Effects of mature redberry juniper on associated herbaceous vegetation

Abstract

The effects of mature redberry juniper (juniperus pinchotii Sudw.) trees on biomass, density and basal cover of associated herbaceous plants were quantified during 1991-1993 on 3 soils in the northern Edwards Plateau. Herbaceous biomass was 1024 to 1876 kg/ha lower and herbaceous plant densities were 64 to 88% lower at the base of redberry junipers than at 6 m beyond their canopy edges. Basal cover of herbaceous plants ranged from 0.0 to 2.0% at juniper stem bases compared to a range of 5.0 to 10.0% 6 m beyond canopy edges. Species richness of grasses and forbs was reduced at juniper stem bases and at mid-canopy on some sites compared to that in the interstitial areas, whereas species richness of shrubs and succulents was greatest beneath juniper canopies. Juniper interference with herbaceous production extended at least 6 m beyond their canopy edges on a shallow, rocky Kimbrough loamy soil, but only to the edge of juniper canopies on a deep Angelo clay loam and to 1 m beyond canopy edges on a deep Tulia loam. Herbaceous biomass increased 4164% at the stem base and 74% 6 m beyond canopy edges 2 yr after mature redberry junipers were killed by soil applications of picloram (4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid) on the Kimbrough soil. Herbaceous biomass responses to juniper control were limited to beneath the dead juniper canopies on the Angelo clay loam and from stem bases to 1 m beyond canopy edges on the Tulia loam. Grass densities increased 293% beneath dead juniper canopies on the Kimbrough soil and 47 and 40% at canopy edges and 1 m beyond canopy edges of dead junipers, respectively, on the Tulia loam. Basal cover of herbaceous species at the base and mid-canopy was 3.5 to 13-fold greater for dead junipers compared to live junipers 2 yr after treatment application. Species richness of grasses was enhanced by juniper control only on the Kimbrough soil. Forb species richness beneath juniper canopies was reduced following juniper control on the Kimbrough and Angelo clay loam soils, whereas there was a trend toward enhanced forb species richness beyond the canopy edges.