LAUSR

Abstract Marginal populations are usually under stressful environmental conditions and may render novel phenotypes due to intense selection resulting from evolutionary and ecological changes. Thus the study of phenotypic variation under contrasting environments and their possible genetic basis is highly relevant particularly under changing climates. In Patagonia the study of populations located at the extremes of a pronounced west-to-east precipitation gradient due the rainshadow effect of the Andes has great importance to understand potential responses to drought that have caused already massive forest decay. Nothofagus pumilio is a winter deciduous tree that dominates high-elevation forests of southern Argentina and Chile. Populations at the driest extreme of the range are considered marginal in contrast to central ones at the humid end. We compare ecophysiological traits under common gardens and responses to a manipulative water deficit experiment of greenhouse-grown N. pumilio seedlings from contrasting precipitation regimes (humid and dry, hereafter central and marginal) to analyze genetically-based and / or plastic differences. During cultivation in common gardens central seedlings outgrew marginal ones in terms of height, basal diameter, and number of leaves. In contrast, plants from marginal populations endured water stress and had higher water use efficiency and relative growth rate than central ones which in turn showed greater susceptibility to desiccation. Given that the experiment was performed under homogeneous conditions, those differences are genetic. These results suggest that water-stress related traits have a genetic basis emphasizing their importance under predicted future altered water balances and intensity of droughts in northwestern Patagonia.