Adaptive vs. plastic ecophysiological responses in dry montane habitats: Nothofagus pumilio, the tree that took over the Southern Andes

Abstract

Mountain areas impose variable selection pressures on plant species in relation with heterogeneous conditions of the physical environment along altitudinal gradients. These together with the regional climate influence widespread species inhabiting such gradients which may develop complex ecophysiological responses. Nothofagus pumilio is a deciduous native forest species with the largest distributional range in Patagonia. It is the dominant species of high-elevation forests where most often occurs in pure stands. At its northern range, inhabits climates of Mediterranean regime, with wet winters and dry summers. We reviewed a series of previous studies on dry forests of N. pumilio at its northern range to provide a synthesis on ecophysiological responses under natural conditions in the field, common gardens, and reciprocal transplants. We evaluated the extent to which ecophysiological and functional traits of N. pumilio result from distinct selection pressures with elevation by means of plasticity or genetically- based adaptation. Photosynthetic rates were higher in plants from the upper altitudinal limit under field conditions which were maintained in the common garden. This suggests a genetic control on net photosynthesis and also that no shortage for carbon assimilation exist at high elevation. Therefore, photosynthetic responses and morphological traits are probably related to nitrogen economy and a shorter growing season at high elevations. In contrast, conductance and stomatal density showed plastic responses which will be advantageous for a deciduous species like N. pumilio given that the growing season coincides with drought. Additionally, plants from contrasting elevations had significant differences in terms of architectural features of individuals, as well as leaf morphology and phenology under the common gardens suggesting strong genetic control. Reciprocal transplants between contrasting elevations indicated that plants of low-elevation origin, which in turn were the most genetically diverse by molecular markers, outgrew high-elevation ones. Bioclimatic data showed that drought and high temperatures result in limited growth and more profused branching. These results suggest that ecophysiological characteristics in N. pumilio combine genetic and plastic responses. Nevertheless, genetically fixed traits will probably limit adjustments particularly of high-elevation plants under changing conditions. On the other hand, plasticity in combination with greater genetic variation of low-elevation plants may favor their performance and thus they may ascend in elevation under warmer climates.