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18 The success of tree recruitment in Mediterranean Quercus ilex forests is threatened by the 19 increasing intensity, duration and frequency of drought periods. Seedling germination and 20 growth are modulated by complex interactions between abiotic (microhabitat conditions) and 21 biotic factors (mycorrhiza association) that may mitigate the impacts of climate change on tree 22 recruitment. To better understand and anticipate these effects, we conducted a germination 23 experiment in a long-term precipitation reduction (PR) field experiment where we monitored 24 seedling establishment and survival, micro-environmental conditions and ectomycorrhizal 25 (ECM) colonization by different mycelia exploration types during the first year of seedling 2 26 growth. We hypothesized that (i) the PR treatment decreases seedling survival relative to the 27 control with ambient conditions, (ii) the underlying mechanisms for seedling survival are better 28 understood with detailed information of microhabitat water and light availability irrespective 29 of the PR treatment, (iii) the PR treatment will favour the development of ECM exploration 30 types with drought-resistance traits such as differentiated rhizomorphs. Contrary to our first 31 hypothesis, seedling survival was lower in control plots with overall higher soil moisture.32 Micro-site light and soil moisture conditions were better predictors of seedling survival and 33 growth than the plot-level PR treatment, confirming our second hypothesis. Furthermore, in 34 line with our third hypothesis, we found that ECM with longer extramatrical mycelia were more 35 abundant in the PR treatment plots and were positively correlated to survival, which suggests a 36 potential role of this ECM exploration type in seedling survival and recruitment. Although 37 summer drought was the main cause of seedling mortality, our study indicates that drier 38 conditions in spring can increase seedling survival, presumably through a synergistic effect of 39 drought adapted ECM species and less favourable conditions for root pathogens. 40 41 Introduction 42The success of seedling establishment is one of the most critical processes determining 43 forest regeneration dynamics and long-term persistence of tree species. In Mediterranean 44 forests, where summer drought is already a major cause of mortality of woody species during 45 early life stages [1,2], the success of tree establishment is expected to deteriorate as a 46 consequence of the ongoing climate change. Climate models for the Mediterranean region 47 predict longer and more severe drought episodes in the near future [3] and the first signs of 48 climate change impacts on forests are already documented as tree dieback and crown defoliation 49 [4,5]. As Mediterranean forests are, for the most part, naturally regenerated, a better 3 50 understanding and quantification of drought effects on tree recruitment processes is needed to 51 better mitigate the impacts of climate change on forest ecosystems. 52 The success of tree recruitment under ...

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