The relative invariability of forest water use (i.e. evaporation or ET) across years is a well-documented phenomenon across many forest types. Interannual invariability in ET has also been observed in longleaf pine (Pinus palustris) ecosystems, despite large fluctuations in precipitation (ppt) and significant variations in soil moisture availability. Quantifying size- and species-specific patterns of transpiration (Et) of dominant trees in response to precipitation may help us determine why stand-level ET is relatively invariable compared to precipitation and other ecosystem fluxes such as carbon exchange. To better understand the apparent disconnect between ET and ppt, we measured sap-flux (Js) during the 2016 growing season in 22 trees at two sites in southwestern Georgia, USA. Selected trees represent seven of the most common tree species (two Pinus, five Quercus) found in longleaf pine ecosystems. The latter part of the 2016 growing season in this region was characterized by a severe drought in which our sites received <6 mm of precipitation in 74 days and no measureable ppt for 44 consecutive days. Thus, we focused our analysis on how Jsin these species responded to drought by comparing sap-flux between the pre-drought period and the subsequent drought period.
We observed strong size- and species-specific differences in both total water use and in Js responses to drought. Overall, Js was higher in longleaf pines than is Quercus spp. However, during the drought pines tended to downregulate Js, especially in smaller trees (i.e. ~20 cm diameter at breast height). Average Js of these smaller pine trees during the drought was 76-80% lower than the pre-drought average Js. Quercus spp. tended to maintain Js at or above pre-drought levels with some trees showing increases in Js (up to 38%) during the drought. Sap-flux in these species reflects typical isohydric (Pinus) and anisohydric (Quercus) behavior observed for these tree genera in other systems. The balance of these two water-use strategies may help explain why stand-level ET in this ecosystem has shown relatively little variation during previous droughts. Finally, these results suggest that large-scale longleaf pine restoration, which includes hardwood removal and control, may help improve ecosystem function during drought by reducing ET during periods of water scarcity.