资源环境科技发展态势分析平台

Plant communities arranged along a gradient are a product of underlying physico-chemical drivers that vary on both spatial and temporal scales. Spatial variation in the underlying drivers along the gradient usually results in the formation of boundaries between adjacent plant communities. However, the structure and composition of these communities may change over time resulting in boundary shifts. In the Everglades, tree islands are complex ecosystems, where plant communities are arranged along hydrologic and soil nutrient gradients. In these islands, temporal changes in hydrologic regime often result in a spatial shift in community composition along the gradient and determine the trajectory of community succession. We examined the interaction between hydrology and vegetation over a 12-year period in three southern Everglades tree islands. We hypothesized that drier conditions in recent decades would result in an increase in the dominance of flood in-tolerant woody plants over herbaceous and flood-tolerant woody species, ultimately causing a shift in the boundaries between plant communities. The boundary between adjacent communities varied from sharp, clearly defined peaks of Bray Curtis dissimilarity to more gradual, diffuse transition zones. In the head portion of tree island, there was little change in vegetation composition. However, in the tail portion of the islands, the relative abundance of flood tolerant species declined, while that of moderately flood-tolerant species increased over the study period. In these islands, the effects of relatively dry conditions in recent decades resulted in small shifts in the boundaries among communities. These results suggest that tree islands are dynamic successional communities whose expansion or contraction over time depends on the strength and duration of changes in hydrologic conditions.