Document: ROB-3-32-5

Seasonal trends in predawn xylem water potential of two co-occurring species on the Southern Edwards Plateau.

WAYNE, E.R.* and O.W.VAN AUKEN

University of Texas, San Antonio, TX 78249-0662 USA ¹

Abstract:
Juniperus ashei seedlings play an important role in the regeneration of Juniperus woodlands and recruitment into grasslands. Seedlings occur under the canopy and at the canopy edge. Associated with these seedlings is a xeric sedge, Carex lanostachys, occurring at both positions. Until recently, no studies have been conducted on J. ashei seedlings or associated species. To gain an understanding of the effect of temporal changes in soil moisture on plant water relations, bedrock soil moisture and predawn xylem water potential (PSI_p) were measured monthly during 1997 under the Juniperus woodland canopy and at the canopy edge. Soil moisture (%) varied temporally at each position (ANOVA, F = 14.18 and F = 33.44, all P-values < 0.0001), but within any month there were no significant differences between the canopy and canopy edge positions. Mean soil moisture in May was 60.02 2.23% (SE) under the canopy (C) and 44.69 1.34% at the canopy edge (CE), in August 22.81 1.12% (C) and 21.91 0.61% (CE), and in December 57.82 6.69% (C) and 44.66 1.74% (CE). Juniperus ashei seedling PSI_p varied during the study under the woodland canopy and at the canopy edge (F = 1452.62 and F = 633.61, all P-values < 0.0001). The PSI_p for canopy seedlings ranged from -0.48 0.01 MPa in May to -6.5 0 MPa in August. Seedling PSI_p at the canopy edge ranged from -0.76 0.05 MPa in May to -6.5 0 MPa in August. In December, canopy seedlings were -0.36 0.03 MPa and canopy edge seedlings were -0.33 MPa. Carex planostachys PSI_p also varied during the study under the canopy and at the canopy edge (F = 1379.71 and F = 14004.62, all P-values 0.0001). The PSI_p for canopy sedge ranged from -0.01 0 MPa in May to -6.5 0 MPa in August. Canopy edge sedge ranged from -0.4 0.03 MPa in May to -6.5 0 MPa in August. In December, canopy sedge and canopy edge sedge were both 0.01 0 MPa. During wet months, C. planostachys has less water stress than J. ashei seedlings, but during summer drought no differences are detected. Predawn PSI of both species decrease in response to declining soil moisture, and increase with soil rehydration. Additionally, the PSI_P of J. ashei and C. planostachys appear to have similar trends in their response to available soil moisture.

Keywords: Juniperus ashei, seedlings, water relations, Carex planostachys

This abstract is being presented at: 10:15 AM in session:
Oral Session #45: Water Relations in Shrubs and Annuals.