Satellite-derived estimates of forest leaf area index in southwest Western Australia are not tightly coupled to interannual variations in rainfall: implications for groundwater decline in a drying climate KEITH R. J. SMETTEM*, RICHARD H. WARING † , JOHN N. CALLOW ‡ , MELISSA WILSON* andQIAOZHEN MU § *Centre for Ecohydrology, School of Environmental Systems Engineering, University of Western Australia, Nedlands, WA 6009, Australia, †Department of Forest Ecosystems and Society, College of Forestry, Oregon State University, Corvalis, OR 97331-5704, USA, ‡School of Earth and Environment, University of Western Australia, Nedlands, WA 6009, Australia, §Numerical Terradynamics Simulation Group, University of Montana, Missoula, MT 59812, USA Abstract There is increasing concern that widespread forest decline could occur in regions of the world where droughts are predicted to increase in frequency and severity as a result of climate change. The average annual leaf area index (LAI) is an indicator of canopy cover and the difference between the annual maximum and minimum LAI is an indi- cator of annual leaf turnover. In this study, we analyzed satellite-derived estimates of monthly LAI across forested coastal catchments of southwest Western Australia over a 12 year period (2000–2011) that included the driest year on record for the last 60 years. We observed that over the 12 year study period, the spatial pattern of average annual satellite-derived LAI values was linearly related to mean annual rainfall. However, interannual changes to LAI in response to changes in annual rainfall were far less than expected from the long-term LAI-rainfall trend. This buffered response was investigated using a physiological growth model and attributed to availability of deep soil moisture and/or groundwater storage. The maintenance of high LAIs may be linked to a long-term decline in areal average underground water storage and diminished summer flows, with an emerging trend toward more ephemeral flow regimes. Keywords: baseflow, climate change, ecohydrology, evapotranspiration, leaf area index, water balance Received 23 January 2013 and accepted 27 February 2013 Introduction In regions of the world where droughts are predicted to increase in incidence and severity as a result of climate change, there is increasing concern over the potential for widespread forest decline (Choat et al., 2012). It has been widely reported that southern Australia is experi- encing an extended period of drought, with southwest Western Australia experiencing significant declines in rainfall since the mid-1970s (Power et al., 2005; Murphy & Timbal, 2007; Bates et al., 2008). There has been an observed decrease in both the fre- quency of daily precipitation and wet-day amounts. This decline in regional precipitation is strongly associ- ated with a marked decrease in moisture content in the lower troposphere, an increase in regionally averaged sea level pressure in the first half of the season, and intraseasonal changes in the regional north–south sea level pressure gradient (Bates et al., 2010). Runoff into Perth’s surface water supply catchments (20 km inland from the coast, directly inland from Perth, Western Australia) has decreased by 50% (since 1975) following a 16% reduction in rainfall (Silberstein et al., 2012). Across the forested water supply catchments of the Darling Plateau, declining rainfall has led to a shift from perennial to ephemeral streams (Bari & Smettem, 2004). A decrease in the annual runoff coefficient (run- off/rainfall) in the last decade suggests that a new hydrologic regime has developed with important impli- cations for future surface water supply to Perth (Petrone et al., 2010; Hughes et al., 2012). The identified synoptic influences on declining rainfall have a regional extent, so forested catchments across the entire south- west of Western Australia may also experience declin- ing rainfall and hence, declines in runoff. Modeled scenarios using input from 15 Global Climate Change Models (GCMs) are also projecting further declines in surface runoff (Silberstein et al., 2012) and lower groundwater levels for forested catchments by 2030 (Ali et al., 2012). At intermediate spatial scales (1000 km 2 ) when the dominant limitation to plant growth is water, the leaf Correspondence: Keithl R. J. Smettem, tel. +61864881692, fax +61864881015, e-mail: smettem@sese.uwa.edu.au © 2013 John Wiley & Sons Ltd 1 Global Change Biology (2013), doi: 10.1111/gcb.12223