PHOTOSYNTHESIS AND WATER-USE EFFICIENCY OF 2 SANDHILL OAKS FOLLOWING ADDITIONS OF WATER AND NUTRIENTS

Water and nutrients are considered the primary factors limiting vegetative growth in many plant communities. We examined the effects of added water and nutrients on photosynthesis and water use efficiency (WUE) of juveniles of Quercus hemisphaerica and Quercus laevis growing together in a sandhill community in South Carolina. Supplemental water and nutrients were added in a 2 x 2 factorial design. Diurnal photosynthesis was determined approximately biweekly from June to October and integrated long-term WUE was evaluated using carbon isotope discrimination at the end of this period. Analysis of diurnal photosynthesis curves indicated no significant differences between species, but differences within species were significant between day of sampling, time of measurement, and treatment. Net photosynthesis of Q. hemisphaerica in watered treatments was significantly greater than photosynthesis in unwatered treatments, primarily at midday on days with high temperatures following prolonged drought. Neither nutrients nor water significantly affected the photosynthesis of Q. laevis, regardless of environmental conditions. Fertilization was found to differentially affect WUE of Q. hemisphaerica and Q. laevis, possibly confounding WUE differences between these two species. Because Q. hemisphaerica is often found in more mesic environments than Q. laevis, and because it responded to the addition of water with an increase in photosynthesis, we concluded that in a sandhill community Q. hemisphaerica juveniles are limited primarily by water rather than by low fertility. Our results indicated that juveniles of Q. laevis, the community dominant, are not as plastic as Q. hemisphaerica in their response to fertilizer and water additions.