Above- and below-ground response by Nothofagus pumilio to climatic conditions at the transition from the steppe-forest boundary to the alpine treeline in southern Patagonia, Argentina

Background: Forest boundaries at sites of natural limitation of tree growth ('treelines') offer a valuable opportunity for studying responses and adaptation mechanisms of tree species to extreme site conditions under recent and future climate. Aim: The study investigated above- and below-ground structure and leaf and root morphology in a Nothofagus pumilio forest in south Patagonia along an elevation gradient from the natural xeric forest-steppe treeline to the alpine treeline with intervening mesic tall forest. Methods: We analysed the stands' above- and below-ground structure and leaf production, as well as leaf and root morphological and chemical traits. We also measured microclimate and soil chemical properties of the forest stands. Results: Microclimate measurements showed a remarkably high temperature lapse rate (c. 1 K 100 m(-1)) along the elevation transect. Soil moisture was most favourable in the mid-altitude tall forest and least favourable at the xeric treeline. Thermal conditions at the alpine treeline were in line with data reported to coincide with the upper treeline position globally. The changes in above- and below-ground structure from the tall forest to the treelines were similar: tree height, stem diameter, and leaf production decreased, while stem density ( number of trees per area) increased towards the treelines indicating a reduction in above- ground biomass investment. However, leaf area index was only reduced at the xeric treeline. In contrast, living and dead fine root mass, as well as the surface area of the fine root system increased markedly towards both treeline sites. Leaf and root morphology did not differ among the stands, but leaves at the treelines had higher nitrogen contents, while foliar N:P ratio decreased with elevation. Conclusions: The findings suggest that the above- and below-ground response to growth conditions at predominantly drought-affected and temperature-driven treelines can be principally similar. However, the identification of the underlying driving factors causing the marked shift in C allocation towards the root system is more obvious at the xeric than at the alpine treeline.