The semiarid southwestern US has been warming in recent decades, and this region is predicted to continue to experience higher day- and nighttime temperatures. In addition to becoming warmer, the southwest is forecasted to become progressively drier and characterized by more variable precipitation patterns with longer inter-storm periods. Uncertainty in how plants and ecosystems will perform under these scenarios has renewed research interest in quantifying plant responses to changing temperatures under varying degrees of water stress in hopes of better anticipating ecosystem-scale responses to climatic change. We quantified the temperature response of the dominant vegetative components within semiarid ecosystems of differing physiographic position and vegetative cover. By repeatedly measuring CO2 uptake across a wide range of temperatures, we quantified leaf scale temperature sensitivity of various growth forms within a mosaic of semiarid ecosystems. We quantified ecosystem scale temperature sensitivity by monitoring responses to variation in temperatures by way of eddy covariance techniques. We also computed changes in leaf and ecosystem scale sensitivities due to periods of precipitation input, and we estimated the role of component fluxes in driving ecosystem-scale responses.
Results/Conclusions
We found that access to groundwater resulted in the temperature sensitivity of a riparian shrubland being nearly half that of an upland shrubland throughout all seasonal periods, emphasizing that a weaker coupling to groundwater resulted in a stronger connectivity between ecosystem performance and precipitation input. Grasses were only able to physiologically outperform woody plants within mixed-vegetation systems when precipitation was relatively abundant and shrubs were only recently established. By maintaining physiological function across a wider range of temperatures throughout periods of limited precipitation, woody plants were acquiring large amounts of carbon while grasses were limited to functioning within a narrow range of temperatures. In terms of ecosystem scale responses, a semiarid grassland was nearly twice as sensitive to temperature as a nearby woodland, and the greater access of a woodland to subsurface water allowed for a relaxation of that ecosystem’s dependence on precipitation for carbon assimilation. Ultimately, the temperature sensitivity and responsiveness to precipitation of each ecosystem behaved like its vegetative components, such that a grassland resembled the temperature sensitivity of a grass, a woodland resembled that of the tree, and a mixed-vegetation system illustrated attributes of the temperature sensitivities of both grasses and trees.
