A PALYNOLOGICAL APPROACH TO RECONSTRUCT PAST EL NIÑO-VARIABILITY; FIRST RESULTS FROM A SUB-TROPICAL FLORIDA WETLAND

South-Florida wetland vegetation patterns are determined by hydroperiod and water depth which in turn depend on precipitation levels. Changes in these parameters immediately result in the adjustment of local plant communities. Hence, shifts in vegetation through time most likely represent changes in local water availability. Over 50% of the winter precipitation in Florida depends on the strength of the El Niño Southern Oscillation (ENSO). Past vegetation changes, reconstructed by means of pollenanalysis, reflect past wetland hydrology. Analysis of well-dated peat sections on high temporal resolution may thus provide valuable clues about past El Niño-variability, magnitude and persistence.

Evaluating the extent of vegetation response to known variations in hydrology, allows calibration and quantification of naturally occurring vegetation changes. Peat profiles from a mixed cypress swamp in Fakahatchee Strand, Florida (U.S.A.) have been examined for pollen and spores. The high resolution records reveal several dry periods, the latest of which can be attributed to wetland drainage starting in 1930 (AMS radiocarbon-dated). An earlier dry phase, approximately between 1750 and 1850 AD, is of the same magnitude as the recorded change caused by human activities in the 20th century. An even larger vegetation change is apparent in the deepest part of the profile, where present-day cypress forest was preceded by wet prairie-fringe vegetation indicating substantially lower water tables and shorter hydroperiods. Additionally, within this dry period spells of high Salix abundance occur indicating short wet phases.

Pollen analysis in fast-accumulating Florida peat sediments provide a sensitive tool for reconstructing past hydrologic conditions. Spatial patterns of South Florida vegetation, surface pollen assemblages and water regime are well studied and provide a solid basis for interpretation of past vegetation changes. Further dating (now in progress) will establish the exact timing and duration of past dry phases and their relation to climatic global events (e.g. Little Ice Age), comparison with other proxies will provide clues about phases of altered ENSO persistence and magnitude.