Spatial and Temporal Patterns of Xylem Water Potential in Juniperus ashei Seedlings

Abstract

Juniperus ashei woodlands are found extensively throughout the Edwards Plateau of central Texas. Prior studies indicate that seed germination and seedling survival in J. ashei was greatest under the canopy whereas growth was greatest at the canopy edge. To investigate the cause of these spatial differences, measurements of percent soil moisture and plant predawn xylem water potential (ψ p ) for J. ashei seedlings were made along transects from under a woodland canopy into an intercanopy patch. A soil moisture gradient was detected, with soil moisture highest under the woodland canopy and decreasing into the intercanopy patch. Soil moisture was lowest during the summer drought along this gradient. Mean soil moisture was highest at 68.4 ′ 7.0% (′SE) under the Juniperus canopy in May 1997 and lowest at 12.5 ′ 1.9% in the intercanopy patch during July 1997. Seedling ψ p under the Juniperus canopy ranged from values of -0.51 ′ 0.02 MPa in May to -6.59 ′ 0.00 MPa in August Plant ψ p at the canopy edge was -0.72 ′ 0.03 MPa in May and -6.50 ′ 0.00 MPa in August. No seedlings were found in the intercanopy patch. Spatial and temporal differences in seedling ψ p between the Juniperus woodland and canopy edge did exist, but differences were small (<0.20 MPa). Temporal decreases in soil moisture were followed by more negative seedling ψ p values. Differences in observed ψ p along this woodland to intercanopy patch gradient probably cause reductions in photosynthesis and growth rates in J. ashei, and are likely due to an interaction of several abiotic factors, including soil temperature, soil depth, soil moisture, surface light levels, and soil organic content.