Principles of terrestrial ecosystem ecology.pdf

occurs near their edges, rather than in the
center, where the overlying air is so stable
that it saturates rapidly and supports a relatively
low evaporation rate. A swamp with
interspersed patches of vegetation would
have greater convective turbulence and overall
evaporation than homogeneous water or moist
vegetation. When ecosystem patches that differ
strongly in energy partitioning are larger in
diameter than the depth of the planetary
boundary layer (greater than about 10km),
they can modify mesoscale atmospheric circulations
and cloud and precipitation patterns
(Pielke and Avisar 1990, Weaver and Avissar
2001).
Seasonal Energy Exchange
Over sufficiently long time scales, energy
outputs are tightly coupled to inputs because
there is little energy storage at Earth’s surface.
Most ecosystems have a limited capacity for
long-term energy storage in vegetation and
surface soils. Consequently, energy losses to the
atmosphere closely track solar inputs on both a
daily and a seasonal basis, although the form in
which this energy is lost (sensible vs. latent heat
flux) varies, depending on moisture availability.
Ground heat fluxes are usually negligible when
averaged over 24h (Oke 1987). Important
exceptions to this generalization are water
bodies such as lakes and oceans, in which the
solar inputs often penetrate to depth, resulting
in substantial warming of the water, and
some high-latitude regions. Water bodies often
absorb substantial energy in spring and early
summer, when the solar angle is greatest,
causing the water to warm. There is a net
release of energy in the autumn, moderating
the temperatures of adjacent terrestrial surfaces
(see Chapter 2). This seasonal pattern of
energy exchange drives the annual or semiannual
turnover of lakes (see Chapter 10). Permafrost
contributes to a seasonal imbalance in
energy absorption and release in cold climates.
In the arctic, for example, 10 to 20% of the
energy absorbed during summer is consumed
by thawing of frozen soil. This energy is
released back to the atmosphere the next
winter, when the soil refreezes (Chapin et al.
2000a).
Water Inputs to Ecosystems 77
Snow-covered surfaces experience threshold
changes in energy exchange at the time
of snowmelt (Liston 1999). The high albedo
of snow-covered surfaces minimizes energy
absorption until snowmelt occurs, at which
time there is a dramatic increase in the energy
absorbed by the surface and transferred to
the atmosphere. This may result in abrupt
increases in regional air temperature after
snowmelt. Leaf out also alters energy exchange
by both changing albedo and increasing evapotranspiration
at the expense of sensible heat
flux.
Water Inputs to Ecosystems
Precipitation is the major water input to most
terrestrial ecosystems. Global and regional controls
over precipitation therefore determine the
quantity and seasonality of water inputs to most
ecosystems (see Chapter 2). In ecosystems that
receive some precipitation as snow, however,
the water contained in the snowpack does not
enter the soil until snow melt, often months
after the precipitation occurs. This causes the
seasonality of water input to soils to differ from
that of precipitation.
Vegetation in some ecosystems, particularly
in riparian zones, accesses additional groundwater
that flows laterally through the ecosystem.
Desert communities of phreatophytes
(deep-rooted plants that tap groundwater),
for example, may absorb sufficient groundwater
that the ecosystem loses more water
in transpiration than it receives in precipitation.
Lakes and streams also receive most
of their water inputs from groundwater or
runoff that drains from adjacent terrestrial
ecosystems. Water inputs to freshwater ecosystems
are therefore only indirectly linked to
precipitation.
In ecosystems with frequent fog, canopy
interception of fog increases the water inputs to
ecosystems, when cloud droplets that might not
otherwise precipitate are deposited on leaf surfaces
and are absorbed by leaves or drip from
the canopy to the soil. The coastal redwood
trees of California, for example, depend on
fog-derived water inputs during summer, when
precipitation is low, but fog occurs frequently.