Throughfall Reduction Causes Small Reductions in Leaf- and Canopy-Level Physiology in a Longleaf Pine Plantation

Longleaf pine (Pinus palustris Mill.) is considered one of the most drought-resistant pines in the southeastern US and could serve as a suitable long-term carbon sink and important species for adapting southern forests to climate change. However, questions remain about the sensitivity of established longleaf pine to reduced water availability over more extended periods. In this study, we determined the effects of 40% throughfall reduction on leaf- and canopy-scale physiology in a 13-year-old longleaf pine plantation in Georgia, USA, over a period of three years (2017-2019). We found that three years of 40% throughfall reduction decreased volumetric soil water content but resulted in only small reductions in leaf-level stomatal conductance and net photosynthesis, and no changes in leaf-scale water use efficiency (instantaneous or isotopically determined). Reduced throughfall caused a similarly small reduction in maximum canopy-level stomatal conductance, but did not reduce whole-tree hydraulic conductance. Moreover, throughfall reduction had no effect on leaf- or canopy-scale stomatal sensitivity to the vapor pressure deficit (VPD). Our results indicate that established longleaf pine trees were not strongly affected by reductions in throughfall and may be relatively resistant to reductions in average rainfall. Less frequent rain and longer dry spells in the southeast US, especially during warmer months, might have stronger negative effects on tree function than changes in total annual rainfall alone.