The role of photosynthesis in the recruitment of juvenile Quercus gambelii into mature Q. gambelii communities

O. W. VAN AUKEN AND J. K. BUSH (Department of Biology, The University of Texas at San Antonio, San Antonio, TX 78249). The role of photosynthesis in the recruitment Of juvenile Quercus gambelii into mature Q. gambelii communities. J. Torrey Bot. Soc. 136: 465-478. 2009-Previous work with Quercus gambelii (Gambel's oak), a widespread oak species From the mountains of the western United States and northern Mexico, showed high juvenile density (< 3 cm in DBH [diameter at breast height] or less than 150 cm in height), but few or no small (3-5 cm in DBH) tree sized plants, suggesting a recruitment bottleneck. We postulate that there Could be several factors preventing the recruitment of juveniles into the adult population, including reduced light levels, herbivory, competition for soil resources, or a combination of these factors. In this Study, we evaluated the response of leaves of Q. gambelii to various light levels. Surface light levels and leaf gas exchange rates were measured for sun (open) and shade (understory) Q. gambelii seedlings in Q. gambelii communities in the Lincoln National Forest, New Mexico, USA. Mean daily photosynthetic flux densities (PFD) in the understory during the monsoon period (wet season) were 40 +/- 46 mu mol m(-2) sec(-1), or 6% of the open community levels (622 +/- 461 mu mol m(-2) sec(-1)); and 124 +/- 109 mu mol m(-2) sec(-1) or 15% of open community levels (835 +/- 597 mu mol m(-1) sec(-1)) after the monsoon. Light response curves indicated that sun plants had significantly higher maximum photosynthetic rates (A(max) = 23.42 +/- 2.58 mu mol CO2 m(-2) sec(-1)) than shade plants (A(max) = 11.21 +/- 1.68 mu mol CO2 m(-2) sec(-1)). At higher light levels, photosynthetic rates were significantly greater for sun plants than shade plants, however at lower light levels, no significant differences in photosynthetic rates were found. The light compensation points (L-ep) for the sun and shade plants were significantly different at 23.3 +/- 2.9 and 13.7 +/- 8.1 mu mol m(-2) sec(-1), respectively. Respiratory rates of the shade plants were significantly lower than the sun plants. There were no significant differences in mean stomatal conductance or transpiration rates for sun Or shade plants at the maximum light levels (2000 mu mol m(-2) sec(-1)) measured. Reduction of photosynthetic rates below the canopy by reduced light levels may prevent juvenile plants from entering the adult population, but coupling reduced photosynthetic rates to a second inhibitory Factor such as competition for soil resources or browsing by Cervus elaphus (elk = red deer) would reduce the possibility of recruitment even lower.