Change detection of riparian woodland vegetation in the Okavango Delta, Botswana

Abstract

Riparian vegetation in the Okavango Delta has extensive economic, social and environmental benefits. However, despite its importance, it is threatened by changes in variation in hydrology, anthropogenic activities, and other environmental conditions. Some riparian vegetation communities may change rapidly in response to both long term and short term disturbances. Long term monitoring of riparian woodland vegetation can help reveal changes in these woodland species, enabling timeous management interventions. In this study we classified, mapped and conducted change detection analysis of the spatial extent of different communities of riparian woodland in the Okavango Delta. Woodland species were identified in the field and their percentage cover estimated. Agglomerative hierarchical cluster analysis in PC-ORD was used to classify woodland vegetation into four communities. These communities were Croton megalobotrys-Hyphaene petersiana, Vachellia erioloba-Gymnosporia senegalensis, Phoenix reclinata-Syzygium cordatum and Vachellia tortilis-Combretum imberbe. Multi response permutation procedures (MRPP) was used to test if there was any significant difference in plant species composition between different vegetation communities. MRPP showed that communities varied significantly (p < 0.05) from each other in composition, which may be indicative of the prevailing environmental conditions in the Delta. Landsat data was used to produce land cover maps at 3 time-steps which were then used in the change detection analysis for riparian woodland vegetation. Phoenix reclinata-Syzygium cordatum areal extent increased from 1991 to 2019, whilst Croton megalobotrys-Hyphaene petersiana decreased. The area covered by Vachellia tortilis-Combretum imberbe and Vachellia erioloba-Gymnosporia senegalensis was fluctuating over the study period. The results suggest that the Phoenix reclinata-Syzygium cordatum community is more susceptible to variations in hydrology whilst the Vachellia erioloba-Gymnosporia senegalensis community is more stable. The Croton megalobotrys-Hyphaene petersiana and Vachellia tortilis-Combretum imberbe communities are also susceptible to prevailing environmental conditions including the hydrology, soil nutrient status as well as the occurrence of elephants in large densities in the Delta and anthropogenic activities. This information will serve as baseline for monitoring of riparian woodland vegetation in the Okavango Delta.