Transient changes in transpiration, and stem and soil CO2 efflux in longleaf pine (Pinus palustris Mill.) following fire-induced leaf area reduction

In 20-year-old longleaf pine, we examined short-term effects of reduced live leaf area (AL) via canopy scorching on sap flow (Q; kg H2O h−1), transpiration per unit leaf area (EL; mm day−1), stem CO2 efflux (Rstem; μmol m−2 s−1) and soil CO2 efflux (Rsoil; μmol m−2 s−1) over a 2-week period during early summer. Rstem and Q were measured at two positions (1.3-m or BH, and base of live crown—BLC), and Rsoil was measured using 15 open-system chambers on each plot. EL before and after treatment was estimated using Q measured at BLC with estimates of AL before and after scorching. We expected Q to decrease in scorched trees compared with controls resulting from reduced AL. We expected Rstem at BLC and BH and Rsoil to decrease following scorching due to reduced leaf area, which would decrease carbon supply to the stem and roots. Scorching reduced AL by 77%. Prior to scorching, Q at BH was similar between scorch and control trees. Following scorching, Q was not different between control and scorch trees; however, EL increased immediately following scorching by 3.5-fold compared to control trees. Changes in EL in scorched trees corresponded well with changes in VPD (D), whereas control trees appeared more decoupled over the 5-day period following treatment. By the end of the study, Rstem decreased to 15–25% in scorched trees at both stem positions compared to control trees. Last, we found that scorching resulted in a delayed and temporary increase in Rsoil rather than a decrease. No change in Q and increased EL following scorching indicates a substantial adjustment in stomatal conductance in scorched trees. Divergence in Rstem between scorch and control trees suggests a gradual decline in stem carbohydrates following scorching. The absence of a strong Rsoil response is likely due to non-limiting supplies of root starch during early summer.