Climate change is expected to cause greater stress on forests by superimposing more extreme droughts on elevated evaporative demand (VPD) due to warming. However, uncertainties about the role of VPD vs. water availability on plant function during periods of water stress are currently large. Plants are known to have a wide range of strategies to deal with drought and elevated VPD, including physiological and structural acclimation. While more extreme droughts and rising VPD could increase plant stress in the future, active acclimation may partially or completely mitigate impacts of climate change. Disentangling the relative impacts of drought and rising VPD on plant function and determining if long-term climate acclimation could influence plant water dynamics is critical both to fundamental understanding of plant function and for improved prediction of climate impacts on terrestrial ecosystems.
In a semi-arid woodland, we subjected mature piñon pine and juniper trees to multi-year precipitation reduction, atmospheric warming, and their simultaneous effects. We report here the response of tree sap flux density (FD) and analyze how acclimation in hydraulic and rooting traits to long-term drying and warming conditions influence these patterns.
This work shows that warmer conditions superimposed on more extreme precipitation regimes will likely modify the utilization of water resources for trees in the future. Stomatal acclimation to global warming is also likely to occur in response to higher VPD. Though acclimation processes seem to develop under a warming and drying climate, reductions in FD were found, suggesting that active adjustments were not sufficient to compensate for the relative decrease in resources. Furthermore, responses in FD to warming superimposed on drying were not exacerbated (i.e. similar reductions as under single stresses) suggesting no additive effects of drought and temperature rise. Overall, our results show that physiological adjustments to precipitation reduction and warming could minimize but not entirely prevent additive effects of warming and drying. Future models should incorporate the response of plants to simultaneous stresses, as adjustments may partially compensate for the additive effects of multiple climate change stresses.