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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 (λ) varied among matrix projections. Mean, periodic, and stochastic projections yielded λ > 1 for nopalera and λ ≈ 1 for grassland. The relative contributions of size classes and demographic processes to changes in λ 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 λ 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.

Abstract. Using size‐distance data we tested the intensity and importance of competition between Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a succulent) in the Chihuahuan desert. We also compared the vertical and horizontal distribution of roots to assess the potential degree of overlap in the use of soil resources. The relationships between sizes and distances of nearest‐neighbour plants suggested that intraspecific competition is generally more important than interspecific competition. However, evidence of stronger inter than intraspecific competition was found in some cases. Species combinations showing significant interspecific competition involved always Opuntia, whereas Larrea and Hilaria did not influence each other. The analysis of the symmetry of competition showed that Opuntia was adversely affected by the presence of Hilaria or Larrea. Although differences were found in the distribution of roots, the results of the size‐distance study support the idea that, (particularly) Opuntia, below‐ground niche differentiation is not sufficiently important to totally avoid the negative effects of plant competition.

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Two field experiments were conducted using three dominant perennial species of the Chihuahuan Desert: Hilaria mutica (a tussock grass), Larrea tridentata (a microphyllous shrub) and Opuntia rastrera (a flat-stemmed succulent cactus). Two hypotheses concerning competition in arid plant communities were tested. (1) Marked resource partitioning with no interspecific competition could be expected since the three species belong to different life-forms, and that plant growth in deserts is basically limited by harsh environmental conditions. (2) Alternatively, resource scarcity (particularly water) will result in strong plant competition. In a 1-year removal experiment, water status and plant growth of the three species were monitored in twelve 10 m × 10 m plots randomized in three blocks and assigned to the following treatments: (a) removal of all species, except H. mutica; (b) removal of all species, except L. tridentata; (c) removal of all species, except O. rastrera, and (d) control without any manipulation. In a watering experiment, under two neighbourhood conditions (growing isolated or in associations of plants of the three species in plots of 20 m), the water status of the three species and the growth of H.mutica and L.tridentata were studied for 32 days after an irrigation equivalent to 30 mm of rain, similar to a strong storm event at the site. In the removal experiment, where plants were free to capture water, no evidence of competition was observed. However, during the watering experiment, in which water was forced into the soil, competitive effects were observed. Associated individuals of L. tridentata had lower xylem water potentials and osmotic potentials (OPs) and produced shorter twigs and less leaves and nodes. Although less pronounced, neighbours also had a negative effect on the OP in O. rastrera. According to these results, the intensity of the interspecific competition for water seems to depend on the level of resource availability in the soil. Thus, the validity of the two hypotheses tested in this study also depends on the level of resources. Competition could be absent or very low in years of low precipitation, as in the year of this study (173 mm against a 25-year average of 264 mm). However, when soil water availability is high, e.g. following heavy rain, the negative interactions between species could be more intense.

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