Direct Effects of Competition on Individual Juniper Plants: A Field Study

Abstract

SUMMARY (1) Two hundred and forty Juniperus pinchotii plants were harvested on each of two sites in western Texas. (2) The competitive influence of herbs and shrubs associated with each juniper plant was quantified in an attempt to explain variability in survival and regrowth of individual coppiced plants. (3) Survival and regrowth were greater on the deep-soiled Rolling Plains site than on tthe shallow-soiled High Plains site. (4) Pre-harvest age or size largely controlled survival and subsequent regrowth of coppiced Juniperus pinchotii plants. (5) Competition from neighbouring shrubs, though significant, explained little of the observed variation in survival or growth. Competition was most common during periods of active Juniperus growth. Competitive influence decreased with increasing distance, but not in a linear manner. (6) Competition from herbaceous plants was not detected. Most studies addressing plant competition in semi-arid regions have investigated spatial pattern, with the hypothesis that competition will convert clumped distributions of plants into random, and eventually, regular ones (Fowler 1986b). Few studies have examined direct effects of competition on individual plants, despite the fact that problems associated with inferring competition from spatial arrangements (Ebert & McMaster 1981; Fowler 1984, 1986b; Silander & Pacala 1985) indicate a need for studies which address direct effects of competition on individual plant performance. The objective of this study was to determine the effect of competing vegetation on survival and growth of a woodland conifer. Competitive status was evaluated for a range of size-classes of several shrub species in a field study. Nearly all field studies of direct interference have been conducted in even-aged woodlands or forests and have ignored the influence of tree size on competitive ability (e.g. Weiner 1984). Cannell, Kothery & Ford (1984) showed that competitive status of individuals was related to tree height in even- aged monocultures of two tree species. Wu et al. (1985) extended the concept of size- related regions of influence to natural stands of vegetation. Penridge & Walker (1986) found that a competition model which included plant size predicted growth better than nearest-neighbour distance or Weiner's interference model. Juniperus pinchotii Sudw., a sprouting evergreen conifer commonly found on limestone and gypsum soils (Correll & Johnston 1970), was selected for this study because of its ability to sprout basally following top removal (Vines 1960). Its resprouting ability is related to the position of the stem bud zone, a region of meristematic tissue near the base of the tree. The bud zone becomes buried with the passage of time as soil and plant