Constraints on the establishment of plants along a fluctuating water-depthgradient

We used simulation modeling to investigate the relative importance of curre nt environmental conditions and factors affecting establishment of differen t plant species on the formation of vegetative zonation patterns. We compar ed the results from a series of six models that incorporated increasing amo unts of information about key factors affecting species' ability to adjust to water-level fluctuations. We assessed model accuracy using aerial photog raphs taken of a 10-yr field experiment. in which 10 wetlands were flooded to 1 in above normal water level for 2 yr, drawn down for or 2 yr, and refl ooded for 5 yr to three different water levels (normal, +0.3 in. +0.6 m). W e compared each model's ability to predict relative areal cover of five dom inant emergent species and to recreate the spatial structure of the landsca pe as measured by mean area of monospecific stands of vegetation and the de gree to which the species were intermixed. The simplest model predicted post-treatment species distributions using log istic regressions based on initial species distributions along the water-de pth gradient in the experimental wetlands. Subsequent models were based on germination, rhizomatous dispersal, and mortality functions implemented in each cell of a spatial grid. We tested the effect on model accuracy of incr ementally adding data on five factors that can alter the composition and di stribution of vegetative zones following a shift in environmental condition s: (1) spatial relationships between areas of suitable habitat (landscape g eometry), (2) initial spatial distribution of adults, (3) the presence of r uderal species in the seed bank, (4) the distribution of seed densities in the seed bank, and (5) differential seedling survivorship. Because replicated, long-term data are generally not available, the evaluat ion of these models represents the first experimental test, of which we are aware, of the ability of a cellular-automaton-type model to predict change s in plant species' distributions. Establishment constraints, such as recruitment from the seed bank, were mos t important during low-water periods and immediately following a change in water depth. Subsequent to a drop in water level, the most detailed models made the most accurate predictions. The accuracy of all the models converge d in 1-2 years after an increase in water level, indicating that current en vironmental conditions became more important under stable conditions than t he effects of historical recruitment events.