Symposium # 9: The Water Limitation: Issues in Plant, Community, and Ecosystem Water Use.
Water is a limiting resource in most ecological systems. The symposium will present an up-to-date overview of the fast changing field of ecological plant water relations, integrating individual plant, community and ecosystem level approaches. At the individual plant level, there have been recent highly publicized challenges to the theory of how water moves through plants (cohesion theory) and the mechanisms of how plants recover from impairment of the water transport capacity (xylem refilling). Two of the speakers will critique these challenges, pointing out that the cohesion theory still stands, but that we need more research into the mechanisms of xylem refilling under conditions where plants are transpiring. Also at the individual plant level, two speakers will present the latest interpretations of stomatal responses to hydraulic vs. chemical signals from roots. Understanding the mechanism of stomatal responses to soil and atmospheric drought increase our ability to predict and manage plant water use. The remaining speakers will address the impact of individual plant water use on community and ecosystem level responses. Hydraulic lift is an apparently widespread phenomenon that redistributes water from one soil region to another via plant roots, and it has major implications for competition and total water use of plant communities. Hydraulic constraints on stomatal conductance may play a central role in the decline of productivity with forest age. Ecosystem level studies of water use indicate the extent to which controls on whole plant water use scale up to regional patterns and also reveal new controls operating at higher levels. Overall, the symposium will provide a summary of current debates and advances in the field of ecological plant water relations for ecologists with interests ranging from physiology to ecosystem biology.