A vegetation-based hierarchical classification for seasonally pulsed floodplains in the Okavango Delta, Botswana

摘要

A classification scheme is presented for seasonal floodplains of the Boro-Xudum distributary of the Okavango Delta, Botswana. This distributary is subject to an annual flood-pulse, the inundated area varying from a mean low of 3 600 km2 to a mean high of 5 400 km2 between 2000 and 2006. A stratified random sample of 30 sites was surveyed for species composition and abundance in March-June 2007, using multiple quadrats along transects orthogonal to the floodplain long axis. A combination of indicator species analysis and ordination was used to derive a hierarchical classification system for floodplains, based on species assemblages. Indicator species analysis was used to identify ecologically meaningful levels of division, at four and nine classes. The four main classes of floodplain were: (1) Dry Floodplain Grassland (main indicators Urochloa mosambicensis, Ipomoea coptica, Chloris virgata and Pechuel-Loeschea leubnitziae); (2) Seasonally Flooded Grassland (Nicolasia costata, Eragrostis lappula, Cyperus sphaerospermus and Setaria sphacelata); (3) Seasonally Flooded Sedgeland (Eleocharis dulcis, Leersia hexandra, Oryza longistaminata and Cyperus articulatus); and (4) Seasonal Aquatic Communities (Sacciolepis typhura, Eleocharis variegata, Fuirena pubescens and Cycnium tubulosum). The resultant dendrogram provides an objective routine for classifying floodplains in the Boro-Xudum distributary in an ecologically meaningful way. This classification will assist in monitoring changes in vegetation resulting from hydrological change.