Environmental Productivity Indices for a Chihuahuan Desert Cam Plant, Agave Lechuguilla

Productivity of Agave lechuguilla, a commercially harvested plant that occurs over vast areas of the Chihuahuan Desert, was measured using conventional dry mass changes in the field and was predicted based on physiological responses to environmental variables in the laboratory. An environmental productivity index (EPI) was constructed as the product of indices for water status, leaf temperature, and photosynthetically active radiation (PAR). Each of these component indices was assigned a maximum value of unity when that variable was not limiting net CO₂ uptake over a 24—h period. Soil water potential, daily air and leaf temperatures, and PAR in the planes of the leaves at the field site in Coahuila, Mexico, could thus be quantitatively described in terms of their effect on net CO₂ uptake. Water status proved to be the most important of the three variables considered, nearly all leaf unfolding occurring during the relatively wet summer and early fall. The rosette habit and low leaf area index (1.23) for the plant groups monitored led to a relatively uniform and high level of PAR over the leaves. Seasonal changes in PAR proportionally affected both the modest daytime net CO₂ uptake and the predominant nighttime net CO₂ uptake for this crassulacean acid metabolism plant, while seasonal variations in temperature had relatively small effects on net CO₂ uptake over a 24—h period. EPI was highly correlated with the number of new leaves unfolding each month in the field (°² °0.83); counting unfolding leaves in a nondestructive method of estimating productivity. For the 1—yr study period EPI averaged 0.28, which led to a predicted annual dry mass gain per unit leaf area of 0.68 kg/m². Field measurements indicated that the actual dry mass gain was about half this value, the difference representing photosynthate needed for constructing and maintaining folded leaves, stem, and roots. The productivity of A. lechuguilla per unit ground area explored by its roots was 0.38 kg.m—2.yr—1, which although much less than for agricultural crops, is still much greater than the average productivity for desert ecosystems.