Intraspecific variation in drought tolerance traits in a Nothofagus species (southern beech) in southern South America

Latitudinal gradients have commonly been used to evaluate intraspecific trait variation. However, studies assessing the mechanisms underlying this variability (i.e., genetic differentiation and/or phenotypic plasticity) are highly limited for tree species, especially for those from the Southern Hemisphere. The aim of this study was to evaluate intraspecific variability in functional traits related to drought tolerance and the underlying mechanisms in Nothofagus obliqua (Mirb.) Oerst. (southern beech), an ecologically and economically important tree species in Chile. We measured 11 functional traits and tree radial growth in six populations growing in a common garden and at their site of origin across a latitudinal gradient in the Andean and Coastal Cordilleras of south-central Chile. There were significant differences in most of the evaluated drought tolerance traits across the gradient, although few of them exhibited a clinal trend. From the studied populations, two located in the northern and southern limits of the Coastal Range were the most drought tolerant, probably due to restrictive site-specific conditions. Only two traits, wood vessel proportion and tree-radial growth, were genetically differentiated with the wettest population having the largest radial growth. Plasticity to local environmental conditions is likely to be the key mechanism for Nothofagus obliqua adaptation to climate change. Finally, a weak but significant trade-off between growth and safety was found across the gradient, with populations from wetter environments growing faster and being less drought-tolerant than populations from drier sites. This is the first study of this type carried out in adult trees of the temperate forests of South America. In a climate change adaptation context, our results demonstrated that assisted migration of provenances cannot be broadly recommended, as some northern populations perform poorly when planted in the south.