Date of Award:

5-1978

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Plants, Soils, and Climate

Committee Chair(s)

James A. MacMahon

Committee

James A. MacMahon

Abstract

Spacing patterns of shrubs were studied at a series of sites in the Mojave and Sonoran Deserts. There were significant numbers of occurrences of both aggregation and regularity in dispersion of individuals. These may be due to vegetative reproduction or environmental heterogeneity, and competition between plants, respectively. Different size classes of shrubs tended to follow the trend of aggregated → random → regular with increasing size. This suggests the increasing importance of competition as the plants grow. Further evidence of competition between plants was provided by the correlations of plant size with the distance to their neighbors. Root systems were extensive enough to abut or overlap each other in the interplant spaces. Most plants tended to have neighbors of the same species rather than other species. Three new methods were devised to measure this aspect of spacing. The first is based on the frequency of co-occurrence of two species in quadrat samples. The other two methods are non-parametric and are based on nearest neighbor samples rather than quadrats. Any number of nearest neighbors may be used for each base plant; one method weights them by their relative proximity to the base plant. Finally, none of these results correlated with climatic gradients through the Mojave and Sonoran Deserts.

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