RELATION BETWEEN MONTHLY GROWTH OF FEROCACTUS ACANTHODES AND AN ENVIRONMENTAL PRODUCTIVITY INDEX

Most productivity studies use destructive harvesting methods, prohibiting continuous plant monitoring under various environmental conditions. Here a nondestructive procedure involving the displacement of areoles from the apex was utilized to study the growth of Ferocactus acanthodes, a common barrel cactus of the Sonoran Desert. Net CO₂ uptake measured under controlled conditions of temperature, water, and photosynthetically active radiation in the laboratory was used to indicate how F. acanthodes would respond to field values of these parameters. For example, net CO₂ uptake over 24 hr was maximal at day/night air temperatures of 23 C/14 C, the mean annual values in the field, and was approximately halved at 11 C/5 C and 32 C/23 C, the monthly extreme values in the field. An environmental productivity index (EPI), constructed as the product of indices for the three environmental variables, indicated the fraction of maximal CO₂ uptake expected. The monthly production of areoles on 33 plants was highly correlated with EPI (r² = 0.81). Areole production for individual plants, however, tended to be in pulses represented by Fibonacci numbers. EPI predicted an annual stem growth of 8% compared with 9 ± 3% measured previously in the field. Thus, morphological and physiological studies can be usefully combined and applied to indicate field productivity of F. acanthodes and, by extension, of other plants.