Bioclimatic Extremes Drive Forest Mortality in Southwest, Western Australia

Abstract: Extreme and persistent reductions in annual precipitation and an increase in the mean diurnal temperature range have resulted in patch scale forest mortality following the summer of 2010–2011 within the Forest study area near Perth, Western Australia. The impacts of 20 bioclimatic indicators derived from temperature, precipitation and of actual and potential evapotranspiration are quantified. We found that spatially aggregated seasonal climatologies across the study area show 2011 with an annual mean of 17.7 °C (± 5.3 °C) was 1.1 °C warmer than the mean over recent decades (1981–2011,- 16.6 °C ± 4.6 °C) and the mean has been increasing over the last decade. Compared to the same period, 2010–2011 summer maximum temperatures were 1.4 °C (31.6 °C ± 2.0 °C) higher and the annual mean diurnal temperature range (T_max−T_min) was 1.6 °C higher (14.7 °C ± 0.5 °C). In 2009, the year before the forest mortality began, annual precipitation across the study area was 69% less (301 mm ± 38 mm) than the mean of 1981–2010 (907 mm ± 69 mm). Using Système Pour l'Observation de la Terre mission 5 (SPOT-5) satellite imagery captured after the summer of 2010–2011 we map a broad scale forest mortality event across the Forested study area. This satellite-climatology based methodology provides a means of monitoring and mapping similar forest mortality events- a critical contribution to our understanding the dynamical bioclimatic drivers of forest mortality events.