Environmental Gradients

Topographic Relationships

The locations of natural communities are strongly influenced byenvironmental gradients, which areprincipally controlled by topography. In the mid-1900s, Robert Whittaker (1956) published his study of the vegetation communities of the Great Smoky Mountains, in which he demonstrated that the different community types respond to a combination of temperature and moisture gradients that depend on elevation and landform. Landform is the overall shape of the local terrain and surrounding terrain. It describes whether a given site is on an south-facing ridge exposed to wind and sun or, in the other extreme, in a north-facing ravine protected from the elements. Similar analyses have been performed for other portions of the southern mountains (for example, Day and Monk 1974).

Both landform and elevation affect temperature and moisture gradients. Higher elevations have cooler climates and receive more precipitation than sites at lower elevations. Convex landforms, such as ridges and peaks, tend to be drier because precipitation that falls on them runs off. In contrast, concave landforms, such as ravines, are wetter because these places receive moisture from the surrounding hillsides. Sideslopes are intermediate along a moisture gradient because they both lose moisture downslope and gain moisture from upslope positions.Variations in temperature and moisture regimes create conditions that favor different plant and animal species.

These gradients can also influence factors such as disease incidence and primary productivity. For example, the dogwood anthracnose fungus that has killed many flowering dogwoods (Cornus florida) in our area is particularly virulent at elevations above 3,000 ft (Wilds 1997). Higher moisture levels at higher elevations favor the growth of this fungal disease. Primary productivity is a basic ecosystem process that is affected by terrain. For example, aboveground productivity ranges 400 - 1200 g/sq.m./yr in the Smoky Mountains. Productivity is highest at low-elevation sites with adequate moisture. It decreases as elevation increases andmoisture availability decreases(e.g., cove to ridge) (Whittaker 1966).

Landform or terrain shape.
Landform or terrain shape	Landform or terrain shape 2	Landform or terrain shape 3
Convex shape = ridge or peak	Planar shape = sideslope or flat	Concave shape = ravine, cove, or valley

Quantitative relationships between community types and topography can be used to produce maps that classify the terrain and predict forest community types (Bolstad and others 1998, McNab 1989, 1993, 1996, Ford an others 2000) Spatial data on landform and elevation can be incorporated into a Geographic Information Systems model to predict the BROKEN-LINK spatial distribution of community types in a landscape. These maps are useful for conservation and land management because they allow the user to visualize which community types are likely to be affected by management practices. They reveal which communities are most abundant or rare and which ones occur in connected blocks or fragmented patches. Moreover, models of predicted community types can be overlaid with maps of land cover (e.g. forest, agriculture, or urban areas) to determine which natural community types are being destroyed or altered by landscape change.

Topography Map

Image5.gif (Key for topographic map)