Morphological and ecophysiological traits shaping altitudinal distribution of three Polylepis treeline species in the dry tropical Andes

Numerous species of the genus Polylepis form the highest treeline in the world, with striking dissimilarities in their upper altitudinal limits. The commonly accepted hypothesis is that growth at a treeline is limited by temperature. Here, using in situ records of various morphological and ecophysiological traits, we aimed to identify other factors influencing altitudinal distribution of three congeneric species from the dry tropical Andes: Polylepis rugulosa, Polylepis tarapacana and Polylepis tomentella. While P. tarapacana and P. tomentella reach their altitudinal limit at around 5000 m asl, P. rugulosa does not thrive above 4300 m, but precipitation is markedly lower in its distribution area. The three species responded to altitude by a change of morphological (e.g. decreased tree height and leaf size) and ecophysiological (e.g. decrease of transpiration rate, nutrient concentration or enrichment in the 13C isotope) traits, and this response was generally more pronounced in P. rugulosa. In comparison with P. tarapacana and P. tomentella, P. rugulosa displayed higher transpiration rates. Waxes from the abaxial (stomatous) leaf side of P. rugulosa were most strongly enriched in 13C. Furthermore, leaves of all species studied here had exceptionally low N and P concentrations. Trade-offs linked to changes in leaf area (e.g. bigger leaves, higher photosynthetic capacity but elevated transpiration) seem to drive differentiation and adaptations to altitude among these three congeneric species. We hypothesize that, while the upper distribution limit of P. tarapacana and P. tomentella is largely driven by low temperature, water is an important additional factor controlling the altitudinal distribution of P. rugulosa. Our results suggest that water stress needs to be taken into account among the factors shaping the altitudinal distribution of tropical treeline species.