Tree Islands of the Everglades

Chapter 12.
Analysis of Tree Island Vegetation Communities
Hydrologic and Fire Impacts Over a Decade

Paul R. Wetzel
email: pwetzel@smith.edu
Department of Botany, Iowa State University.
Present address: Department of Biological Sciences, Smith College, Northampton, MA 01063 USA.

From Tree Islands of the Everglades edited by Fred H. Sklar (South Florida Water Management District) and Arnold G. van der Valk (Iowa Lakeside Laboratory)

Abstract

This chapter analyzes vegetation changes on tree islands surveyed in Water Conservation Area 3A between 1977-1986. Presence/absence was measured annually on 24m x 3m plots located on the head of each island. Based on a 15 year hydrologic record, the islands were divided into two groups: frequently flooded and frequently dry islands. In 1981 the frequently flooded islands experienced a drawdown, while the frequently dry islands experienced a drawdown and fire. After fire disturbance, frequently dry islands had a greater number of herbaceous species than frequently flooded islands. The lower number of herbaceous species on frequently flooded islands was maintained even after the flooded islands experienced drier conditions during drawdown. Ordination analysis of all species revealed that of the 15 environmental factors examined, the hydrological gradient represented by the 10 year mean water level and the time period since the last fire disturbance were most important, explaining about 45% of the variation of the data. A separate analysis of woody species frequency correlated most strongly with time since last burn and the difference between the maximum and minimum water levels over 15 years (extreme drought and flooding events) and the average amount of time that the islands were flooded. Woody species may respond to hydrologic parameters over a longer time frame, but long-term hydrologic data (greater than 15 years) was not available for analysis. Drawdown on the frequently flooded islands enabled rapid recruitment of some woody species from the seed bank. Based on the ordination analysis, species were divided into general groups along low, moderate, or high water and time since last burn parameters. From these life history traits, island vegetation composition was predicted under different environments.

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