Preliminary growth rates and a proposed age-form classification for the Joshua Tree, Yucca brevifolia (Agavaceae)

This paper presents the first published comparative growth-rates for the Jo shua Tree, Yucca brevifolia, measured in the field in three plots in the Mo jave Desert in 1975, 1985, and 1995. Numerous variables were measured: heig ht growth-rates varied among populations, years, and age-form classes; leaf -cluster number, stem-heights, and diameters increased. Height increments a veraged about four cm/yr for all locations and time intervals. Based on our data, we predict the ultimate height and leaf-cluster number for mature pl ants at our site in the Cima Dome area. We conclude that a 20-year study of this long-lived species only begins to provide insights into its demograph y. A long-lived desert perennial requires demographic data obtained over a lon g time-period. This is the first published study to monitor population dyna mics for this species over a 20-year period. Without access to growth rings , we developed "age-form" classes to reflect stages in the species' life hi story and as one basis for comparison. Few plants died and no recruitment w as evident. The populations had a fairly constant age-form structure over t he 20-year interval.