Discussion
More than a decade of measurements documented generally consistent ways that novel environmental conditions associated with climate change influenced the trade-off of water loss vs. carbon gain among 21 boreal and temperate species Overall, g1 decreased in response to both reduced rainfall and warming, driven largely by stomatal responses to soil drying in both cases, effectively increasing the water use efficiency of plants by maintaining stomata less open (H1, H2). The direction of these responses to experimental manipulations was uniform across all species despite differences in averageg1 associated with species-specific adaptations (i.e., drought tolerance) and associations (i.e., climate of origin or phylogeny) (H4). We also found that the combination of warming plus reduced rainfall (H3) had an additive effect. Moreover, the warming and reduced rainfall effects were consistent across years, sites, and species (Figure 1, 5) providing strong support for these responses as a general prediction. These results suggest that projected future warming and reduced summer rainfall will likely move boreal and temperature species to a more conservative water spending stomatal behavior; likely helping plants ameliorate drought stress but at a carbon cost.
The unified optimization theory predicts g1 to have a small increase in response to warming, largely because it is related to Γ* (Medlyn et al., 2011), which is dependent on temperature (Bernacchi et al., 2001). Hence, a neutral or near neutral effect of temperature ong1 was found previously ( Nijs et al. , 1997; Duursma et al., 2013; or Gimeno et al., 2016). Our results support that prior work but only when water was abundant (Table 5 and Figure 4). This is firm evidence that the weak neutral to positive direct effect of temperature on g1 was overwhelmed by the stronger effect warming had on water limitation, through an increase in evapotranspiration demand (Seager et al.,2014; Wang et al., 2014); Reich et al., 2018).