25 An increase in temperature and water deficits caused by the ongoing climate change 26 might lead to a decline growth rates and threaten the persistence of tree species in 27 drought-prone areas within the Mediterranean Basin. Developmental instability (the 28 error in development caused by stress) may provide an index of the adaptability of 29 woody plants to withstand climatic stressors such as water shortage. This study 30 evaluated the effects of drought stress on growth variables in

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... stands of a 31 Mediterranean oak (Quercus faginea) exposed to differing climatic conditions (xeric, 32 mesic and cooler) along an altitudinal gradient in northeastern Spain, in two climatically 33 contrasting years (wet and dry years). Two indices of developmental instability, 34 fluctuating and translational asymmetries, which reflect environmental stress, were 35 measured in leaves and current-year shoots respectively. We also measured branch 36 biomass and fractal complexity of branches as indicators of the species' performance. 37 After a period of drought the individuals' at the most xeric site presented lower 38 developmental instability and less branch biomass than did the individuals from the 39 mesic and cooler sites. We interpret that difference as an adaptive response to drought 40 which reflects a trade-off between maintenance of homeostasis and growth when water 41 is scarce. The study demonstrated that developmental instability constitutes a useful 42 index to assess the degree of adaptation to stressful environmental conditions. The 43 assessment of developmental instability in sites and years with contrasting climatic 44 conditions provides a means of quantifying the capacity of plants to develop plastic 45 adaptive responses to climatic stress. 46 47