Height Distributions of Two Species of Cacti in Relation to Rainfall, Seedling Establishment, and Growth

Abstract

In 3 populations of Ferocactus acanthodes and 2 of Carnegiea gigantea, multiple discrete peaks in the height distribution were observed, suggesting that seedling establishment was intermittent. To identify periods of establishment, the relationship between stem height and age was determined for each site, based on observed growth rates in the field, gas-exchange data and weather records. The average yearly growth for the globular F. acanthodes was relatively constant at .apprx. 9 mm yr-1, but for the club-shaped C. gigantea, it increased with age from 2 mm yr-1 in the 1st yr to 44 mm yr-1 at 13 yr. In years suitable for establishment, seedlings grow to sufficient size that stored water is not depleted by cuticular transpiration during the ensuing drought. The pattern of such suitable years over the last 3 decades correlated with the measured height distributions when the relation between stem height and age was considered. At a Sonoran Desert site, major peaks in the height distribution were centered at 0.05 m and 0.19 m, which corresponded to suitable conditions for establishment in 1976 and 1959, respectively. Rainfall records from various weather stations indicated that both species occurred where at least 10% of the years are suitable for seedling establishment.