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 (A L) via canopy scorching on sap flow (Q; kg H2O h−1), transpiration per unit leaf area (E L; mm day−1), stem CO2 efflux (R stem; μmol m−2 s−1) and soil CO2 efflux (R soil; μmol m−2 s−1) over a 2-week period during early summer. R stem and Q were measured at two positions (1.3-m or BH, and base of live crown—BLC), and R soil was measured using 15 open-system chambers on each plot. E L before and after treatment was estimated using Q measured at BLC with estimates of A L before and after scorching. We expected Q to decrease in scorched trees compared with controls resulting from reduced A L. We expected R stem at BLC and BH and R soil to decrease following scorching due to reduced leaf area, which would decrease carbon supply to the stem and roots. Scorching reduced A L 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, E L increased immediately following scorching by 3.5-fold compared to control trees. Changes in E L 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, R stem 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 R soil rather than a decrease. No change in Q and increased E L following scorching indicates a substantial adjustment in stomatal conductance in scorched trees. Divergence in R stem between scorch and control trees suggests a gradual decline in stem carbohydrates following scorching. The absence of a strong R soil response is likely due to non-limiting supplies of root starch during early summer.