Hi all, I am currently writing one of my dissertation chapters. I collected plant community data in a grassland ecosystem along with environmental variables . One of the questions I am addressing is: "How do the dominant grasses respond to the aspect?" Aspect was the direction that the slope faced. In the linked graphs (http://sweb.uky.edu/~mpsrin2/aspect_fig.pdf) I have displayed aspect in two ways: (1) in degrees as measured by compass bearings, Fig. 2,and (2) converted to linear scale using: A' = cos (45 - A) + 1, where A is the aspect in degrees (Beers et al, 1966, Journal of Forestry), Fig.3. The resulting index values range from 0 to 2 (0 = SW, 1 = SE and NW, 2 = NE), see Fig. 1. I have fitted regression lines after determining the appropriate fit. One of my concerns is: in Fig. 3, the hump around A'=1.5 could either correspond to N or E, as both take the value 1.5 (see Fig.1). So this index of aspect does not allow me to interpret if the plants are more abundant at N or at E facing slopes. Fig 2. allows me to distinguish data from the different aspects, and it is easier to explain; but the explanatory variable here is circular, and I am concerned whether it is correct to apply regressions on circular data. I showed these graphs to some of my colleagues and I got mixed responses. I would like to know which representation is more appropriate? Right now I am leaning towards Fig. 2, but I am concerned about the statistical appropriateness. I will include the explanatory Fig. 1 with either graph that I choose to finally use. Also, if there is a better way to display/ analyze this, please let me know.
Thanks, Madhu Srinivasan Department of Biology University of Kentucky
