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).