Carbon isotope ratios of crassulacean acid metabolism species in relation to climate and phytosociology.

The δ 13C, ‰ values of the two principal, widely distributed CAM species in Big Bend National Park, Texas, were examined and related to phytosociologic and macroclimatic parameters. Samples of Agave lecheguilla and Opuntia engelmannii were collected along an elevational gradient covering 1370 m, and δ values for both total and insoluble fractions were measured. Linear regressions were used to correlate changes in δ values with changes in community importance and mean annual precipitation for both species to determine whether or not they became increasingly C3-like in their photosynthetic mode at more mesic, upper elevations. In addition, δ values were measured for other Agave and Opuntia species from different locations and for different seasons and for A. lecheguilla following a short laboratory experiment. Results show that there are very small changes in δ values for both species along the pronounced temperature and precipitation gradients associated with elevational change. The ranges in δ values for O. engelmannii are only 1.5‰ (n=7) and 1.2‰ (n=6) for total and insoluble fractions respectively. The ranges in values for A. lecheguilla are 2.6‰ (n=9) and 3.1‰ (n=9). The results for other species confirm this low variability. We conclude that the species examined are obligate CAM species and that wide elevational distributions are not due to shifts in photosynthetic mode.