Overstory-understory relationships along flooding gradients in Everglades tree islands

Jay P. Sah, Michael S. Ross and Susana Stoffella, Institute of Environment, Florida International University, Miami, FL, Michael S. Ross, Department of Earth and Environment, Florida International University, Miami, FL, Pablo L. Ruiz, South Florida/Caribbean Network, National Park Service, Palmetto Bay, FL, Ximena Mesa, Department of Biological Sciences, Florida International University, Miami, FL

Background/Question/Methods
In a forest ecosystem, overstory structure usually has a strong influence on understory plant community composition. However, the strength of such an effect varies in space and time, resulting in distinct understory vegetation assemblages along existing environmental gradients. In the Everglades, tree islands are isolated woody vegetation patches within a matrix of marshes and wet prairies. In these islands, plant community structure and composition vary along topographic gradients, and they are impacted by periodic disturbances, such as tropical storms and fire. In our study, we addressed the question: how do canopy cover and hydrology interact to influence understory species composition and diversity along a flooding gradient? We hypothesized that, a) Variation in understory plant community composition along a hydrologic gradient will also depend on the overstory structure and composition, and b) Canopy cover (shade) influences understory species composition more in elevated portions of the topographic gradient, with shorter periods of inundation, than in areas with prolonged hydroperiod. We used vegetation composition data collected in a series of nested plots along transects in 12 tree islands and in 24 permanent plots on 16 islands to examine the overstory-understory interaction. Variance partitioning in Redundancy Analysis (RDA) followed by correlation analysis between environmental variables and the major axis of RDA were used to determine the relative importance of hydrology and canopy cover on understory vegetation composition.

Results/Conclusions
The understory vegetation composition changed along the hydrologic gradient from forbs- to graminoid-dominated herbaceous vegetation. Canopy cover and hydrology interacted in their influence on understory composition. In the elevated portion of the islands, changes in canopy cover were relatively insensitive to hydrologic variation, but were periodically influenced by disturbances. Thus, the tree cover had strong effects on understory. In the frequently flooded portions of the islands, where tree cover was minimum, herbaceous vegetation composition was mostly determined by hydrology. Spatio-temporal changes in understory vegetation in response to hydrologic variations and canopy disturbance suggest their importance in tree island dynamics. Our study has important implications for maintenance and restoration of ecologically imperiled Everglades tree islands.