Eco-physiological Differences between High and Low Elevation CAM Species in Big Bend National Park, Texas

Abstract

There are distinct high and low elevation assemblages of crassulacean acid metabolism (CAM) plants in Big Bend National Park, Texas. The elevational distribution of the CAM photosynthetic pathway community importance is bimodal; while the pathway is most dominant in low elevation desert scrub communities, it is also important in high elevation woodlands. The principal members of these groups (A gave lecheguilla at low and Opuntia engelmannii at high elevations) were examined in both the laboratory and the field to describe differences in gas exchange patterns between the two assemblages. Primary production potential, expressed as CO2 assimilation capacity and diurnal acidification, and water economy, expressed as transpiration and transpiration ratio (TR), were examined. Low elevation CAM species have lower CO2 assimilation potential but have more efficient water utilization and conservation mechanisms, while high elevation CAM species have higher CO2 assimilation potential but less efficient water conservation. These differences are discussed in light of interspecific competitive ability with plants using alternate photosynthetic pathways (C3 and C4), macroclimatic differences along the elevational gradient and basic distributional patterns.