Agroecosystem water use in the context of global change
Ecosystem water use (i.e evapotranspiration; ET) is a complex process that is highly regulated by most vegetation types, yet also quite sensitive to changes in abiotic factors. As a result, ET is going to be affected by global change, specifically increasing CO2, temperature and O3 along with changes in humidity and precipitation. Indeed some studies suggest that changes have already occurred due to changes in CO2 and temperature since the industrial revolution. In this presentation, we will summarize the key biotic and abiotic factors that influence ecosystem water use and discuss how these factors are predicted to be altered under global change. We will summarize evidence that ecosystem water use has already been altered by global change, focusing primarily on unmanaged ecosystems. We will also review manipulative empirical studies of changes of managed ecosystem water use and water use efficiency (WUE) under carefully controlled perturbed CO2 and O3 concentrations. We will examine modeling studies that project the impacts of these changes and conclude with a discussion about knowledge gaps and feedbacks in ecosystem water use in the context of global change.
In unmanaged ecosystems, long-term isotope studies suggest that increasing CO2 has decreased ecosystem water use relative to gains in productivity, thereby increasing WUE. However, when factors such as increasing temperature and vapor pressure deficit are considered it appears the gains in WUE may have been largely offset. In managed systems, increases in CO2 from ~380 to 550 ppm caused an average decrease in ET of ~10% and increase in WUE of ~20%. In a similar experiment, increasing O3 from 40 ppb to ca 80 ppb also decreased ET, but decreased WUE. Modeling studies are able to capture some of these dynamics, and indicate that much of the increase in WUE at elevated CO2 would be lost with a concurrent ~2 °C temperature increase. Because changing ET affects the flux of moisture as well as heat into the atmosphere these changes in ecosystem water use could result in feedbacks that are not currently well characterized in modern earth system models. Future research efforts should work to incorporate these complexities to better understand their effects on regulation of climate by ecosystems.