Ecosystem Process Models

Ecosystem process models offer mechanistic representations of the interactions between the physical environment and biological function over space and time. Models vary in terms of (1) the specific processes and interactions that are included, (2) the complexity of process representation, and (3) the delineation of spatial linkages.

Historically, ecosystem process models have fallen into two classes: those that model the cycling of carbon, water, and nutrients (Biogeochemical models) and those that model species competition (Competition and competition models); although more recently, coupled models of bio-geochemical cycling and species competition have been developed. Models can also be divided into those that are applied to terrestrial (e.g., forest models) versus aquatic (e.g., estuary models) systems. Figure 2 illustrates how key ecological processes can be distributed and linked over time and space in a terrestrial-based ecosystem model. Note that this is a daunting task and consequently few models incorporate detailed representation of all of these processes. For a given process, models use either field-based empirical relationships (empirical models) or theoretical principles to predict the change in system state or process rates in response to variation in environmental controls (forcing functions). For example, some models estimate photosynthesis based on empirical relationships between net photosynthesis and the availability of energy and water; while other models attempt to resolve controls on physiological mechanisms of carbon assimilation within an individual leaf and then scale to whole-plant estimates.

Landscape modeling is mostly concerned with the two horizontal dimensions, but it is often necessary to consider vertical ecological processes and interactions as well. The extent of the vertical profile may range from the top of the canopy through the rooting zone and underlying soil with varying levels of resolution considered. For example, some models represent the canopy as a single unit, while more complex models estimate light interception and attenuation and the associated carbon and moisture fluxes for different layers within the canopy. Similarly, some models represent the soil as a single bucket with a specific moisture content and water-holding capacity. Other models utilize physical

Saturated soil

-n-Saturated throughflow-

Figure 2 An example of vertical processes and spatial interactions that may be represented in ecosystem process modeling approaches. Adapted from RHESSys - Regional Hydro-Ecologic Simulation System.

principles to model the movement and storage of water through multiple soil layers. Some modeling studies have shown that the degree of separation of vegetation or soils into multiple layers can affect estimates of energy transfer, water usage, and biogeochemical fluxes. Ultimately, there will always be a tradeoff between the detail included in process representation and the availability of data needed to parametrize and evaluate a given model. Further, while complex models are often necessary to capture relevant feedbacks and interactions, the analysis of model error and sensitivity becomes problematic in more complex models (model analysis).

Ecosystem process models that consider one or more attributes of landscape spatial heterogeneity may be regarded as landscape models. Most ecosystem process models explicitly account for spatial differences in key environmental controls, including radiation, moisture inputs, and temperature. Spatial differences in soil and vegetation characteristics, topography, chemical inputs (e.g., atmospheric nitrogen deposition), and other factors may also be included for specific models. The scale at which relevant heterogeneity in environmental controls must be represented is a critical issue for ecosystem process models (see discussion of scale below). Spatial differences in ecosystem and biophysical processes also give rise to gradients between landscape patches. Models differ in terms of (1) whether fluxes between patches (due to these gradients) are included and (2) the methods and assumptions used to characterize these lateral fluxes. A variety of models couple distributed hydrology with ecosystem processes and consequently include patch-to-patch transfer of water and dissolved nutrients as a function of hydrologic gradients (watershed models). Explicit modeling of other lateral fluxes, such as the movement of soil organic matter due to erosion or nutrient redistribution by animals, is rarely included in ecosystem process models.

Figure 3 Network model of a hypothetical landscape. Each green habitat patch is represented by a node at the centroid of the patch (black dot). Lines between nodes represent potential dispersal movement, or connectance, between pairs of patches. Two potentially separate populations are shown connected by a 'stepping stone' patch. An isolated patch with no potential dispersal pathways is also shown.

Figure 3 Network model of a hypothetical landscape. Each green habitat patch is represented by a node at the centroid of the patch (black dot). Lines between nodes represent potential dispersal movement, or connectance, between pairs of patches. Two potentially separate populations are shown connected by a 'stepping stone' patch. An isolated patch with no potential dispersal pathways is also shown.

models hold great promise for conservation applications concerned with identifying those patches whose loss or degradation would most drastically alter ecosystem function.

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