4.2 Js respond to VPD
Although J s in all species were related to VDP (Table 3), it is worth noting that the relationships between growing season VPD and J s in 2015 in the riparian buffer were stronger than in 2016 (i.e., higher r2 values), likely because the soils in the watershed were drier and the evaporative potential was significantly higher in 2015 than in 2016 (Domec et al., 2009). Growing season VPD in 2015 was significantly higher (by 5%) than the long-term VPD average for the watershed, and growing season VPD in 2016 was significantly lower (by 10%) than the long-term average (Boggs et al., 2016). The variation suggests that VPD likely compensated for any potential drought-induced reduction in J s in 2015 for some species in this study, as would be supported by Oishi et al., (2010).
We found the interaction between VPD and zone was significantly related to Js in loblolly pine, sweetgum, and Virginia pine but not related in red maple, tulip poplar, and white oak (Table 5). Emanuel et al. (2010) reported that water stress conditions could decouple species J s from VPD and become increasingly more dependent on soil moisture. Species specific transpiration responds differently to soil moisture depletion for several reasons including deep-water access even when shallow soil water is available (Oren and Pataki 2001; Lanning et al., 2020).