Principles of terrestrial ecosystem ecology.pdf

74 4. Terrestrial Water and Energy Balance
example, have a lower albedo than deciduous
forests, and grasslands with large amounts
of standing dead leaves have relatively high
albedo. The albedo of a complex canopy is less
than that of individual leaves, because much of
the light reflected or transmitted by one leaf is
absorbed by other leaves and stems. For this
reason, deep, uneven canopies of conifer
forests have a low albedo. Changes in ecosystem
albedo explain in part why high-latitude
regions are projected to warm more rapidly
than low latitudes. As climate warms, snow and
sea ice will melt earlier in the spring, replacing
a reflective snow-covered surface with a dark
absorptive surface. This process, together with
the resulting change in temperature, is referred
to as the snow (or ice) albedo feedback. Over
longer time scales, the northward movement of
trees into tundra causes an additional reduction
in regional albedo in winter because the dark
forest canopy masks the underlying snowcovered
surface.With the low sun angles typical
of high latitudes, this effect is significant even
with sparse canopies. As the treeline moves
north, the land surface absorbs more energy,
which is then transferred to the atmosphere,
causing a positive feedback to regional
warming (Foley et al. 1994). Albedo also varies
diurnally; it is about twice as high in early
morning and evening as at midday. The diurnal
changes in absorbed radiation are therefore
greater than one would expect from diurnal
variations in incoming radiation.
Ecosystem Radiation Budget
The amount of longwave radiation emitted
by an object depends on its temperature and
its emissivity, a coefficient that describes the
capacity of a body to emit radiation. Most
absorbed radiation is emitted (emissivity
about 0.98in vegetated ecosystems), so differences
among ecosystems in longwave radiation
balance depend primarily on the temperature
of the sky, which determines Lin, and the surface
temperature of the ecosystem, which determines
Lout.
Rnet = ( Kin -Kout)+
( Lin -Lout)
= ( 1 -a)
K + s e T -e
T
( )
4 4
in sky sky surf surf
(4.2)
where a is the surface albedo, s is the Stefan-
Boltzman constant (5.67 ¥ 10 -8 Wm -2 K -4 ), T is
absolute temperature (K), and e is emissivity.
Clouds are warmer than space and effectively
trap longwave emissions from the surface, so
ecosystems receive more longwave radiation
under cloudy than under clear conditions. This
explains why cloudy nights are warmer than
clear ones and why deserts are generally cold
at night, despite the high inputs of solar energy
during the day.
Longwave radiation emitted by the ecosystem
depends on surface temperature, which,
in turn, depends on the quantity of radiation
received by the surface and the efficiency with
which this energy is transmitted into the air and
soil. Surfaces that absorb a large amount of
radiation, due to high solar inputs and/or low
albedo tend to be warmer and therefore emit
more longwave radiation. Dry surfaces and
leaves with low transpiration rates also tend to
be warm because they are not cooled by the
evaporation of water. Desert sands, recent burn
scars, and city pavements, for example, are
generally hot. Similarly, a well-watered lawn is
much cooler than an ecosystem that is dry or is
dominated by plants with low transpiration
rates. Because L out is a function of temperature
raised to the fourth power (Eq. 4.2), surface
temperature has a powerful multiplying effect
on L out.
Canopy structure also influences surface
temperature and surface energy exchange
through its effect on the efficiency of energy
dissipation. The irregular surface of vegetation
slows down airflow unevenly, creating mechanical
turbulence. Tall uneven canopies such as
conifer forests are aerodynamically more rough
than are short smooth canopies. The mechanical
turbulence generated by airflow across the
vegetation surface creates eddies, which sweep
down into the canopy, transporting bulk air
inward and canopy air out. These eddies transfer
energy away from the surface and mix it
with the atmosphere (Jarvis and McNaughton
1986). Air flowing across short, smooth
canopies such as grasslands or crops tends to be
less turbulent, so these canopies are less efficient
in shedding the energy that they absorb—
that is, they are less tightly coupled to the bulk