CARBON-ISOTOPE DISCRIMINATION AND WATER-USE EFFICIENCY IN NATIVE PLANTS OF THE NORTH-CENTRAL ROCKIES

Stable carbon isotope composition was determined on leaves of woody pl ants sampled along an 800-km transect on the western flank of the Rock y Mountains at altitudes ranging from 610 to 2650 m above mean sea lev el. Discrimination decreased by 1.20 +/- 0.11 parts per thousand (mean +/- 1 SE) per km of altitude (n = 15, F-1,F-13 = 127.8, P < 0.0001). The change in discrimination was just sufficient to maintain a constan t CO2 partial pressure gradient from ambient air to the intercellular spaces within the leaf for both deciduous (P = 0.60) and evergreen (P = 0.90) species. However, the CO2 gradient so maintained was significa ntly steeper among evergreen (11.31 +/- 0.14 Pa) than among deciduous (9.64 +/- 0.14 Pa) species (t = 8.4, 27 df, P < 0.0001). As a conseque nce, the evergreens had lower discrimination than the deciduous specie s at any given altitude. After the data were corrected for altitude, f urther analysis revealed significant differences in discrimination and in CO2 partial pressure gradient among species. Thuja plicata (wester n red-cedar), a scale-leaved evergreen, had lowest mean discrimination (16.67 +/- 0.50 parts per thousand, n = 4) and the steepest CO2 gradi ent from ambient to intercellular spaces (14.5 +/- 0.5 Pa). Larix occi dentalis (western larch), a deciduous conifer, had the highest discrim ination (20.95 t 0.34 parts per thousand, n = 9) and the flattest CO2 gradient (8.3 +/- 0.4 Pa), A simple model of water-use efficiency pred icted that evergreen species would average 18 +/- 2% higher in water-u se efficiency at any given altitude and that mean water-use efficiency would triple across a 2000-m altitude gradient. The difference betwee n evergreen and deciduous species is attributable to variation in the CO2 partial pressure gradient, but the tripling with altitude was almo st exclusively a consequence of reduced evaporative demand.