Calculating Species Importance Values

Overview:
Plant species vary in their responses to environmental factors. A given species will have a unique set of tolerances to environmental variables, such as light, temperature, moisture, and nutrients. At the community level, these differences in tolerances will cause various species to have competitive advantages, depending on the nature of those environmental factors.

We will examine the distribution of tree species with respect to terrain properties. Several major environmental variables are impacted by terrain, including light, temperature and moisture. Steep slopes and ridgetops tend to be drier due to drainage. Temperature drops with elevation. Sunlight intensity and duration varies with aspect (i.e. the angle of the slope plane with respect to north). Solar radiation is greatest for south facing slopes in the Northern Hemisphere. Soil depth and nutrient concentrations typically decrease with elevation, although this relationship can be quite complex depending on hillslope geomorphology.

In any terrestrial watershed, then, we will expect to see vegetation controlled by terrain attributes. We hypothesize that tree species distributions will differ with respect to hillslope position (streamside vs ridgetop). As a first step, you must decide on your research question.

Using data gathered at our study site, we will analyze vegetation distribution and abundance using the point-quarter sampling method. We will estimate the density, frequency, and coverage of mature trees and calculate the importance values. Then, we will compare these values to another site. The methods used come from Chap 3 in Brower, Zar and von Ende (1997).