Environmental and anthropogenic influences on the distribution, structure, and floristic composition of mangrove forests of the Gulf of Urabá (Colombian Caribbean)

Abstract

The Gulf of Urabá is the largest sea inlet along the Colombian Caribbean coast, and is highly influenced by freshwater currents that carry substantial quantities of sediment, shaping the coastal geomorphology, and vegetation species composition and distribution. Despite regional low tides and pore-water salinity levels, we hypothesised high variability in mangrove vegetation along the coasts due to local environmental variation, coupled with a recognised physiographic classification. Mangrove forests were mapped and classified following analysis of aerial photographs. Based on 87 circular 500m2 plots, we assessed the structure, floristic composition, and environmental attributes of mangroves. Three mangrove forest types were identified, according to the physiographic classification based on location; riverine, basin, and fringe; and a mangrove type in a highly disturbed stage. Significant differences among mangroves were found; therefore our hypothesis was accepted. Rhizophora mangle, Laguncularia racemosa, and Avicennia germinans exhibited the highest importance values (IVI) in fringe and riverine mangroves; and in basin mangroves, A. germinans showed the highest IVI, with the importance order of these species inverted. Small highly disturbed mangrove fragments were dominated by R. mangle. Tree diameters and heights were asymmetric unimodally distributed, suggesting large forest disturbances related to timber overexploitation, and poor conservation status. Differences among mangrove types were related to soil salinity, granulometry, and nutrient concentrations. The sediment loads transported by rivers, soil nutrient concentrations, and vegetation structure revealed high fluvial and anthropogenic influences on forests, and these conditions are promoting colonisation by alluvial forest species.