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 Juniperus ashei and its role in spatial differences of seedlings beneath the woodland canopy and canopy edge.

Wayne, E.^*,1, Jurena, Paul¹, Van Auken, O.¹, ¹ University of Texas at San Antonio, San Antonio, Texas

ABSTRACT- The central Edwards Plateau, Texas, is dominated by Juniperus ashei woodlands which comprise >40% of the canopy cover at ca. 1500 trees/ha. Prior studies indicate that most seedling germinations occur beneath the Juniperus woodland early winter through early spring with reductions seen along a woodland to intercanopy patch gradient. Survival is reported to be highest beneath the woodland canopy and decreased at the canopy edge, but growth rates are highest at the canopy edge. Little is known of the light response of J. ashei seedlings and its affects on spatial differences in germination, growth and survival. In a series of studies, yearly mean photosynthetic flux density (PPFD) was measured beneath the canopy at 346.11±96.81 mol m^-2 s^-1 and 411.84±120 mol m^-2 s^-1 at the canopy edge. Soil moisture beneath the canopy was 34.82±3.72% and 28.00±3.39%. The light response of J. ashei seedlings was also measured and the effect of drought on seedling physiology. Photosynthetic rates, conductance, and transpiration rates for hydrated seedling were 4.56±0.53 mol CO₂ m^-2 s^-1, 0.06±0.01 mol H₂O m^-2 s^-1, and 1.11±0.14 mol H₂O m^-2 s^-1 at 1500 mol m^-2 s^-1 PPFD, respectively. The light compensation point for J. ashei seedlings was between 25 and 50 mol m^-2 s^-1 PPFD. Amax (90%) was 4.48±0.03 mol m^-2 s^-1 at 750 mol m^-2 s^-1 PPFD. The photosynthetic rate of seedlings declined rapidly and significantly below 700 mol m^-2 s^-1 PPFD (P <0.001). Preliminary results indicate that soil moisture and photosynthetic rates decline during drought but conductance and transpiration were unaffected. Juniperus ashei seedlings appear tolerant of low light and drought conditions. In addition, they do not appear to have a mechanism to control water loss. The cause for increased growth of J. ashei seedlings at the canopy edge is likely due to the increased availability of light, while the increase in survival of seedlings beneath the canopy may be due to greater water availability.

KEY WORDS: Juniperus ashei, light response, water stress