Ecosystem Resilience to the Millennium Drought in southeast Australia (2001-2009): Possible Mechanisms and Implications for Water Resources Management

Abstract:

The Millennium Drought was the worst drought on record for southeast Australia. Several previous studies have revealed that the ecosystem greenness during the major growing season was not strongly influenced by the water deficit in this multi-year drought. In this study, we explore the possible mechanisms of this ecosystem resilience to the severe drought by integrating optical satellite-observed leaf area index (LAI) data, microwave satellite-observed vegetation optical depth (VOD) data, and the process-based ecohydrological model (Ecohydro-SiB: e.g., Sawada et al. [2014]). During the beginning of the 21st century, the microwave VOD, which can penetrate the canopy to some extent, significantly decreases in southeast Australia while the LAI observation shows no such response, which implies that vegetation lost a lot of water stored in the tissue of whole aboveground biomass while they did not greatly lose the photosynthetically active part of biomass. This observation suggests that the carbon allocation fraction for leaf biomass greatly increases under the severe droughts to maintain the LAI although the total amount of aboveground vegetation water content (and aboveground biomass) is decreased. We demonstrate that this hypothesis is consistent to the result of our ecohydrological model applications. Although our modeled LAI significantly decreases in the Millennium Drought when we use the default model parameter that is widely used by the dynamic vegetation modeling communities, the model can reproduce the observed resilience when we chose the model parameter which makes the carbon allocation more sensitive to the light and water availability. In addition, we found that a CO2 fertilization effect, which induces the increase of water use efficiency, cannot fully explain the satellite-observed ecosystem resilience according to our model sensitivity study. The effect of this ecosystem resilience on our available water resources is also discussed by analyzing our simulated runoff.