Gas exchange and low temperature resistance in two tropical high mountain tree species from the Venezuelan Andes

Abstract

Temperature may determine altitudinal tree distribution in different ways: affecting survival through freezing temperatures or by a negative carbon balance produced by lower photosynthetic rates. We studied gas exchange and supercooling capacity in a timberline and a treeline species (Podocarpus oleifoliusand Espeletia neriifolia, respectively) in order to determine if their altitudinal limits are related to carbon balance, freezing temperature damage, or both. Leaf gas exchange, leaf temperature-net photosynthesis curves and leaf temperature at which ice formation occurred were measured at two sites along an altitudinal gradient. Mean CO2 assimilation rates for E. neriifolia were 3.4 and 1.3 µmol·m -2 ·s -1 , at 2 400 and 3 200 m, respectively. Mean night respiration was 2.2 and 0.9 µmol·m -2 ·s -1 for this species at 2 400 and 3 200 m, respectively. Mean assimilation rates for P. oleifolius were 3.8 and 2.2 µmol·m -2 ·s -1 at 2 550 and 3 200 m, respectively. Night respiration was 0.8 µmol·m -2 ·s -1 for both altitudes. E. neriifolia showed similar optimum temperatures for photosynthesis at both altitudes, while a decrease was observed in P. oleifolius. E. neriifolia and P. oleifolius presented supercooling capacities of -6.5 and -3.0 °C, respectively. For E. neriifolia, freezing resistance mechanisms are sufficient to reach higher altitudes; however, other environmental factors such as cloudiness may be affecting its carbon balance. P. oleifolius does not reach higher elevations because it does not have the freezing resistance mechanisms. © 2000 Editions scientifiques et medicales Elsevier SAS