Forests in the southern Appalachians are moving toward a novel ecosystem state, characterized by rising temperatures, increasing variability in the timing and intensity of precipitation events, and altered stand structure. These changes are expected to affect ecological processes and ecosystem services, such as surface water supply changes to the southeastern US from these headwater catchments. Currently, hydrologic models are derived from a narrow range of forest age, structure and species composition with limited environmental variability; and may not be relevant to future forests. Our objective was to quantify the sensitivity of transpiration to environmental drivers among dominant canopy species of varying age and height.
We measured sap flow during the growing seasons from 2013 to 2015 using heat ratio and thermal dissipation sap flow probes in forest stands of 10, 35, 85, and 200 years of age located within the Nantahala National Forest of western North Carolina. At each site, trees representing the dominant canopy species were sampled, including Liriodendron tulipifera, Betula lenta, Acer rubrum, Quercus spp. and Carya spp. At each site we measured open-field precipitation and PPFD, and vapor pressure deficit (D), and within each stand we measured D and soil moisture.
With increasing stand age, up to 35-yrs, trees were taller and had greater basal area; after 35-yrs basal area (BA) stabilized at ca. 30 m ha-2. Among sites, Liriodendron was most dominant in the 35-year old stands (64% of BA), whereas Quercus were most abundant in the oldest stand (58% of BA). Daily sap flow increased with D, for some species more than others. During typical mid-July conditions when D was high, daily sap flux was ~85% higher in Liriodendron than in Quercus, 145 versus 78 g H2O cm-2 day-1 sapwood area, respectively. Additionally, species with ring-porous xylem anatomy, such as Quercus, had less sapwood area per unit BA than diffuse porous species. Furthermore, based on increasing trends of species with diffuse porous xylem anatomy in this region, we anticipate that future forests may use more water than current forests, potentially reducing streamflow. However, since many of these diffuse porous species strongly regulate leaf water loss through stomatal closure, we also expect forests to be more sensitive to dry air or soil. Future analyses will compare tree-level water use responses to Dand soil moisture, which can independently affect stomatal closure.