4.1 Js across species and zones
In the dry year, most species water use rates were sensitive to the topography, and their J s values increased with increasing soil water availability in the zones (Table 2). For example, growing season J s in loblolly pines in the riparian buffer was 14% higher than J s in loblolly on the mid-hillslope and 65% higher thanJ s in loblolly on the upland-hillslope. In addition, growing season J s in white oak in the riparian buffer was 12 % higher than J s in white oak on the upland-hillslope. By contrast, growing seasonJ s in tulip poplar and red maple in the riparian buffer was 12-13% lower than J s in poplar and maple on the mid-hillslope. This suggests that tulip poplar and red maple may have utilized shallow soil water in the riparian buffer early in the growing season with no relative increase in transpiration rates when compared to the mid-hillslope zone, and may have been water-stressed later in the season as there was a sharp decline and wide fluctuations in soil moisture in the buffer when compared to the mid-hillslope (Hawthorne and Miniat 2016). In addition, there was likely enough energy and soil water in the soil profile for poplar and maple to maintain water use rates on the mid-hillslope during a period when mid-hillslope soil moisture was higher than the riparian buffer.
We observed a soil moisture shift from a wet state to a dry state halfway through the 2015 growing season in the riparian buffer, and soil moisture remained in a dry state until late September (i.e., mid-hillslope moisture was higher than riparian buffer moisture from July 2015 to September 2015). This depletion of soil water in the buffer zone likely reduced J sin maple and poplar trees. This switch between wet and dry states in the buffer was not as pronounced in 2016, likely because it was a wet year. Thus, riparian buffer and mid-hillslope red mapleJ s were not significantly different and not influenced by the topography in the wet year. In addition,J s in white oak was statistically the same in the buffer and upland zones in the wet year, 96 vs. 101 g cm-2 day-1, indicating no sensitivity to zones. Hawthorne and Miniat (2016) also found variations in species-based transpiration responses to climatic variation and topographic position, and they suggest that variability in species responses to drought may lead to complex shifts in species composition.