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Abstract:
The anthropogenic climate change is producing a reduction of the precipitation and a change in its regime in regions such as the Mediterranean Basin, which will be intensified during the next decades. Warmer temperatures are amplifying the effects of droughts by enhancing soil evaporation and increasing the atmospheric evaporative demand. In recent years, there has been an increase in reports of forest declines related to drought. They involved widespread crown defoliation and mortality and were usually related to a single extreme drought episode. However, other large-scale changes in forest structure and composition have been linked to the progressive reduction of water availability. Co-occurring woody species may show contrasting drought sensitivity, so increasing drought may produce shifts in species composition and deep modifications of ecosystems’ structure and function. Therefore, understanding species-specific water-use strategies may help elucidate which physiological and/or morphological traits play a key role in determining the effects of droughts on forests. In this sense, drought experiments are a useful tool, but they are usually of short duration while woody species are long-lived organisms. Changes in plant communities occurred on the short-term may not be representative of the future performance of individuals and communities. For this reason, long-term experiments can provide better insights on the dynamics of forests. In this thesis, I examined the effect of long-term (13 years) experimental and natural droughts on holm oak (Quercus ilex) forests. I studied how drought affects the movement of water from belowground to the atmosphere at the individual level in co-occurring species with different drought sensitivity. At the stand-level, I evaluated whether the effect of experimental drought on growth and mortality rates changes over time and after extreme drought events. I also performed a quantitative synthesis on the use of groundwater by plants as estimated with stable water isotopes. The results of this thesis confirm that co-occurring species present contrasting water-use strategies; the tree Quercus ilex may be out-competed by the tall shrub Phillyrea latifolia in the driest sites since the latter transpires larger amounts of water during dry periods. Indeed, the growth and mortality rates of P. latifolia were not affected by experimental drought while Q. ilex showed higher mortality rates and both Q. ilex and A. unedo showed lower growth rates. However, I report that the effect of the experimental drought is reduced over time. Relatively higher mortality of stems in the drought plots reduce competition between stems and offsets the effect of treatment. The experiment also produced changes in the plant water sources that contribute to dampen the effect of the drought treatment. I also reviewed the sequence of drought responses of holm oak forests at different time and organizational scales. Q. ilex is a species with mechanisms and traits that allow it to survive and recover from droughts, but extremely long droughts produce on it extensive crown damage. It can still resprout vigorously after such episodes and can acclimate to persistent drier conditions, but it is unclear whether more recurrent extreme droughts can affect its resilience. Finally, I report the widespread occurrence of groundwater uptake by plants. Groundwater is critical regardless of the landscape position, climate and plant anatomy, although plants in riparian ecosystems and dry climates use relatively more groundwater. Summing up, tree individuals and forests have mechanisms and processes related to water use that help them acclimate to drier conditions over time. Nonetheless, it has to be elucidated to which extent this acclimation can offset the negative effects of increasing drought.
