Transpiration in recovering mixed loblolly pine and oak stands following wildfire in the Lost Pines region of Texas

Depending on severity, wildfire alters stand biomass, tree species distribution, and age, which may modify stand transpiration (E _t ) and the amount of water available to other parts of the hydrologic cycle. Our objective was to determine how wildfire severity affected E _t in mixed pine/oak (Pinus taeda L./Quercus stellata Wangehn., Quercus marilandica Muenchh.) stands in the Lost Pines eco-region (Bastrop, TX, USA). Transpiration was estimated for mature pines and oaks at unburned and moderately burned sites and oak resprouts and pine saplings at a severely burned plot. On average, mature pines had 36% greater sap flux rates (J _s ) than mature oaks in the unburned and moderately burned stands. Under low moisture stress, regenerating pines had greater J _s than resprouting oaks, but J _s quickly decreased as soil moisture declined. By contrast, mature pines were unaffected by dry periods. Pines contributed most to E _t at the unburned and moderate stands. Conversely, oak E _t dominated the severely burned stand, contributing over 95%. Transpiration was greatest at the moderately burned stand (2.02 mm day ⁻¹ ), followed by the unburned (1.44 mm day ⁻¹ ), and the severely burned stands (0.46 mm day ⁻¹ ). Despite greater J _s in resprouts and saplings, reductions in total sapwood area resulted in lower stand-level daily E _t at the severe site. Although severe fire decreased stand transpiration through reductions in vegetation density, individual oak resprouts appear to thrive, undeterred by high vapour pressure deficit. Without pine planting, oaks will likely dominate severely burned stands that could result in shifts to local hydrology and microclimate.