View article

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 oleifolius and 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 …