Comparison of photosynthetic responses of Ashe juniper and live oak on the Edwards Plateau, Texas



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Ashe juniper (Juniperus ashei Bucholz) has encroached into the historical grasslands of the Edwards Plateau. This area is environmentally sensitive as it serves as the recharge zone for the Edwards aquifer, providing large municipalities such as Austin, San Antonio, and San Marcos with water. The increased tree density may impact local water budgets, but the trees may have the capability of sequestering a greater amount of carbon than the historic grasslands. An understanding of what regulates gas exchange and water relations at the leaf level of the two dominant tree species, Ashe juniper and live oak (Quercus virginiana P. Mill. Var. fusiformis), is important to assess the impact of juniper encroachment on the aquifer. Photosynthesis and transpiration were measured in four juniper and four oak trees throughout an entire year. Juniper consistently had lower carbon assimilation rates, transpiration, and conductance values than oak. Oak exhibited greater seasonal variation and seemed less dependent on precipitation to maintain gas exchange. Canopy position in live oak regulates leaf level photosynthesis to a higher degree than in Ashe juniper. Gas exchange of both species decline as water becomes limited, but juniper consistently exhibits lower and steadier rates throughout the year than oak. Juniper does not respond quickly to erratic precipitation events. The consistent low rates of gas exchange and stomatal responses in juniper could indicate shallower rooting structure and/or limitation of hydraulic conductivity, as well as photosynthetic capacity. The higher rates of photosynthesis, transpiration, and stomatal conductivity exhibited by live oak during drought suggest a deeper rooting pattern than Ashe juniper. Light response curves were computed for three juniper and three oak trees that were marked and sampled at three different seasons. Juniper light-compensation and light-saturation estimates, at different canopy positions, were poor indicators of differences in photosynthetic capacity between the two species. Patterns of light responses of juniper and oak did not follow responses of previous studies assessing differences in photosynthetic capacity through light-response curves. Computing the light curves was difficult due to low tree activity and lack of responses to changes in light, especially in juniper.