Future climate change predictions for the southeastern U.S. include increases in temperature and evaporative demand, which will increase tree transpiration and soil water depletion. Longleaf pine (Pinus palustris Mill.) forests in the southern U.S. may serve as a pathway to increase the resilience of southern forests to changing climate. The objective of this research was to examine the effects of experimentally induced drought on physiological and phenological processes and productivity of longleaf pine to better understand drought adaptation and resilience. The impacts of 40% throughfall reduction (TR40) versus ambient throughfall (TR0) on leaf-level physiology, leaf water relations, tree and stand-level water use and phenological development of a 12-year-old longleaf pine plantation were studied over one year. Physiological measurements included leaf stomatal conductance (gs), net photosynthesis (Pnet), leaf water potential (ΨL), canopy stomatal conductance (GS) and stand transpiration (EG). Phenological measurements included shoot and needle elongation. Leaf-level measurements were made on upper canopy foliage approximately every three weeks. Sap flow was measured on five trees in each of the six plots for 12 months.
Results/Conclusions
Mean monthly soil moisture (5-15 cm depth) was decreased by 40% under throughfall exclusion troughs, as compared to a similar location in the TR0 treatment. Throughfall reduction decreased gs by 25% (from 60 to 48 mmol m-2 s-1), and leaf transpiration by 26% (1.3 to 1.1 mmol m-2 s-1), but had no significant effect on Pnet. Leaf water use efficiency (WUE, calculated as the ratio of Pnet to gs) was increased 15% by TR40 treatment. A trend for a reduction in mean predawn ΨL in response to TR40 treatment was observed, but midday ΨL was similar between treatments, indicating that improved WUE limited plant water stress. A significant main treatment effect was observed for mean monthly midday GS. Averaged across all months, GS was decreased 15% (from 29 to 24 mmol m-2 s-1) by TR40 treatment. Throughfall reduction significantly decreased monthly EG from July through November 2017. Preliminary analysis indicates that trees under TR40 treatment had decreased shoot elongation, but needle elongation was similar between treatments. These results indicate an impact of experimental drought on leaf and canopy-level water use of longleaf pine. Results will be used to better understand drought adaptive capacity and resilience of longleaf pine forests.