Assessment of Forest Resistance and Resilience to the Millennium Drought in Australia

The resistance and resilience of forest ecosystems to drought extremes are important for predicting ecosystem dynamics and their carbon cycle feedbacks to climate change. Compared to manipulated drought experiments at local scales, remote sensing enables drought assessment over large areas at various resolutions. In addition, multi-source remote sensing data including optical, microwave, and Lidar improve the monitoring of forest dynamics by offering diverse biochemical and biophysical information but these data have seldom been used altogether to assess the resilience and resistance of forests to extreme drought. The recent climate extreme event known as the Millennium Drought (MD) in Australia provides an opportunity to examine forest resistance and resilience to extreme drought. This study employed multi-source remote sensing datasets and an event-based drought assessment method to explore cumulative effects along long or repeat drought exposure and to quantify post-drought recovery in Australian forest ecosystems. Key vegetation properties used here include fraction of photosynthetically absorbed radiation (FPAR, AVHRR), photosynthetic vegetation cover (PVC, MODIS), canopy optical density (Vegetation Optical Depth), upper canopy water content and structure (QuikSCAT backscatter), and aboveground biomass carbon (ABC). Results show that the frequency of cumulative effects as measured with ABC was highest in Eucalypt Woodlands but lower in Acacia Open Woodlands, which showed higher frequency of cumulative effects in FAPR, PVC, and canopy density. In addition, Eucalypt Woodlands and Eucalypt Open Woodlands generally required longer time period to reach full recovery and had a higher frequency of persistent decline than Acacia Forests and Woodlands and Acacia Open Woodlands. The low resistance and resilience of Eucalypt Woodlands to drought extremes implies a high risk of drought mortality in these forests and carbon release in response to future drought.