Department Seminar: Lucas Vargas Zeppetello

Title: Why do ecosystems have optimum temperatures?

Abstract: The observed relationships between temperature, vapor pressure deficit, and gross primary productivity at the ecosystem scale are known to be non-linear; productivity increases with both temperature and vapor pressure deficit up to a critical threshold known as the optimum, above which it declines dramatically. In this talk, I will present a new hypothesis that explains ecosystem optimum values for temperature and vapor pressure deficit by considering the joint influence of water stress on both plant biology and energy exchange between Earth's land surface and the overlying atmosphere without incorporating any direct temperature or humidity stress on photosynthesis or transpiration. Using eddy covariance observations, idealized theory, and a model that includes both a biochemical representation of photosynthesis and a physical representation of land-atmosphere coupling, I demonstrate that neither biotic temperature stress nor stomatal closure induced by high vapor pressure deficit are required to explain the existence of ecosystem optimum values for temperature or vapor pressure deficit; the influence of ecosystem-scale water stress on stomatal closure is sufficient to explain the observed behavior.