Integration of demographic annual variability in a clonal desert cactus

Spatial and temporal variability influence the structure and dynamics of perennial plant populations. In order to investigate the consequences of this environmental heterogeneity on population and life history traits of a perennial plant with a complex life history, four size-classified population matrix models were employed (i.e., annual, mean, periodic, and stochastic simulations) in a clonal cactus, Opuntia rastrera, in the Chihuahuan Desert, Mexico. This species was studied over a seven-year period in two contrasting, neighboring habitats (nopalera and grassland). The specific aims were: (a) to assess the effect of annual environmental variability on the long-term dynamics of both populations; and (b) to estimate the relative contribution of sexual reproduction and clonal propagation to the populations' rates of increase. Projections from each model provided complementary information on population dynamics and life history in the two habitats. The finite rate of population growth (lambda) varied among matrix projections. Mean, periodic, and stochastic projections yielded lambda > 1 for nopalera and lambda approximate to 1 for grassland. The relative contributions of size classes and demographic processes to changes in lambda differed widely between years and habitats for annual matrices. In contrast, elasticities of periodic matrices showed a stable habitat-dependent pattern. The proportional change in lambda produced by sexual recruitment and clonal propagation showed wide spatial variation in which the most distinctive difference between habitats was the predominance of clonal recruitment at the nopalera and seedling recruitment at the grassland. Elasticities also showed temporal variation whereby clonal propagation decreased as precipitation increased, while sexual reproduction and growth tended to increase with precipitation. The striking spatial and temporal differences found in the structure, dynamics, and life history traits of O. rastrera are aptly summarized by the periodic analyses. These differences reflect both the varying influence of the selective pressures operating on this species and the ways in which demographic plasticity deals with them.