Growth Rates of Stenocereus thurberi and Lophocereus schottii in Southern Arizona

Abstract

Annual stem and plant growth rates of Stenocereus thurberi and Lophocereus schottii in southern Arizona from 1970 through 1983 were related to plant size, age, and meteorological conditions with analysis of variance and regression analysis. In both species, size was the major factor in growth rates. Annual growth rates of individual stems decreased as stem size increased, whereas annual growth rates of individual plants increased as both plant size and stem number increased. Mean plant growth rates (±SD) were 0.07 ± 0.06 m/yr for S. thurberi <1 m tall and 0.62 ± 0.30 m/yr for plants >5 m tall. Corresponding mean plant growth rates for L. schottii were 0.06 ± 0.12 and 0.88 ± 0.67 m/yr. Age-size relationships derived from the plant size-growth models indicated that S. thurberi reached heights of 0.1 m in ca. 8 yr, 1 m in 19 yr, and 10 m in 45 yr. Lophocereus schottii attained heights of 0.1 m in 14 yr, 1 m in 27 yr, and 10 m in 42 yr. Interannual variation in mean growth rates of all plants in each sample was strongly related to both winter precipitation and freeze frequency. For both species, mean growth rates tended to be greater in growing seasons after unusually wet winters and lower after relatively dry winters. Freeze frequency was most strongly related to the mean growth rate the following summer in S. thurberi and to the mean growth rate two summers later in L. schottii. The more prolonged effect of severe winters on L. schottii growth rates resulted from the greater sensitivity of this species to subfreezing temperatures and its occurrence on flat terrain often affected by cold air drainage in the study area.