Principles of terrestrial ecosystem ecology.pdf

when the water vapor condenses to form clouds
or precipitation. The energy released to the
atmosphere by condensation typically occurs
some distance downwind from the point at
which the water evaporated. Ecosystem structure
influences the efficiency with which sensible
heat and latent heat are transferred to
the atmosphere. Wind passing over tall uneven
canopies creates mechanical turbulence that
increases the efficiency of heat transfer from
the surface to the atmosphere (see Chapter 4).
Smooth surfaces, in contrast, tend to heat up
because they transfer their heat less efficiently
to the atmosphere, only by convection and not
by mechanical turbulence.
The effects of vegetation structure on the
efficiency of water and energy exchange influence
regional climate. Between 25 and 40% of
the precipitation in the Amazon basin comes
from water that is recycled from land by evapotranspiration
(Costa and Foley 1999). Simulations
by climate models suggest that, if the
Amazon basin were completely converted from
forest to pasture, South America would have a
permanently warmer drier climate (Shukla et
al. 1990). The shallow roots of grasses would
absorb less water than the deep tree roots,
leading to lower transpiration rates (Fig. 2.11).
Pastures would therefore release more of the
absorbed solar radiation as sensible heat, which
directly warms the atmosphere.The simulations
also suggests that warming and drying of air
caused by widespread conversion from forest to
pasture would reduce the transport of moisture
from the adjoining oceans, causing a permanent
Evapotranspiration (mm d -1 )
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3
0
Surface temperature ( o C)
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Vegetation Influences on Climate 33
reduction in precipitation—conditions that
favor persistence of pastures over forests.These
simulations do an excellent job of exploring
the consequences of such vegetation effects on
processes that are well understood. There are
still many uncertainties, however. Changes in
cloudiness, for example, can have either a positive
or a negative effect on radiative forcing,
depending on the clouds’ properties and height.
Because these models do a poor job of simulating
the processes that produce clouds, the
simulations should be viewed as a way to synthesize
the net effect of the processes that we
understand, rather than as predictions of the
future.
At high latitudes, tree-covered landscapes
absorb more solar radiation before snow melt,
due to their low albedo, than does snowcovered
tundra. Model simulations suggest that
the northward movement of the treeline 6000
years ago could have reduced the regional
albedo and increased energy absorption sufficiently
to explain half of the climate warming
that occurred at that time (Foley et al. 1994).
The warmer regional climate would, in turn,
favor tree reproduction and establishment at
the treeline (Payette and Filion 1985), providing
a positive feedback to regional warming
(see Chapter 12). Predictions about the impact
of future climate on vegetation should therefore
also consider ecosystem feedbacks to
climate.
Albedo, energy partitioning between latent
and sensible heat fluxes, and surface structure
also influence the amount of longwave radia-
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Forest Pasture Forest Pasture Forest Pasture
Figure 2.11. Simulations, using a general circulation
model, of changes in evapotranspiration, surface air
temperature, and precipitation that would occur if
Precipitation (mm d -1 )
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4
the rain forests of South America were replaced by
pasture (Shukla et al. 1990).