Climate models for the southeastern United States predict increased drought severity and frequency, with potential negative impacts on forest health and regional stream flow. The restoration of longleaf pine [Pinus palustris Mill. (Pinacea)] woodlands, historically the dominant ecosystem across the region, has been proposed as a means to mitigate drought impacts. To better understand the effects of drought on the longleaf pine ecosystem, we measured rates of water use, or sap-flux (Js), of two functional groups comprised of seven tree species (two Pinus spp. and five Quercus spp.) at two sites (mesic and xeric) during the 2016 and 2017 growing seasons. The latter part of the 2016 growing season was characterized by a severe, record-breaking drought during which our sites received < 2 mm of rainfall over 52 days. Conversely, rainfall during the same period in 2017 (~ 57 mm) was closer to average. To understand responses of pines and oaks to drought, we measured short-term effects of the drought by quantifying immediate changes in Js during the drought. We also modeled tree sap-flux response to environmental variables during growing seasons of both years to identify potential delayed effects of the drought.
Results/Conclusions
We observed strong differences between functional groups in sap-flux responses to drought, although these differences varied considerably depending on tree size class, species, and site. During the drought, pines tended to downregulate Js, especially in smaller trees (<20 cm dbh). During the same time period the following year, Js was ~ 100% higher in smaller pines. However, mature pines at the mesic site showed potential legacy-effects of the drought with Js rates 25% – 40% lower throughout the growing season following the drought. The response of Quercus species to the drought varied, but generally showed no decline in sapflux during the drought. In 2017, Js of several Quercus species was 27% - 55% higher during the early growing season, relative to 2016. Short-term drought response in these species reflects typical isohydric (Pinus) and anisohydric (Quercus) behavior observed for these tree genera in other systems, and may help inform watershed restoration priorities. However, the lowered sap-flux rates in longleaf pine following the drought also suggest that severe drought may affect pine Js for a much longer time period relative to oaks, and raises concerns about future forest health.