn )
0 0
Conifer Deciduous Conifer Deciduous Conifer Deciduous
Figure 12.6. Sensible and latent heat fluxes from deciduous and conifer boreal forests (Baldocchi et al.
2000).
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