Functional characteristics of the arborescent genus Polylepis along a latitudinal gradient in the high Andes

Polylepis is a genus restricted to the Andean Mountain Range, naturally occurring above the upper continuous forest limit. The purpose of this work was to integrate and compare functional characteristics in terms of water and carbon relations and low temperature resistance mechanisms in different Polylepis species along a latitudinal gradient. The studied species were P. sericea in Venezuela, P. tarapacana in Bolivia and P. australis in Argentina. Seasonal measurements of leaf water and osmotic potentials, stomatal conductance, CO2 assimilation and respiration rates, and injury and freezing temperatures were compared. There is a gradient, in terms of functional attributes, along the environmental range. P. tarapacana is the most tolerant species to water stress, while P. sericea avoids the less harsh conditions of its habitat through osmotic adjustments and cell wall elasticity changes. Mean CO2 assimilation rates were higher in P. australis (9 mu mol.m(-2).s(-1)) campared to P. sericea (5 mu mol.m(-2).s(-1)) and P. tarapacana (3.mu mol.m(-2).s(-1)). Mean night leaf respiration rates were similar for all species (1-2.mu mol.m(-2).s(-1)). In terms of low temperature resistance, P. sericea shows daily osmotic adjustments and a moderate supercooling capacity (-9 degrees C). The other two species rely on freezing tolerance in order to survive the more extreme low temperature conditions. The functional attributes described in this study for the different species in a wide environmental range may explain some aspects of their success along the latitudinal and altitudinal gradients.