Ecological Applications, 25(4), 2015, pp. 1054–1071 Ó 2015 by the Ecological Society of America What determines tree mortality in dry environments? a multi-perspective approach MICHAEL DORMAN, 1,7 TAL SVORAY, 1 AVI PEREVOLOTSKY, 2 YITZHAK MOSHE, 3 AND DIMITRIOS SARRIS 4,5,6 1 Department of Geography and Environmental Development, Ben-Gurion University of the Negev, Beer-Sheva 84105 Israel 2 Department of Agronomy and Natural Resources, Agricultural Research Organization, Volcani Center, Bet Dagan 50250 Israel 3 Keren Kayemeth LeIsrael, Southern Region, Gilat Afforestation and Soil Conservation, Gilat Doar-Na Hanegev 85410 Israel 4 Faculty of Pure and Applied Sciences, Open University of Cyprus, 2252 Latsia, Nicosia, Cyprus 5 Department of Biological Sciences, University of Cyprus, 1678 Nicosia, Cyprus 6 Division of Plant Biology, Department of Biology, University of Patras, 265 00 Patras, Greece Abstract. Forest ecosystems function under increasing pressure due to global climate changes, while factors determining when and where mortality events will take place within the wider landscape are poorly understood. Observational studies are essential for documenting forest decline events, understanding their determinants, and developing sustainable management plans. A central obstacle towards achieving this goal is that mortality is often patchy across a range of spatial scales, and characterized by long-term temporal dynamics. Research must therefore integrate different methods, from several scientific disciplines, to capture as many relevant informative patterns as possible. We performed a landscape-scale assessment of mortality and its determinants in two representative Pinus halepensis planted forests from a dry environment (;300 mm), recently experiencing an unprecedented sequence of two severe drought periods. Three data sources were integrated to analyze the spatiotemporal variation in forest performance: (1) Normalized Difference Vegetation Index (NDVI) time-series, from 18 Landsat satellite images; (2) individual dead trees point-pattern, based on a high-resolution aerial photograph; and (3) Basal Area Increment (BAI) time-series, from dendrochronological sampling in three sites. Mortality risk was higher in older-aged sparse stands, on southern aspects, and on deeper soils. However, mortality was patchy across all spatial scales, and the locations of patches within ‘‘high-risk’’ areas could not be fully explained by the examined environmental factors. Moreover, the analysis of past forest performance based on NDVI and tree rings has indicated that the areas affected by each of the two recent droughts do not coincide. The association of mortality with lower tree densities did not support the notion that thinning semiarid forests will increase survival probability of the remaining trees when facing extreme drought. Unique information was obtained when merging dendrochronological and remotely sensed performance indicators, in contrast to potential bias when using a single approach. For example, dendrochronological data suggested highly resilient tree growth, since it was based only on the ‘‘surviving’’ portion of the population, thus failing to identify past demographic changes evident through remote sensing. We therefore suggest that evaluation of forest resilience should be based on several metrics, each suited for detecting transitions at a different level of organization. Key words: aerial photography; Aleppo pine; dendrochronology; drought; Landsat; Pinus halepensis; remote sensing; semiarid; tree rings. INTRODUCTION Drought-induced forest mortality Changes in climate are taking place at an increasing rate across the globe, and are likely to continue into the future (Mora et al. 2013). The uncertainty of how forest ecosystems will respond to projected climatic changes is an on-going challenge. For instance, the physiological mechanisms through which drought drives tree mortal- ity are a rapidly growing research interest (McDowell et al. 2008, 2011, Anderegg et al. 2012). However, mechanistic understanding of tree mortality processes is yet insufficient for prediction of timing, or spatial pattern, of tree die-off events (McDowell et al. 2011, 2013a). Therefore, observational studies are essential for documenting the decline events that are already underway (Allen et al. 2010), understanding their determinants, and developing forest management plans to ensure that forest ecosystems services are sustained (Anderegg et al. 2013a). Particularly, factors and processes determining when and where mortality events will take place within the wider landscape are poorly Manuscript received 12 April 2014; revised 30 July 2014; accepted 2 October 2014. Corresponding Editor: W. J. D. van Leeuwen. 7 E-mail: michael.dorman@mail.huji.ac.il 1054