BROWSING AND TREE SIZE INFLUENCES ON ASHE JUNIPER UNDERSTORY

Ashe juniper (Juniperus ashei, Buckholz) is increasing on most sites a cross the Edwards Plateau of Texas, It is the purpose of this investig ation 1) to document the influence of Ashe juniper tree size on unders tory vegetation and 2) to evaluate how the interaction between tree si ze and browsing by domestic goats and white-tailed deer modifies overs tory/understory relationships. Trees were randomly selected from 2 lon g-term treatments (browsed and unbrowsed) and analyzed with univariate analysis of covariance and multivariate repeated-measures analysis, W ithout browsing, Ashe juniper is more abundant and its individual infl uence increases as the size of the tree increases; trees with a canopy diameter < 6.0 m expressed minimal influence on understory vegetation compared to larger trees, When browsers are present at sufficient sto cking rates to create a browse line on large trees, encroachment of As he juniper is slowed, rate of increase of all woody species is reduced , and large trees cause a shift in species composition directly under the canopy, however cover of all herbaceous species is not reduced, Im mediately under the canopy of small browsed trees, herbaceous cover is lower than for unbrowsed trees, Environmental variables responsible f or these patterns were litter depth and light penetrating the canopy w hen the sun is at an angle (during the winter), The increased cover of several herbaceous species under the canopy of large browsed trees an d at the canopy edge of browsed and unbrowsed trees, indicates the imp ortance of the interaction between canopy cover and the presence of a browse line, Browse lines on large trees enhance growth and production of cool season species, such as Texas wintergrass (Stipa leucotricha Trin, & Rupr,) and reduce negative influences (low light, thick leaf l itter, etc) on other herbaceous species, At this level of browsing man y other palatable species could be reduced or lost from the plant comm unity.