384
might be the relation between ecosystem productivity and climatic indices (Rosenzweig
1968; Leith 1975;·Box 1978), or the regularities
in decomposition rates across ecosystems
(Meentemeyer 1978).
2. Optimality: The concept that processes in ecosystems will (over time) tend to optimize (or, in
some cases, maximize) such attributes as leaf
area index, net primary productivity, or internal
energy fluxes. Two frequently invoked principles are limiting or critical factors-the concept
that a particular internal variable, such as soil
nitrogen or canopy water flux, is the keystone
factor controlling the fluxes and changes in the
terrestrial ecosystems, and stoichiometry-the
concept that physical and chemical processes in
ecosystems will tend to maintain certain ratios
in the concentrations of critical nutrient elements, carbon and water.
3. Equilibrium seeking: The assumption, either
built into the model formulations or used to estimate model parameters, that ecosystems seek
an equilibrium. Models based on conservation
of mass, energy, etc. maintain balances of input
and outputs, and lead naturally to an assumption
of equilibrium-seeking behavior.
Many of this class of models "scale-up" the response of processes as they are understood at the
smaller scale to the landscape- or regional-system
response. For example, one might assume that the
fluxes of heat, water, and carbon dioxide (C02) associated with the functioning of a single leaf are
duplicated by the sum of the responses of the billions of individual leaves comprising a vegetated
landscape. In some cases, the underlying assumption of homogeneous landscape models is that the
mathematical structure of particularly biophysical
and chemical reactions at the landscape level resemble those observed at a detailed level (perhaps
with some differences in the model parameters).
These models are usually the type of ecological
models linked to other models of ocean or atmospheric dynamics to assess the feedbacks among
these major Earth systems (Ojima 1992).
There is a rich array of models of this class. For
example, most models of element cycling in watersheds or other ecosystems tend to view the processes as being homogeneous within the system of
definition. Indeed, traditional ecosystem models
Herman H. Shugart
were often referred to as point models because they
simulated the dynamics of ecosystems with no explicit references to spatial heterogeneity. Two types
of these models that have been widely applied in
the context of changing environmental conditions
are those that simulate a regional plant canopy and
those that are focused on the storage and transfer
of material.
Canopy Process Models
Plant physiologists have made considerable progress in understanding the dynamic response of individualleaves to their environment. Much of this
work was initially developed looking at the response of leaves (over minutes or seconds) in small
chambers in which the temperature, vapor pressure
deficit and other important variables for understanding leaf function were measured and/or controlled. Canopy process models (e.g., Running and
Coughlan 1988) simulate the flux of CO2 and water
from plant canopies over time scales of seconds to
a day. In general, these models are extensions of
leaf-level models of photosynthesis (e.g., Farquhar
et al. 1980) and transpiration (e.g., Penman 1948;
Monteith 1972) applied to whole canopies. The
models do not consider individual plants, but view
the canopy as a single, multilayer unit with a fixed
structure (i.e., leaf area). Photosynthesis and transpiration are simulated by estimating microclimatic
variation and stomatal conductance for the canopy
(or canopy layers).
Material Transfer Models
One "standard" ecosystem model is the compartment model of the transfer of material or energy
into an ecosystem, through the components of an
ecosystem and, eventually, out of the ecosystem.
More complex, often nonlinear material transfer
models were developed during the International
Biological Program (IBP) in the early 1970s. The
IBP models were a heterogeneous collection of
models and modeling approaches that emphasized
understanding natural productivity. Many of these
models have evolved toward present day applications. An example is the CENTURY model, which
was developed as a soil-process-related section of
the IBP Grassland Biome ELM model (Parton
1978). The CENTURY model (Parton et al. 1987)
was developed to simulate soil organic matter dy-
might be the relation between ecosystem productivity and climatic indices (Rosenzweig
1968; Leith 1975;·Box 1978), or the regularities
in decomposition rates across ecosystems
(Meentemeyer 1978).
2. Optimality: The concept that processes in ecosystems will (over time) tend to optimize (or, in
some cases, maximize) such attributes as leaf
area index, net primary productivity, or internal
energy fluxes. Two frequently invoked principles are limiting or critical factors-the concept
that a particular internal variable, such as soil
nitrogen or canopy water flux, is the keystone
factor controlling the fluxes and changes in the
terrestrial ecosystems, and stoichiometry-the
concept that physical and chemical processes in
ecosystems will tend to maintain certain ratios
in the concentrations of critical nutrient elements, carbon and water.
3. Equilibrium seeking: The assumption, either
built into the model formulations or used to estimate model parameters, that ecosystems seek
an equilibrium. Models based on conservation
of mass, energy, etc. maintain balances of input
and outputs, and lead naturally to an assumption
of equilibrium-seeking behavior.
Many of this class of models "scale-up" the response of processes as they are understood at the
smaller scale to the landscape- or regional-system
response. For example, one might assume that the
fluxes of heat, water, and carbon dioxide (C02) associated with the functioning of a single leaf are
duplicated by the sum of the responses of the billions of individual leaves comprising a vegetated
landscape. In some cases, the underlying assumption of homogeneous landscape models is that the
mathematical structure of particularly biophysical
and chemical reactions at the landscape level resemble those observed at a detailed level (perhaps
with some differences in the model parameters).
These models are usually the type of ecological
models linked to other models of ocean or atmospheric dynamics to assess the feedbacks among
these major Earth systems (Ojima 1992).
There is a rich array of models of this class. For
example, most models of element cycling in watersheds or other ecosystems tend to view the processes as being homogeneous within the system of
definition. Indeed, traditional ecosystem models
Herman H. Shugart
were often referred to as point models because they
simulated the dynamics of ecosystems with no explicit references to spatial heterogeneity. Two types
of these models that have been widely applied in
the context of changing environmental conditions
are those that simulate a regional plant canopy and
those that are focused on the storage and transfer
of material.
Canopy Process Models
Plant physiologists have made considerable progress in understanding the dynamic response of individualleaves to their environment. Much of this
work was initially developed looking at the response of leaves (over minutes or seconds) in small
chambers in which the temperature, vapor pressure
deficit and other important variables for understanding leaf function were measured and/or controlled. Canopy process models (e.g., Running and
Coughlan 1988) simulate the flux of CO2 and water
from plant canopies over time scales of seconds to
a day. In general, these models are extensions of
leaf-level models of photosynthesis (e.g., Farquhar
et al. 1980) and transpiration (e.g., Penman 1948;
Monteith 1972) applied to whole canopies. The
models do not consider individual plants, but view
the canopy as a single, multilayer unit with a fixed
structure (i.e., leaf area). Photosynthesis and transpiration are simulated by estimating microclimatic
variation and stomatal conductance for the canopy
(or canopy layers).
Material Transfer Models
One "standard" ecosystem model is the compartment model of the transfer of material or energy
into an ecosystem, through the components of an
ecosystem and, eventually, out of the ecosystem.
More complex, often nonlinear material transfer
models were developed during the International
Biological Program (IBP) in the early 1970s. The
IBP models were a heterogeneous collection of
models and modeling approaches that emphasized
understanding natural productivity. Many of these
models have evolved toward present day applications. An example is the CENTURY model, which
was developed as a soil-process-related section of
the IBP Grassland Biome ELM model (Parton
1978). The CENTURY model (Parton et al. 1987)
was developed to simulate soil organic matter dy-