干旱区生态环境知识资源中心

Most of the grassland communities in arid and semiarid regions of the world have changed their structure in recent times. The even distribution of grassy species is now interrupted by clusters of woody or succulent species. Where this two-phase mosaic prevails, interspecific competition within the cluster is an important biotic process structuring these plant communities. In the southern Chihuahuan Desert of Mexico, we studied shrub encroachment using two different methods; as an individual-based process using plant association dynamic techniques and, as a continuum of species-by-species replacement occurring at the landscape level and inferred from a principal component analysis of species cover. At the individual scale, the competitive process was measures in the canopies of woody and succulent species as a function of their symmetry and vigor. From these data, we inferred a replacement order of species in these clusters, in which taller and denser canopied species replace over time those with shorter and more open canopies. in our study, larrea tridentata or Prosopis spp. invade open areas in the grassland. next Acacia schaffneri replaces mesquite and serves as a focal point for the establishment of arborescent opuntia spp. Finally, Yucca spp. enter the system, replacing any of the previously established species. This replacement process occurs within a patchy environment anf is driven by non-equilibrium dynamics, causing co-existence of the species temporally in the form of a cyclical succession. in comparison, the multivariate analysis of species cover shows a similar pattern to that found using plant association dynamic techniques. However, these analyses are static and it is not possible to infer from them the causal processes leading to the dynamic structure of the plant community. in addition, they tend to emphasize the correlative role of abiotic interactions rather than biotic interactions, such as interspecific competition, as possible mechanisms leading to community structure.