Superimposed on these disturbed landscapes are strong environmental gradients (ie. changes in temperature, vegetation, wind, etc.) which occur in response to variation in elevation, topography, aspect, and soils. For example, at Coweeta, mean annual temperature decreases by almost 5 degrees C from lower to higher elevations, while precipitation increases by 30 percent. These gradients cause a difference in forest communities along topographic and elevational gradients (Figure 1). Temporal variation can also be substantial. For example, in the mid-1980's Coweeta experienced a record drought which resulted in the death of a large number of canopy trees. Taken together, spatial (ex. canopy gaps) and temporal variation in environmental driving variables creates a complex pattern. Some sites, because of their position along environmental gradients, are more prone to certain disturbances (i.e. ice damage occurs more frequently at cooler and higher elevations, windthrow is more likely on exposed slopes, drought mortality is more frequent on ridge sites and on coarse textured soils, and fires are more likely on dry, warm, low-elevation southern exposures).
In combination with these natural disturbances and
environmental variation, southern Appalachian ecosystems have been
substantially influenced by humans for at least the last 6,000 years. Our
activities have altered the atmospheric environment (i.e.
acidic deposition and
ozone) which has affected both aquatic and terrestrial
ecosystems. Fire exclusion, the
extirpation of
large
predators, the introduction of non-native species, and direct and
indirect manipulations to increase and maintain high populations of game
species such as deer, grouse, turkey and trout, have further changed these
ecosystems.
To understand how this variation effects populations and ecosystem
processes, we are continuing to use a combination of long-term
measurements, experiments, and modeling to characterize temporal and
spatial variation in environmental conditions and interactions with
disturbance from the plot to the region (Figure
2). As we continue to
expand our research effort to the entire southern Appalachian region, we hypothesize that land use is more
important than environmental heterogeneity in influencing populations and
processes.
To learn more:
P-
P-IB. Human-caused land cover change during the last half-century
P-IC. Socio-economic drivers of land-use change
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