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.