Browsing and tree size influences on Ashe juniper understory.

Abstract

Ashe juniper (Junius ashei, Buckholz) is increasing on most sites across the Edwards Plateau of Texas. It is the purpose of this investigation to document the influence of Ashe juniper tree size on understory vegetation and to evaluate how the interaction between tree size and browsing by domestic and white-tailed deer modifies overstory-understory relationships. Trees were randomly selected from 2 long-term treatments (browsed and unbrowsed) and analysed with univariate analysis of covariance and multivariate repeated-measures analysis. Without browsing, Ashe juniper is more abundant and its individual influence 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 stocking rates to create a browse line on large trees, encroachment of Ashe juniper is slowed, rate of increase of all woody speces is reduced, and large trees cause a shift in species composition directly under the canopy, however cover of all herbaceous species is not reduced. Immediately under the canopy of small browsed trees, herbaceous cover is lower than for unbrowsed trees. Environmental variables rpsponsible for these patterns were litter depth and light penetrating the canopy when the sun is at an angle (during the winter). The increased cover of several herbaceous species under the canopy of large browsed trees and at the canopy edge of browsed and unbrowsed trees, indicates the importance 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 litter, etc.) on other herbaceous species. At this level of browsing many other palatable species could be reduced or lost from the plant comunity.