Response of Mid-Rotation Loblolly Pine (Pinus taeda L.) Physiology and Productivity to Sustained, Moderate Drought on the Western Edge of the Range

The productivity of the approximately 11 million ha of loblolly pine plantations in the southeastern USA could be threatened by decreased water availability in a future climate. To determine the effects of sustained drought on leaf gas exchange, whole-tree water use, and individual tree growth, we examined the response of loblolly pine trees to 100% throughfall exclusion cumulatively spanning the sixth and seventh growing seasons of a plantation in southeastern Oklahoma. Throughfall exclusion reduced volumetric soil water content for 0-12 cm soil depth from 10.8% to 4.8% and for 12-45 cm soil depth from 24.2% to 15.6%. Compared to ambient throughfall trees, leaf water potential of the throughfall exclusion trees became more negative, -0.9 MPa vs. -1.3 MPa for predawn measurements and -1.5 MPa vs. -1.9 MPa for midday measurements. Throughfall exclusion did not significantly reduce leaf gas exchange or tree water use. However, throughfall exclusion significantly reduced leaf biomass by 21% and stem volume growth by 23%. These results indicate that sustained drought may cause downward shifts in leaf quantity to conserve water rather than reducing leaf-level water use.