PALAEO-BIOCOMPLEXITY: PAST MANGROVE ECOSYSTEM DYNAMICS AND ELEMENTAL CYCLING IN BELIZE, CENTRAL AMERICA DURING THE HOLOCENE EVIDENT FROM POLLEN, STABLE ISOTOPE AND STOMATAL ANALYSES

Multiple proxies of past ecosystems, including stable carbon isotopes, pollen and stomata, have been applied in the tropics to understand past woodland – grassland ecological and physiological dynamics. In many instances this type of vegetation transition represents a significant difference in ecosystem structure, ecology and physiology. Superficially modern mangrove ecosystems seem comparatively simple, due to low plant diversity (e.g. with only three dominant tree species in Belize – Avicennia germinans, Rhizophora mangle and Laguncularia racemosa all of which use the C₃ photosysthetic pathway). However, findings from a NSF funded biocomplexity study are illustrating that this assumption is incorrect. The three dominant tree species have very different ecologies; R.mangle can survive high tidal ranges, A. germinans favors more elevated topography, L. racemosa can exist in extreme salinities. Moreover, fertilization experiments have shown that differences in the stand structure of R.mangle (dwarf vs. tall) represent dramatic differences in nutrient limitation. Here we present a multi-proxy approach applied to continuous cores of mangrove peat from cays off the coast of Belize, to examine intra and inter island ecosystem dynamics. AMS ¹⁴C dates indicate that the cores represent ~8,000 calendar years. Pollen data illustrate that habitats currently dominated by R.mangle were significantly altered and conditions were drier from between 1500 and 3200 calendar years BP. A taphonomic study of R.mangle leaves preserved in the peat, indicate that stable carbon and nitrogen isotope analyses have the potential to illustrate shifts in the ecosystem stand structure and nutrient limitation. d¹³C values from R.mangle leaves vary between –23.7‰ (dwarf) and –28.0‰ (tall) in the cores. d¹⁸O of R.mangle leaves, which vary between 15.9‰ and 20.0‰ in the cores, support the carbon isotope data, implying that the water-use-efficiency of R.mangle varied in the past. R.mangle leaves preserved in the peat also retain intact stomata, the relative abundance of which can potentially be used to examine the response of mangroves to changes in atmospheric CO₂ concentrations throughout the Holocene. By examining multiple proxies of peat underlying a ‘simple’ mangrove ecosystem we document complex palaeo-ecological dynamics.