The role of hydrology in the distribution and composition of ground cover vegetation in subtropical cypress domes of central Florida
During the last century, the hydrology and biotic structure of many wetlands have changed profoundly. As increased effort is placed on the conservation and restoration of wetlands in response to climate change and urbanization, a thorough understanding of linkages between biotic and abiotic factors is of paramount importance. This study describes variation in both hydrology and ground cover plant communities and their interrelationships in geographically isolated, subtropical cypress domes near Tampa, Florida. Data were collected from 19 wetlands at three study locations (Cypress Creek, Green Swamp, and Starkey) over a 19 year period with hydrology being monitored on a monthly/daily basis and ground cover being sampled annually prior to the onset of the summer wet season. A distance based redundancy analysis was utilized to show the amount of annual variation in ground cover communities and to assess which hydrologic parameter(s) and lag time(s) were most correlated with these changes.
Overall, mean annual hydroperiod, mean annual water depth, and annual maximum water depth were 51% inundation (6 months), 0.16m, and 0.46m, respectively. All three hydrologic parameters varied considerably among wetlands and years. Likewise, ground cover vegetation communities varied considerably although different wetlands fluctuated in different patterns. Most wetlands displayed annual variation in the structure and dominance of the existing plant community; however, some plant communities displayed such dramatic changes that they changed to a different alternative stable state. No single hydrologic parameter was correlated with vegetation changes when all samples were investigated collectively; however, most wetlands (90%) were correlated with at least one hydrologic variable. Most often these changes were correlated with hydroperiod (63% of wetlands), followed by average water depth (53%), and maximum water depth/precipitation volume (42%). The hydrologic conditions 12 and 24 months prior to monitoring were less correlated with ground cover communities fluctuations that the hydrology 12 to 36/48 months prior to sampling. This study suggests that while cypress domes are relatively stable in canopy strata, their hydrology and ground cover communities are both extremely diverse and in a constant state of flux. As a result, ground cover species, not trees, appear to be more indicative of specific, recent hydrologic patterns and should be the focus of management efforts to describe the current conditions and potential responses to human and natural perturbations.