Contrasting holm oak provenances show different field performance but similar resilience to drought events eight years after planting in a Mediterranean environment

Abstract

Diversification of genetic plant material in forest plantations is viewed as a promising strategy to promote forest adaptation and resilience to ongoing climate change. However, there is an intense debate about whether foreign populations might outperform local ones under new climatic conditions. Unfortunately, long-term experiments using contrasting provenances are scarce, especially in the Mediterranean region. In this context, the evaluation of the resistance and resilience of individuals from different provenances to drought can help to forecast their performance under different climate change scenarios, and therefore to establish guideline regarding plant material selection in reforestation projects. We evaluated the performance (survival and drought) and drought sensitivity of Quercus ilex saplings from two contrasting provenances growing during eight years in a common garden experiment. For this, we used a combination of dendroecological methods and water-relation measurements, such as leaf water potential (Ψpd), cuticular transpiration (Ec), relative water content at the point of stomatal closure (RWCc) and specific leaf area (SLA). We also compared the resilience and resistance, in terms of radial growth to the intense drought event of 2012 between saplings from both provenances. Our results suggest a lack of idiosyncratic physiological response and growth sensitivity to drought between provenances. However, saplings from the drier provenance showed a superior performance in terms of survival and growth. Survival was greater in saplings from the dry provenance (100 vs. 91 %). Mean annual basal area increment was also greater in saplings from the dry provenance (158.8 ± 13.5 vs. 96.2 ± 8.4 mm2), which resulted in greater diameter eight year after planting (47.5 ± 2.8 vs. 38.3 ± 2.3 mm). The lower values of Ψpd, Ec and SLA in the summer of the first two years after planting suggest that climatic conditions after planting rather than isolated drought events was the most critical period for the success of these saplings. In view of our results, the selection of plant material for forest plantation should be carefully evaluated in forest restoration projects, while priority should be given to those actions oriented to increase the early survival of local Q. ilex seedlings.