Date of Award:

5-1-1979

Document Type:

Thesis

Degree Name:

Master of Science (MS)

Department:

Biology

Department name when degree awarded

Life Sciences:Biology

Committee Chair(s)

James A. MacMahon

Committee

James A. MacMahon

Committee

Ivan G. Palmblad

Committee

Martyn M. Caldwell

Abstract

Local and broad geographic patterns of distribution of Sonoran desert subtrees were studied on four bajadas along a southward gradient of increasing moisture. Subtrees are non-riparian desert trees, generally small in stature but having a definite trunk. Percent vegetation cover (m2 of plant canopy/10000 m2) and average size (m2 per plant) of widely distributed species were used to compare environmental favorability among sites. Percent cover of subtrees and total perennials (subtrees plus all other perennials) generally increases up bajadas and southward. Cover of shrubs is variable on bajadas. Subtree importance is most closely correlated with mean summer rainfall (r2= 0.83) and its common logarithmic transformation (r2 =0.85) while variation in mean annual precipitation explains changes in cover of shrubs (r2 =0.98). Perennials as a whole, composed largely of subtrees and shrubs, are correlated with both summer (r2 =0.92) and annual rainfall (r2 =0.91). Increases in soil coarseness up bajadas (as subtree cover increases) and northward (as subtrees decrease in importance), suggest that soil texture interacts with precipitation to regulate subtree abundance. Average size of plants increases with soil coarseness in the more mesic areas but decreases on the rockiest soil where rainfall is minimal. Lifeform cover diversity of the four study sites increases with precipitation while species cover diversity varies with soil coarseness and biseasonality of rainfall. Species diversity does not vary with soil coarseness as reported in the literature. Regression equations adequately predict total perennial cover on bajadas (within 5%) but are poor predictors of subtree cover (within 34%). Sampling methods and vegetation sparseness may explain the latter. Climatic patterns and vegetation characteristics of the northwest Sonoran-Mojave transition indicate that limited summer precipitation and low winter temperatures interact to limit the subtree range.

Download

Share

COinS

Copyright for this work is retained by the student. If you have any questions regarding the inclusion of this work in the Digital Commons, please email us at .