Markovian Dynamics of Simple and Complex Desert Plant Communities

Abstract

Many simple plant communities in extremely arid regions of the Sonoran Desert [Arizona, USA] consist essentially of two shrub species, Ambrosia dumosa and Larrea tridentata. These species exhibit completely different recruitment patterns and rates of population turnover. In these communities, the majority of space is devoid of perennial plant cover, and Ambrosia readily colonize this open space. Reruitment of Larrea occurs less frequently and is largely limited to areas beneath the canopies of large, older Ambrosia indviduals. The dynamics of these systems can be modeled as Markov chains. Probabilities of transitions among states (open space, space covered by Ambrosia canopies, and space covered by Larrea canopies) are computed from data on spatial and temporal patterns of recruitment and mortality. Although the stationary distributions of states predicted by a homogeneous Markov chain are similar to those observed in undisturbed vegetation at two sites, a nonhomogeneous chain more accurately describes vegetational change following disturbance. The dynamics of species-rich plant communities in less xeric parts of the Sonoran Desert appear to be qualitatively similar. One shrub species in particular, A. deltoidea, is capable of colonizing open space, whereas for most other shrub, tree, and succulent species, recruitment is largely limited to areas beneath canopies of other plants, especially canopies of A. deltoidea. Although seemingly more complex, a digraph model showing the transitions among states possesses the same fundamental characteristics as does a digraph of the simple two-species system. A diverse set of mechanisms underlie these dynamics and include, in part, factors that influence seed distributions, germination success, and patterns of post-germination mortality. Even in the simple two-species systems, it is likely that no individual mechanism such as competition can be singled out as the predominant determinant of dynamics and, hence, of the patterns of community structure.