Growth and physiological sapling responses of eleven Quercus ilex ecotypes under identical environmental conditions

Studies with holm oak indicate that genetic variation may result in substantial differences in drought tolerance among its ecotypes. However, few trials have studied this variation under common environmental conditions. This study aimed to assess physiological and morphological responses of holm oak saplings for 11 ecotypes that represent a longitudinal transect across south-central Spain (Andalusia). Drought resistance was assessed by measuring growth, xylem water potential, chlorophyll fluorescence, and photosynthesis. Possible relationships among morphological and physiological responses across ecotypes were determined using Pearson product-moment correlations and multiple linear regressions. The response variables were used in multivariate analyses including discriminant function analysis, principal component and cluster analyses. Last, we used sparse Partial Least Squares regression (sPLS) to analyse the relationships between the morpho-physiological responses and biophysical parameters of the parent locations. Our results indicate that Q. ilex ecotypes growing in a common garden setting exhibited substantial variation in morphological and physiological traits. At the end of the growth trial (65 months post-planting), basal diameter, leaf area, and midday water potential were higher in Q. ilex ecotypes from western sites compared to eastern sites across Andalusia. PCA and clustering revealed clear morphological and physiological differentiation in response to gradients of geographical and ecological variation in ecotype origin. Variables that were related to the water regime of the ecotypes, such as seasonal precipitation and evapotranspiration, showed stronger correlations with ecotype responses. Consequently, eastern ecotypes were more likely to spread in response to projected increases in temperatures and declines in summer precipitation; however, western ecotypes would likely decrease in response to hotter and drier summers.