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Ben-Gurion University of the Negev (2014)

Drought-induced mortality in Pinus halepensis planted forests : a study on several spatial scales

Dorman Michael

Titre : Drought-induced mortality in Pinus halepensis planted forests : a study on several spatial scales

Auteur : Dorman Michael

Etablissement de soutenance : Ben-Gurion University of the Negev

Grade : Doctor of Philosophy (PhD) 2014

introduction
Forest ecosystems function under increasing stress due to global climate changes that may lead to large-scale tree mortality. However, factors determining when and where mortality events will occur within the wider landscape are poorly understood. For example, the following aspects of forest response to drought are associated with substantial uncertainty : (a) Response to consecutive droughts along climatic gradients (b) Time-scales of climatic conditions effect on tree growth, and their spatiotemporal variation with respect to water balance (c) Local environmental influences, in particular those of competition intensity, on tree growth and mortality risk Observational studies are thus necessary for documenting forest decline events, understanding their determinants, and developing sustainable management plans. A central obstacle towards achieving these goals, however, is the fact that mortality is often patchy across a range of spatial scales and is usually characterized by long-term temporal dynamics. Research must therefore integrate various methods and sources of information, from several scientific disciplines, to capture as many relevant informative patterns as possible ; however this has rarely been done. The improvement of the present study, over previous efforts in the above-mentioned directions, was twofold. First, the forests studied here (planted Pinus halepensis in Israel) are located along a wide climatic gradient, yet comprise a relatively homogeneous man-made ecosystem (even-aged monoculture), reducing the confounding effects of species composition on drought response and isolating the climatic gradient effect. In addition, the forests have recently experienced an unprecedented sequence of drought periods, resulting in substantial mortality. Second, a multi-perspective approach was applied to provide a more comprehensive view of patterns associated with forest decline. Forest performance was simultaneously observed on three distinct scales, by integrating : (1) Normalized Difference Vegetation Index (NDVI) time-series, from satellite remote sensing ; (2) individual dead trees point-pattern, from a high-resolution aerial photograph ; and (3) Basal Area Increment (BAI) time-series, from dendrochronological sampling.

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