Graduate Thesis Or Dissertation
 

Relationships between vegetation and terrain variables in southeastern Arizona

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  • A study to determine the relationship between plant species and eight terrain variables and between thirty-one vegetation types and the terrain variables was conducted in a 4, 000 square mile area south and east of Tucson, Arizona. The eight terrain variables included elevation, parent material, macrorelief, landform type, drainage density, slope angle, slope aspect, and solar radiation index, a derivative of slope angle and slope aspect. The term "terrain variable" was chosen to describe several easily measured and identified properties of the landscape. Data was collected from 250 field sample sites which were selected on the basis of parent material and elevation from within the study area. Floristic data collected consisted of a listing of species at the sampled site and estimates of species cover and prominence. Elevation, parent material, macrorelief, landform type, slope angle, and slope aspect were also determined at each field sample site. Drainage density and solar radiation index were determined in the laboratory. The data were analyzed qualitatively using graphs and tables in order to determine general associations between the species and terrain variables, and between vegetation types and the terrain variables. Stepwise discriminant analysis (BMDO 7M) was also used to quantitatively analyze the data. Computer runs employing stepwise discriminant analysis used individual species to discriminate groups of terrain variables and terrain variables to discriminate vegetation types. Analyses showed that individual species had broader terrain variable amplitudes than did vegetation types. Consequently it is concluded that plant species are not as closely related to terrain variables as are vegetation types. Those species which are most closely related to the terrain variables include Cercocarpus breviflorus, Mortonia scabrella, Quercus emoryi, and Sporobolus airoides. Those species which are considered least closely related to the terrain variables include Acacia constricta, Fouquieria splendens, Opuntia phaeacantha, O. spinosior, and Prosopis juliflora. Stepwise discriminant analysis showed that elevation and macrorelief were the best discriminants of the vegetation types. Stepwise discriminant analysis defines an "average" set of terrain variables for each vegetation type. It then identifies the set of terrain variables of each field sample site (observation) with one of the "average" sets of terrain variables of a vegetation type regardless of correlation of the vegetation types. Using this method, one-half of the observations were identified with the correct vegetation type. Thus, all eight terrain variables interacting together did not perfectly discriminate the twenty-five vegetation types. Part of the reason for the "failure" was the similarity among vegetation types. I found that many of the incorrect identifications involved closely related vegetation types.
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