Transpiration is affected by a number of environmental variables including radiation, temperature, humidity, wind, and soil moisture. Understanding how and when these variables affect transpiration, and how they interact through time and space, is important for understanding both plant and whole-ecosystem responses to extreme weather. We compared tree water use during three events that spanned an extremely broad range of spatial scales. These events included a stand-level (~130 ha) prescribed fire, multiple hurricanes that broadly affected much of the southeastern U.S. either directly or indirectly, and a solar eclipse that affected much of the continental United States from Oregon to Georgia. Our measurements were taken in a mature, second-growth longleaf pine (Pinus palustris) woodland in southwestern Georgia managed with frequent prescribed fire. Focal species included two Pinus species and five Quercus species. We used thermal dissipation probes to measure sap-flux at either 1-minute (fire and eclipse) or 15-minute (hurricane) intervals. Sap-flux data were coupled with data from a local climate station to help explain the temporal patterns in tree-level transpiration.
Results/Conclusions
We found effects on tree-level transpiration from all three events, but the effect size varied greatly. The smallest effect was during the prescribed fire. We observed elevated sap-flux for less than five minutes as the flames moved through the stand. We observed larger effects during the solar eclipse. A >90% decrease in radiation, a 2 °C drop in air temperature, and a 46% decrease in vapor pressure deficit caused a proportional (to VPD) decline in transpiration. We observed the greatest effects from Hurricane Harvey when the storm was centered ~1200 km from the study site. Wind direction across the region shifted direction due to the extreme low pressure in the storm. Northerly winds brought in dry air atypical of summers at the study site where prevailing southerly winds generally bring in humid air from the Gulf of Mexico. This change increased daily transpiration significantly for three days. While this analysis was primarily a data exploration exercise, these results show somewhat surprising effects of these events. From a practical perspective, we show that a single prescribed fire has minimal effect on tree water use; however, this system is very sensitive to changes in humidity from shifts in wind direction.