Boro and Xudum Floodplain Vegetation Data 2007

Study Extent

The study covered the seasonally inundated southern parts of Okavango Delta – Two distributaries in the Delta were selected for detailed study: the Boro and neighboring Xudum systems. These systems are characterized by pronounced seasonal fluctuations in response to the annual pulse, and thus exhibit a wide range of hydroperiod variation (Dinçer et al. 1987). It was a once-off campaign study aimed at collecting, analysing and interpreting data on floodplain macrophyte distributions as a function of hydroperiod, to facilitate the development of empirical species distribution models.

Sampling

The work described here was developed against a baseline of previous work described in detail in Wolski and Murray-Hudson (2006). Floodplains were stratified into 5 frequency strata based on the interpretation of Landsat TM and Enhanced Thematic Mapper (ETM) images of annual maximum flood extent from 1989 to 2007 (see Wolski and Murray-Hudson 2006). 30 sites were randomly selected from these strata (6 in each stratum), and a minimum of 30 square metre plots spaced 20 metres apart along transects was surveyed at each site. All plant species in each plot were recorded and their relative abundance was rated according to a modified Braun-Blanquet ranking.

Quality Control

Species were identified in the field where possible; unknowns were given field names and pressed in the field; those which could not be identified from the Peter Smith Herbarium collection at the Okavango Research Centre were sent to the Royal Botanical Gardens, Kew, United Kingdom for identification. Nomenclature followed Germishuizen and Meyer (2007). Relative abundance estimates were made by consensus of at least two field surveyors, and a one-day calibration exercise was carried out at the beginning of the field work to ensure consistency. Data entry was done by M. M-H and F. M-H into a bespoke Microsoft Access database; reading of field sheets and typing was done alternately, and data were cross-checked with field sheets after all had been transcribed.

Method steps

1. Step 1 involved the selection of random sites for vegetation sampling. This was based on historic hydroperiod - a flood frequency map derived from remote sensing which assigned a frequency to each pixel. the frequency map was stratified into 5 strata of approximately equal area, and in each stratum 6 sites were selected by randomising the pixel numbers. Step 2 involved doing surveys of the vegetation at each site by laying out transects orthogonal to the long axis of each floodplain, and enumerating plant species within 1 square metre plots at 20 metre intervals along these transects. Species-area plots from sampling carried out beforehand indicated that a minimum of 25 square metres should be sampled. A minimum of 30 plots was thus surveyed at each site. All species in each plot were recorded and their relative abundance estimated according to a Braun-Blanquet classification.

The Boro and Xudum distributary systems to the west of Chief's Island and east of the western Buffalo Fence.