PARENT SESSION
Oral Session #17: Plant Ecology: Water Relations.
Presiding: W. Pockman
Monday, August 5. 1:00 PM to 3:45 PM. Coconino Meeting Room, TCC.

Light response of Carex planostachys, an understory species in Juniperus ashei woodlands.

Jurena, Paul^*,1, Wayne, E.¹, Van Auken, O¹, ¹ University of Texas at San Antonio, San Antonio, Texas

ABSTRACT- Carex planostachys is a herbaceous species common to limestone soils of the Edwards Plateau of central Texas. This species is mainly distributed beneath the canopies of Juniperus ashei, a dominate tree species which comprises >40% of the canopy cover in this region. Prior studies have shown that C. planostachys, along with J. ashei, are tolerant of drought conditions common to this region and are able to recover their predawn xylem water potential after an extended drought. In addition, yearly mean photosynthetic photon flux densities (PPFD) measured under J. ashei canopies and at the canopy edge ranged from 346.11±96.81 mol m^-2 s^-1 and 411.84±120 mol m^-2 s^-1, respectively. Soil moisture beneath the J. ashei canopy was 34.82±3.72% and 28.00±3.39% for the canopy edge. However, little information is available on the light response of C. planostachys and how this affects its growth, survival, and distribution. Photosynthetic rates, conductance, and transpiration rates for hydrated C. planostachys plants were 3.16±0.40 mol CO₂ m^-2 s^-1, 0.04±0.01 mol H₂O m^-2 s^-1, and 0.76±0.01 mol H₂O m^-2 s^-1 at 1500 mol m^-2 s^-1 PPFD, respectively. The light compensation point for C. planostachys was between 5 and 10 mol m^-2 s^-1 PPFD. Amax (90%) was 2.52±0.10 mol CO₂ m^-2 s^-1 at 273 mol m^-2 s^-1 PPFD. The photosynthetic rate of C. planostachys declined rapidly and significantly below 250 mol m^-2 s^-1 PPFD (P<0.001). The low light compensation level for C. planostachys, along with its drought tolerance, allows for its survival and growth beneath J. ashei canopies.

KEY WORDS: Carex planostachys, Light response, Juniperus ashei, xylem water potential