Influence of flood variation on seasonal floodplain vegetation communities in the Okavango Delta, Botswana

Abstract

This study investigated the influence of flooding variation on floodplain vegetation in the Nxaraga Lagoon seasonal floodplains by sampling community composition and soil nutrient content in 1997, when flood levels were unusually low, and again in 2010 when flood levels were unusually high. In each of the eight vegetation zones identified in 1997, five plots were randomly selected. Flooding duration in each plot was estimated as the number of weeks in which the plots were inundated. Multi-response permutation procedures (MRPP) analysis was used to compare vegetation communities under low and high floods. Soil pH, Na, Mg, K, Ca and P content were analysed. The influence of environmental variables on species distribution was investigated using non-metric multi-dimensional scaling (NMS) ordination. Total plant species richness decreased from 88 during low flood to 53 during high flood, and community co-dominant species changed during high flood. Mean percentage cover decreased for some species, but increased for others. Hydroperiod increased in all zones. During the high flood period the content of Na and K was highest in Zone 8 (tertiary floodplain); Ca, Mg and pH content was highest in Zones 6 and 8 (secondary and tertiary floodplains, respectively), while P was highest in Zone 5. Hydroperiod, Na and pH were found to be correlated with, and are therefore thought to influence, community composition and distribution.

Keywords:

• flooding depth
• flooding duration
• high flood
• low flood