| Summary: | <i>Research Highlights:</i> A small, long-term decrease in the water availability in a Mediterranean holm oak forest elicited strong effects on tree stem growth, mortality, and species composition, which led to changes in the ecosystem function and service provision. <i>Background and Objectives:</i> Many forest ecosystems are increasingly challenged by stress conditions under climate change. These new environmental constraints may drive changes in species distribution and ecosystem function. <i>Materials and Methods:</i> An evergreen Mediterranean holm oak (<i>Quercus ilex</i> L.) forest was subjected to 21 consecutive years of experimental drought (performing 30% of rainfall exclusion resulted in a 15% decrease in soil moisture). The effects of the annual climatic conditions and the experimental drought on a tree and shrub basal area increment were studied, with a focus on the two most dominant species (<i>Q. ilex</i> and the tall shrub <i>Phillyrea latifolia</i> L.). <i>Results:</i> Stem growth decreased and tree mortality increased under the experimental drought conditions and in hot and dry years. These effects differed between the two dominant species: the basal area of <i>Q. ilex</i> (the current, supradominant species) was dependent on water availability and climatic conditions, whereas <i>P. latifolia</i> was more tolerant to drought and experienced increased growth rates in plots where <i>Q. ilex</i> decay rates were high. <i>Conclusions:</i> Our findings reveal that small changes in water availability drive changes in species growth, composition, and distribution, as demonstrated by the continuous and ongoing replacement of the current supradominant <i>Q. ilex</i> by the subdominant <i>P. latifolia</i>, which is better adapted to tolerate hot and dry environments. The consequences of these ecological transformations for ecosystem function and service provision to human society are discussed.
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