RESPONSE OF EVERGLADES WETLANDS TO 20TH CENTURY WATER MANAGEMENT – IMPLICATIONS FOR FUTURE EVERGLADES PLANT COMMUNITIES

A fundamental question in Everglades restoration is the susceptibility of wetland vegetation to changes in water quantity and quality. Specifically, we need to determine how much change the ecosystem can tolerate and whether the system can return to a pre-development state once hydrologic conditions are restored. We explored these questions using pollen, geochronological, and geochemical evidence from sediment cores collected in sawgrass marshes, sloughs, and tree islands in the greater Everglades ecosystem.

Marsh cores indicate that water depths were greater and hydroperiods were longer before the 20th century. The typical pattern is a greater abundance of slough vegetation (indicating long hydroperiod) in older parts of the core and increased abundance of weedy species and sawgrass (shorter hydroperiod) since 1900 AD. The impact of natural climatic fluctuations over the past few thousand years also is evident in these cores, illustrating significant vegetational changes in response to decadal- to centennial-scale dry intervals followed by a return to wet conditions.

Tree-island communities matured over the past 500-3,000 years on sites drier than the surrounding marsh. These communities were fairly stable until the 20th century. After canal construction diverted water from sheet flow in the early part of the century, tree islands in the southern Everglades expanded. Further compartmentalization of the system in the mid-20th century had even greater impacts. In the northernmost Everglades (Loxahatchee NWR), species composition on strand islands changed markedly. In Water Conservation Area 2A, extremely high water levels were maintained for over a decade, resulting in loss of many trees and tree islands in that area. Such changes were unprecedented in the history of the tree islands, and their response to restored hydrology will vary depending on their location in the Everglades.

Past responses of the greater Everglades ecosystem provide a guide to the ability of the system to recover from past perturbations. In light of the fragmentation of the system, local changes in substrate thickness and character, and water quality changes imposed on the system, these data may be used to predict both the general pattern and rapidity of community response to different restoration strategies