How Changing Ecosystem Structure and Climate Alter Water Availability and its Influence on Carbon Dioxide Exchange in Semiarid Regions

Global-scale studies indicate that semiarid regions strongly regulate the long-term trend and interannual variability of the terrestrial carbon sink, but we lack understanding of how changing ecosystem structure and climate might alter the response of ecosystem carbon exchange to water availability in these areas. Therefore, we investigate the response of ecosystem carbon dioxide (CO2) exchange to changes in ecosystem composition and climate forcing using data from a network of eddy covariance sites in semiarid southern Arizona. Four of the sites are typical upland ecosystems with strong soil water limitations and varying abundance of shrubs, trees and grasses. These are contrasted with a riparian woodland site having a stable and accessible groundwater source, allowing us to investigate the competing effects of atmospheric water vapor demand and soil moisture limitation on surface conductance. Previous results suggest that evapotranspiration (ET) partitioning between transpiration and abiotic evaporation may differ across the upland sites, leading to differences in ecosystem water use efficiency (GEP/ET). Therefore, we examine how ET partitioning changes seasonally and across sites using a novel approach that employs the ET-axis intercept in a linear regression of seasonally summed ET and GEP, and we compare our approach with field measurements and a previously published method employing the high-frequency eddy covariance and leaf-level water use efficiency data. We also investigate the role of changing climate in the form of increasing atmospheric CO2 concentrations, [CO2], on water availability by looking for evidence that the 5-8% increase in [CO2] over the 12 -14 years of eddy flux measurements has altered ecosystem water use efficiency, which has been shown to be increasing in wetter, forested regions.