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.