Species composition and functional structure of herbaceous vegetation in a tropical wetland system

Abstract

Understanding driving factors of spatial heterogeneity in plant species com- position and functional structure is a key step towards wetland ecosystems restoration and biodiversity conservation. We surveyed 60 randomly selected plots of 20 9 50 m each and collected 12 explanatory variables to understand potential impacts of natural environ- mental conditions, human disturbance and spatial position of plots variables on plant functional groups (PFGs) and species composition within grasslands of the Kilombero Valley Floodplain wetland, Tanzania. Ordination analyses were used to identify important vegetation gradients and establish significant natural environmental conditions, human disturbance and spatial position of plots correlates. Partial redundancy analysis (RDA) and partial canonical correspondence analysis (CCA) were used to determine the individual and shared effects of these three sets of explanatory variables on the PFGs and species composition, respectively. In total, 115 plant species, including 22 weed and 3 invasive species, and five PFGs were registered. Annual graminoid was the most abundant, whereas perennial forb the least abundant PFG with 50 and 1 % relative covers, respectively. Overall, spatial position, altitude, total organic carbon, cow dung, distance to the river and distance to kraal (cattle enclosure near human settlement commonly made of wood materials for animal protection) were important descriptors of both PFGs and overall species composition. Separate CCA of only weed and invasive plant species showed that some species, particularly the invasive Polygala paniculata were strongly associated with cow dung, indicating that present increase in cattle numbers may result in future problems associated with this species. Intensification of human activities and alteration of natural environmental conditions associated with these factors should be discouraged to maintain plant species composition and functional structure for wetland restoration and sustainable biodiversity conservation.