264
processes along with the ability to simulate changes
in vegetation type. Neilson (1995) proposes that,
after minor enhancements, the model be used for
simulating the effects of alternative land-use patterns on vegetation and water resources.
As described by Neilson (1995), MAPSS is calibrated for overall accuracy and would be expected to
perform less well for specific sites. Because the model
operates on mean monthly climate, effects of year to
year climate variability such as extreme events cannot be incorporated. In addition, the fire disturbance
rules are inadequate for altered climate and fire in
closed forests. The effect of fire is also too severe,
removing all woody vegetation (Neilson, 1995).
The model has been used to compare worldwide
changes in LAI and terrestrial water balance among
five different general circulation models under predicted climate conditions where current CO2 levels are doubled (2 X CO2) to determine if there are
regionally consistent sensitivities to climate change
(Neilson and Marks, 1994). The model was one of
six models (including the BIOME-BGC and CENTURY models discussed next) implemented to
compare simulations over the conterminous United
States under current climate and under a range of
climate change scenarios provided by three general
circulation models (VEMAP Members, 1995).
18.3 Models of
Ecosystem Function
18.3.1 FOREST-BGC and BIOME-BGC
FOREST-BGC was originally developed to simulate forest stand development through careful calculation of a daily plant water budget (Running and
Coughlan, 1988; Running and Gower, 1991). It has
been generalized to other biomes in the model
BIOME-BGC (Running and Hunt, 1993; Hunt et
aI., 1996). Both FOREST-BGC and BIOME-BGC
simulate hydrologic, carbon, and nitrogen cycles
for generalized ecosystems (Hunt and Running,
1992; Running and Hunt, 1993). The models have
been used to map photosynthesis, respiration, evapotranspiration, decomposition, nitrogen mineralization, and biospheric carbon exchange at point, regional, and global scales (Running and Hunt, 1993;
Hunt et aI., 1996).
BIOME-BGC runs on both daily and annual time
steps concurrently. The daily model simulates photosynthesis, autotrophic and heterotrophic respiration, and a hydrologic budget. The annual time step
allocates carbon and nitrogen among leaves, coarse
and fine roots, litter, and soil. Inputs to the models
Ecosystem Structure and Function Modeling
include daily temperature and precipitation data
from weather stations, biome type and LAI derived
from remote sensing imagery, topography (slope,
elevation, aspect), and soils data (texture, depth,
water holding capacity) (Nemani et at., 1991; Hunt
et aI., 1996).
The use of LAI in the models to define the
canopy makes acquisition of an extensive database
of tree heights and diameters unnecessary (Running
et aI., 1989). The strong control of model processes
by climate is an advantage, because daily climate
data are among the most readily available data
globally (Running and Hunt, 1993). Use of the
models is limited for some applications by the entities simulated (e.g., changes in stand carbon over
time), which may not address some ecological assessment needs. In addition, the models are computationally intensive and cannot be used to predict changes in species distribution or internal
changes in LAI (Friend et aI., 1993).
FOREST-BGC has been used to evaluate forest
processes in response to site quality (McLeod and
Running, 1988; Korol et aI., 1991), as well as air
pollution and 2 X CO2 effects on forests (Kremer,
1991; Running and Nemani, 1991). More recently,
BIOME-BGC has been used to estimate global net
terrestrial carbon exchange and atmospheric CO 2
concentrations and global-scale responses of vegetation to 2 X CO2 (Hunt et aI., 1996). Attempts
have been made to integrate some aspects of
individual-based models with the process-based
components of FOREST-BGC. One such model,
HYBRID (Friend et aI., 1993), is discussed in the
section on individual-based models. Another
model, FIRE-BGC, combines properties of an individual-based simulation model, FIRESUM
(Keane et aI., 1989, 1990a, 1990b), and FORESTBGC (Keane et aI., 1996b). FIRE-BGC is a mechanistic, individual tree succession model in which
tree growth, organic matter decomposition, litterfall, and other processes are simulated using detailed physical relationships. Mechanistic components of FOREST-BGC provide the framework for
FIRE-BGC, with FIRESUM routines added and
modified to utilize FOREST-BGC information.
The model includes fire and its effects on ecosystem components in a spatial context, as well as effects of insects and disease.
18.3.2 RHESSys
RHESSys, a combination of mapped data and land
surface characteristics with integrated hydrologic
and ecological models, allows exploration of landscape processes on a watershed scale (Band et at.,
processes along with the ability to simulate changes
in vegetation type. Neilson (1995) proposes that,
after minor enhancements, the model be used for
simulating the effects of alternative land-use patterns on vegetation and water resources.
As described by Neilson (1995), MAPSS is calibrated for overall accuracy and would be expected to
perform less well for specific sites. Because the model
operates on mean monthly climate, effects of year to
year climate variability such as extreme events cannot be incorporated. In addition, the fire disturbance
rules are inadequate for altered climate and fire in
closed forests. The effect of fire is also too severe,
removing all woody vegetation (Neilson, 1995).
The model has been used to compare worldwide
changes in LAI and terrestrial water balance among
five different general circulation models under predicted climate conditions where current CO2 levels are doubled (2 X CO2) to determine if there are
regionally consistent sensitivities to climate change
(Neilson and Marks, 1994). The model was one of
six models (including the BIOME-BGC and CENTURY models discussed next) implemented to
compare simulations over the conterminous United
States under current climate and under a range of
climate change scenarios provided by three general
circulation models (VEMAP Members, 1995).
18.3 Models of
Ecosystem Function
18.3.1 FOREST-BGC and BIOME-BGC
FOREST-BGC was originally developed to simulate forest stand development through careful calculation of a daily plant water budget (Running and
Coughlan, 1988; Running and Gower, 1991). It has
been generalized to other biomes in the model
BIOME-BGC (Running and Hunt, 1993; Hunt et
aI., 1996). Both FOREST-BGC and BIOME-BGC
simulate hydrologic, carbon, and nitrogen cycles
for generalized ecosystems (Hunt and Running,
1992; Running and Hunt, 1993). The models have
been used to map photosynthesis, respiration, evapotranspiration, decomposition, nitrogen mineralization, and biospheric carbon exchange at point, regional, and global scales (Running and Hunt, 1993;
Hunt et aI., 1996).
BIOME-BGC runs on both daily and annual time
steps concurrently. The daily model simulates photosynthesis, autotrophic and heterotrophic respiration, and a hydrologic budget. The annual time step
allocates carbon and nitrogen among leaves, coarse
and fine roots, litter, and soil. Inputs to the models
Ecosystem Structure and Function Modeling
include daily temperature and precipitation data
from weather stations, biome type and LAI derived
from remote sensing imagery, topography (slope,
elevation, aspect), and soils data (texture, depth,
water holding capacity) (Nemani et at., 1991; Hunt
et aI., 1996).
The use of LAI in the models to define the
canopy makes acquisition of an extensive database
of tree heights and diameters unnecessary (Running
et aI., 1989). The strong control of model processes
by climate is an advantage, because daily climate
data are among the most readily available data
globally (Running and Hunt, 1993). Use of the
models is limited for some applications by the entities simulated (e.g., changes in stand carbon over
time), which may not address some ecological assessment needs. In addition, the models are computationally intensive and cannot be used to predict changes in species distribution or internal
changes in LAI (Friend et aI., 1993).
FOREST-BGC has been used to evaluate forest
processes in response to site quality (McLeod and
Running, 1988; Korol et aI., 1991), as well as air
pollution and 2 X CO2 effects on forests (Kremer,
1991; Running and Nemani, 1991). More recently,
BIOME-BGC has been used to estimate global net
terrestrial carbon exchange and atmospheric CO 2
concentrations and global-scale responses of vegetation to 2 X CO2 (Hunt et aI., 1996). Attempts
have been made to integrate some aspects of
individual-based models with the process-based
components of FOREST-BGC. One such model,
HYBRID (Friend et aI., 1993), is discussed in the
section on individual-based models. Another
model, FIRE-BGC, combines properties of an individual-based simulation model, FIRESUM
(Keane et aI., 1989, 1990a, 1990b), and FORESTBGC (Keane et aI., 1996b). FIRE-BGC is a mechanistic, individual tree succession model in which
tree growth, organic matter decomposition, litterfall, and other processes are simulated using detailed physical relationships. Mechanistic components of FOREST-BGC provide the framework for
FIRE-BGC, with FIRESUM routines added and
modified to utilize FOREST-BGC information.
The model includes fire and its effects on ecosystem components in a spatial context, as well as effects of insects and disease.
18.3.2 RHESSys
RHESSys, a combination of mapped data and land
surface characteristics with integrated hydrologic
and ecological models, allows exploration of landscape processes on a watershed scale (Band et at.,