Linking the Water and Carbon Economies of Plants in a Drying and Warming Climate

Purpose of Review Harsher abiotic conditions are projected for many woodland areas, especially in already arid and semi-arid climates such as the Southwestern USA. Stomatal regulation of their aperture is one of the ways plants cope with drought. Interestingly, the dominant species in the Southwest USA, like in many other ecosystems, have different stomatal behaviors to regulate water loss ranging from isohydric (e.g., piñon pine) to anisohydric (e.g., juniper) conditions suggesting a possible niche separation or different but comparable strategies of coping with stress. The relatively isohydric piñon pine is usually presumed to be more sensitive to drought or less desiccation tolerant compared to the anisohydric juniper although both species close their stomata under drought to avoid hydraulic failure, and the mortality of one species (mostly piñon) over the other in the recent droughts can be attributed to insect outbreaks rather than drought sensitivity alone. Furthermore, no clear evidence exists demonstrating that iso- or anisohydric strategy increases water use efficiency over the other consistently. How these different stomatal regulatory tactics enable woody species to withstand harsh abiotic conditions remains a subject of inquiry to be covered in this review. Recent Findings This contribution reviews and explores the use of simplified stomatal optimization theories to assess how photosynthesis and transpiration respond to warming (H), drought (D), and combined warming and drought (H+D) for isohydric and anisohydric woody plants experiencing the same abiotic stressors. It sheds light on how simplified stomatal optimization theories can separate between photosynthetic and hydraulic acclimation due to abiotic stressors and how the interactive effects of H+D versus H or D alone can be incorporated into future climate models. Summary The work here demonstrates how field data can be bridged to simplified optimality principles so as to explore the effect of future changes in temperature and in soil water content on the acclimation of tree species with distinct water use strategies. The results show that the deviations between measurements and predictions from the simplified optimality principle can explain different species’ acclimation behaviors.