Marl Prairie Vegetation Response to 20th Century Hydrologic Change

Abstract

We conducted geochronologic and pollen analyses from sediment cores collected in solution holes within marl prairies of Big Cypress National Preserve to reconstruct vegetation patterns of the last few centuries and evaluate the stability and longevity of marl prairies within the greater Everglades ecosystem. Based on radiocarbon dating and pollen biostratigraphy, these cores contain sediments deposited during the last ~300 years and provide evidence for plant community composition before and after 20 century water management practices altered flow patterns throughout the Everglades. Pollen evidence indicates that pre-20 century vegetation at the sites consisted of sawgrass marshes in a peat-accumulating environment; these assemblages indicate moderate hydroperiods and water depths, comparable to those in modern sawgrass marshes of Everglades National Park. During the 20 century, vegetation changed to grassdominated marl prairies, and calcitic sediments were deposited, indicating shortening of hydroperiods and occurrence of extended dry periods at the site. These data suggest that the presence of marl prairies at these sites is a 20 century phenomenon, resulting from hydrologic changes associated with water management practices. Introduction During the 20 century, the hydrology of the greater Everglades ecosystem was altered to accommodate agricultural and urban needs, significantly altering the distribution and composition of plant and animal communities throughout the wetland (Davis and others, 1994; Light and Dineen, 1994; Lodge, 2005). Changes in both the timing and amount of water flowing through the extensive wetland system have been correlated with reduced numbers of tree islands and altered distribution and community composition of tree islands, sawgrass ridges, sloughs, and other marsh types throughout the system (Bernhardt and others, 2004; Willard and others, 2001a, 2006). Marl prairies occupy higher-elevation sites on either side of Shark River Slough (Fig. 1) and differ from most Everglades wetlands in occurrence of a calcitic substrate and short hydroperiods; these sites typically are dry for an average of nine months per year (Davis and others, 2005). The unique hydrologic and ecologic character of this habitat allows it to have the greatest plant species diversity of the Everglades, well-developed periphyton mats, and unique faunal assemblages. Concern about negative impacts of anthropogenic stressors has led to development of conceptual models to restore marl prairie habitats within an adaptive management framework (Davis and others, 2005). Central to restoration planning is determination of the pre-drainage distribution of marl prairies to predict their likely response to anticipated restoration strategies. Although marl prairie response to changes associated with the Central and South Florida Project (C&SF Project) in the mid-20 century have been documented by field studies, little is known about impacts of hydrologic changes earlier in the century, which include construction of the Tamiami Trail, Hoover Dike, and other water control structures. We designed this pilot study to determine whether proxy evidence preserved in solution holes from marl prairies yields adequate data to evaluate temporal and spatial changes in marl prairie communities, to reconstruct pre-drainage (pre-20 century) and post-drainage plant communities, and to determine whether observed ecosystem changes are correlated with hydrologic alteration of the wetland ecosystem. Marl Prairie Habitat and Community Within the ~6,000 km of wetlands comprising the greater Everglades ecosystem lies a mosaic of vegetation types, including tree-islands, mangrove forests, cypress swamps, marl