Species differ in g1 but respond predictably in their response to soil water limitation and warming
We hypothesized that species would vary g1 in ways largely related to their individual adaptations to drought. We found that g1 did vary among species, but the responses to novel experimental conditions did not differ among them. Overall, g1 was only modestly higher in species with greater drought tolerance and the g1 of species with greater drought tolerance did not respond differently from less tolerant species to variation in VWC, in disagreement with prior reports in the literature (e.g., Gimeno et al., 2016; Héroultet al., 2013 and Zhou et al., 2013). Differences in our result versus those previously reported could be due to the extent to which drought is a primary stress in these contrasting ecosystems, interspecific trait differences that modulate the value of conservative stomatal behavior, or both. In boreal systems, although drought can occur, chronic low temperatures except for a short time window in mid-summer, and low nutrient availability, are also important, and may dampen the strength of selection for g1 in relation to drought adaptation. Additionally, trait differences and acclimation may increase some species tolerances to drought without a need of compromising water use. For example, oaks are known to develop deeper root systems (e.g., Abrams, 1990) and thus increase access to water. Oaks and maples have relatively higher wood density, compared to lower wood density species (e.g., gymnosperms),which has been associated with greater drought tolerance (Greenwood et al., 2017). Moreover, modeling work by Mrad et al., (2019) demonstrates that either aggressive or conservative behavior in water use might be related to acclimation of the rooting zone to competition for water, and little is known about such differences for our species.
Despite individualistic variation in average g1among species, there was consistent movement towards more conservative (i.e., water-saving) stomatal behavior in response to both rainfall reduction and warming, likely as a response to soil drying. This shift was consistent among species, sites, and canopy conditions, and observed across more than a decade of experimental responses. This strongly suggests a consistent acclimation by northern temperate and boreal species that would be beneficial in terms of ameliorating soil drought, but at a carbon cost. It is also unclear just how much additional soil drought this trade-off will offset, and whether those carbon costs translate into adverse impacts on growth or survival.