H3: reduced rainfall and warming will have an additive effects on
g1 because the primary mechanism of both warming and
reduced rainfall effects on g1 will be via the same
pathway, of reduced VWC.
Modeling shows that plants can be differentially acclimated to low soil
moisture in ways consistent with so-called aniso- and iso-hydric
behavior (i.e., slow vs. fast decline of gs in
response to changing environemental conditions)(Mrad et al.,2019). Since plants in warmer regions tend to experience greater
evaporative demand and soil water deficits during periods of low
precipitation, we expect them to have more isohydric behavior and
conservative water use strategies and thus we hypothesize H4:that
in species adapted to either drier and/or warmer conditions
g1 will be both lower on average and less responsive to
varying VWC than in species adapted to more mesic or cooler conditions .
To test these hypotheses, we collected Anet andgs data over the span of 11 years (2009-2019) in
a warming and rainfall manipulation experiment (B4WarmED; Boreal Forest
Warming at an Ecotone in Danger, e.g., Reich et al., 2015, 2018;
Rich et al., 2015). This data set consists of 17,727 measurements
that were collected from roughly mid-June to the end of September in
each growing season during two to five independent survey campaigns each
1-2 weeks long. A minority of these data have been used in prior
publications (Reich et al., 2015, 2018), none of which examined
questions of marginal water costs of carbon gain.