Drought severity and frequency are predicted to increase in the southeastern United States due to climate change. While climate variability leads to large variations in some ecosystem processes, such as productivity, interannual variability of forest water use is typically very small. Reasons for this may include climate feedbacks, and/or a shift to understory transpiration during drought. Our objectives were to quantify responses of understory trees to climate variability and better understand the relative importance of the understory to the invariability of stand-level water use. The study was conducted at The Jones Center at Ichauway, in southwestern Georgia, USA. The site is dominated by mature Pinus palustris (Mill.) and managed with frequent (2 year return interval) prescribed fire. Fire was excluded from our specific sites for 15 years leading to establishment of native hardwoods including Diospyros virginiana L. (common persimmon), several Quercus species, and others in the understory. We measured tree responses to climate using heat balance sap-flux (Js) gauges, and compared tree responses to meteorological variables (soil moisture, solar radiation, and vapor pressure deficit) during a drought year and a year with normal rainfall. Measurements were repeated at two sites representing a soil moisture gradient (mesic vs. xeric).
Results/Conclusions
The latter part of the 2016 growing season was characterized by a severe, late growing-season drought. Our sites received <6 mm of rainfall over 72 days (versus a normal mean of ~260 mm). Conversely, 15% more rain fell during the 2017 growing season, with no prolonged dry periods. Comparing D. virginiana sap-flux data from autumn 2016 and 2017, we observed a 15% decline in Js in the xeric site and a 38% decline in the mesic site relative to vapor pressure deficit during the drought period. These data demonstrate that understory hardwoods in both sites reduced water use during drought, but hardwoods in mesic soils respond much more strongly to drought. Analysis is ongoing to scale-up tree level sap-flux to total understory water use under variable climate conditions, but these results indicate that understory trees are not compensating for loss of canopy transpiration during drought.