Principles of terrestrial ecosystem ecology.pdf

Vapor pressure is the partial pressure
exerted by water molecules in the air. The
driving force for evaporation is the difference
in vapor pressure between the air
immediately adjacent to an evaporating
surface and that of the air with which it
mixes. The air immediately adjacent to an
evaporating surface is approximately saturated
at the temperature of the surface.
The vapor pressure deficit (VPD) is the
glaciers and ice caps. A rising sea level endangers
the coastal zone, where most of the major
cities of the world are located. Given the key
role of water and energy in ecosystem and
global processes, it is critical that we understand
the controls over water and energy exchange
and the extent to which they have been
modified by human actions.
Surface Energy Balance
Solar Radiation Budget
The energy absorbed by a surface is the balance
between incoming and outgoing radiation.
Here we focus on ecosystem-scale radiation
budgets, although the general principles apply
at any scale, ranging from the surface of a leaf
to the surface of the globe (see Fig. 2.2). The
two major components of the radiation budget
are shortwave radiation (K), the high-energy
radiation emitted by the sun, and longwave
radiation (L), the thermal energy emitted by all
bodies (see Chapter 2). Net radiation (Rnet) is
the balance between the inputs and outputs of
shortwave and longwave radiation, measured
as watts per meter squared (W m -2 ).
R = ( K -K
)+ ( L -L
)
net in out in out
(4.1)
On a clear day, direct radiation from the sun
accounts for most of the shortwave input to an
ecosystem (see Fig. 2.2). Additional input of
shortwave radiation comes as diffuse radiation,
which is scattered by particles and gases in
the atmosphere, or as reflected radiation from
clouds and surrounding landscape units such as
lakes, dunes, or snowfields. At noon on a clear
Surface Energy Balance 73
difference between actual vapor pressure
and the vapor pressure of air at the same
temperature and pressure that is saturated
with water vapor. This term is loosely used
to describe the difference in vapor pressure
between air immediately adjacent to an
evaporating surface and the bulk atmosphere,
although strictly speaking, the air
masses are at different temperatures.
day in a nonpolluted atmosphere, direct radiation
accounts for 90% of incoming shortwave
radiation to an ecosystem, and diffuse radiation
becomes proportionately greater on cloudy
days or near dawn or dusk when sun angles are
lower. For Earth as a whole, direct and diffuse
radiation each account for about half of incoming
shortwave radiation (see Fig. 2.2).
The proportion of the incoming shortwave
radiation that is absorbed depends on the
albedo (a) or shortwave reflectance of the
ecosystem surface. Ecosystem albedos vary at
least 10-fold, ranging from highly reflective surfaces
such as fresh snow to dark surfaces such
as wet soils (Table 4.2). Conifer canopies, for
Table 4.2. Typical values of albedo of major surface
types on earth.
Surface type Albedo
Oceans and lakes 0.03–0.10 a
Sea ice 0.30–0.45
Snow
Fresh 0.75–0.95
Old 0.40–0.70
Arctic tundra 0.15–0.20
Conifer forest 0.09–0.15
Broadleaf forest 0.15–0.20
Agricultural crops 0.18–0.25
Grassland 0.16–0.26
Savanna 0.18–0.23
Desert 0.20–0.45
Bare soil
Wet, dark 0.05
Dry, dark 0.13
Dry, light 0.40
a Albedo of water increases greatly (from 0.1 to 1.0) at
solar angles