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Marine Ecology Progress Series

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MEPS 272:77-92 (2004)  -  doi:10.3354/meps272077

Human-impacted mangroves in Gazi (Kenya): predicting future vegetation based on retrospective remote sensing, social surveys, and tree distribution

F. Dahdouh-Guebas1,2,*, I. Van Pottelbergh1, J. G. Kairo3,1, S. Cannicci4, N. Koedam1

1Biocomplexity Research Team, Laboratory of General Botany and Nature Management, Mangrove Management Group, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium
2Uitgegeven met steun van de Universitaire Stichting van België, Egmontstraat 11, 1000 Brussels, Belgium
3Kenya Marine and Fisheries Research Institute, PO Box 81651, Mombasa, Kenya
4Dipartimento di Biologia Animale e Genetica ŒLeo Pardi¹, Università degli Studi di Firenze, Via Romana 17, 50125 Firenze, Italy

ABSTRACT: Gazi Bay, Kenya, covers an area of 18 km2, and its mangroves are degraded. We present a quantitative and qualitative evaluation of the degradation of vegetation structure and dynamics of mangrove communities over a period of 25 yr, using aerial photography in a geographical information system (GIS), combined with ground-truth data for different vegetation layers, and with faunal and environmental factors. Retrospective analysis and understanding of current practices were aided by interviews with Gazi village elders and by field observations of mangrove tree stumps. GIS-based vegetation maps were combined with data obtained using the point-centred quarter method (PCQM), an accuracy analysis was performed, and forestry parameters were derived from the resultant PCQM data. In addition to general human-induced degradation of vegetation structure and floristic composition of the seaward mangrove zone, a particular sandy beach is expanding at the expense of mangrove, whereas the back mangrove zone has undergone minor changes. Aerial photographs of 1992 and current field data show an apparent zonation of 6 different monospecific or mixed mangrove communities, with a high importance of Rhizophora mucronata in each community and each vegetation layer. Retrospective vegetation structure was combined with correspondence analyses on the PCQM data derived for adult, young and juvenile trees in order to make predictions. Present dynamics initiated by anthropogenic degradation of mangroves continues, even though human impact has diminished. We predict that under a Œno impact scenario¹, the sandy ridge will continue to expand, that this will speed up under a Œmangrove cutting scenario¹, and that a scenario altering the complex topography will lead to a major re-organisation of the mangrove and terrestrial vegetation structure.

KEY WORDS: Mangroves · Dynamics · Aerial photography · Macrobenthos · Multivariate analysis · Forecasting · Interviews · Kenya

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