Gradient models describe how physical drivers of ecological patterns and processes vary as a function of landscape position (Zonation). These models typically focus on the small set of drivers that control plant establishment, growth, and mortality in terrestrial systems: light, heat, water, and nutrients. Heterogeneity in light and soil nutrients is frequently at too fine a scale to be captured effectively over large spatial extents. Thus, temperature and water availability are of primary interest to the landscape ecologist. Heat provides the energy for plant metabolic processes, while water maintains cell functions and transports nutrients. At the ecosystem level, gradients in these resources influence such fundamental processes as photosynthesis, respiration, and nutrient uptake, and act as primary constraints in models of forest productivity, species composition, flooding, erosion, and fire.
Gradient models are often rather simple in formulation and were among the first to incorporate spatial heterogeneity at a resolution that was useful to natural resource management. Many of these models are purely statistical and take advantage of the strong correlation between environmental gradients of interest and more easily measured proxy variables (Statistical methods). Because the proxy data are spatially structured, model output can be mapped. For example, maps of temperature variability in mountainous settings are often based solely on the relationship between temperature and elevation as quantified in a lapse rate model. Similarly, distributed-parameter models are commonly used to develop estimates of soil moisture variability from terrain attributes such as local slope angle and upslope area. These types of models are quite effective for representing ecological processes that have spatial structure but are not dependent upon spatial interactions. They are considerably less effective for modeling processes with a high spatial contagion (i.e., what happens at one point in space is dependent upon the dynamic state of neighboring points), such as the spread of fire or many other disturbances.
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