Carbon dioxide exchange and transpiration in species of Echinocereus (Cactaceae), as related to their distribution within the pinaleno mountains, Arizona.

1. Three species of Echinocereus are distributed over an altitudinal gradient within a southeastern Arizona mountain range; Echinocereus Fendleri (1120-1365 m), E. ledingii (1515-1970 m), and E. triglochidiatus (2180-2880 m). No overlap of species exists, suggesting that physiological mechanisms may be involved in establishing species altitudinal limits. 2. Simultaneous analysis of CO exchange and transpiration rates of representative plants showed species differences in water economy. 3. Increased nighttime transpiration along with massive dark carboxylation of atmospheric CO established the existence of mechanisms for crassulacean acid metabolism, including nocturnal stomatal opening. 4. Mean transpiration and total water loss with time (determined by weighing) increased with elevation of origin of the respective species. 5. A comparison of net assimilation to transpiration loss with time showed E. Fendleri to be the most efficient, E. ledingii somewhat less efficient, and E. triglochidiatus the least efficient in terms of net production and concurrent water economy. 6. Environmental measurements showed gradients of increasing temperatures, and decreasing precipitation and soil moisture with descending elevation, suggesting that lower elevational limits of the three species are determined according to the respective abilities of these to maintain a positive carbon balance and simultaneous favorable regulation of water loss.