Effects of Soil Characteristics and Disturbance on Floodplain Vegetation in Upper Tana River, Kenya

Abstract

Floodplains of rivers in semi-arid areas of Africa are important areas for agriculture, livestock grazing and wildlife conservation. However, floods and anthropogenic disturbances affect soil properties, which subsequently influence natural plant community structure, and create favourable conditions for invasive species to establish. The dynamics of tropical African floodplain vegetation in relation to flooding regime, soil characteristics and farming activities has not been widely investigated and hence is not well understood. The purpose of this study was to investigate how floodplain vegetation responds to changes in flood regime, soil characteristics and different levels of human disturbance. The objectives were to (1) determine the variation in vegetation structure of plant species with the cross-sectional floodplain gradient and across the main land use types; (2) determine the influence of soil properties on the distribution of tree species; (3) determine the effects of herbaceous defoliation and cultivation on emergence, survival and growth of Acacia tortilis (Forssk.) Hayne and Prosopis juliflora (Sw.) DC. seedlings on and off the floodplain and (4) determine the effects of site characteristics and soil treatments on emergence, survival and biomass of Acacia tortilis and Prosopis juliflora seedlings. Vegetation and soil data were collected along transects running perpendicular to the river towards the land in the upper Tana River floodplain. The transects cut across different vegetation types and land uses, namely; wildlife conservation, livestock grazing and irrigated agriculture. An experiment was set up to determine the effect of defoliation and cultivation on emergence, survival and growth of Acacia tortilis and Prosopis juliflora seedlings on and off the floodplain. Another experiment was set up to determine the effects of site characteristics and different soil treatments on emergence, survival and biomass of the two species. The mean densities of the woody and non-woody species, the basal areas and canopy covers of the tree species and soil properties were compared using a t-test for between vegetation found inside and outside the floodplain. Analysis of variance (ANOVA) was used to compare the vegetation and soil data based on various land use types. Canonical Correspondence Analysis (CCA) was used to relate the soil properties to the distribution of tree species using Importance Value (IV 200) as the measure of abundance. The experimental data were subjected to a three-way ANOVA and the group means compared using Fisher‘s Protected Least Significant Difference, both at 5% significant level. For further analysis, the vegetation data were separated into three groups: species occurring in the floodplain only, those that occurred outside the floodplain only and those that occurred both inside and outside flood plain. The mean basal area (t (349) = -4.54, P ˂ 0.001) and the canopy cover (t (349) = -3.87, P ˂ 0. 01) of trees were significantly higher outside than inside the floodplain. The densities of the forbs (t(106) = 4.44, P ˂ 0.01) and the perennial grasses (t(58) = 2.33, P = 0.02) were significantly higher inside the floodplain. The invader Prosopis juliflora had also significantly higher mean density (t = 5.44, P ˂ 0.01) and lower mean basal area (t = -2.24, P = 0.03) inside than outside the floodplain. Prosopis juliflora contributed 33.2 and Acacia tortilis (19.65) to IV 200. These were also the highest importance values inside and outside the floodplain respectively. The basal area of Prosopis juliflora was significantly higher in wildlife conservation areas (F = 2.61, P = 0.04) than in any other land use type. Its seedling density was also significantly higher in irrigated agricultural areas (F = 3.13, P = 0.05) than in any other land use type. Irrigated agricultural areas contributed the least to IV 200. Moisture (t = 5.92), pH (t = 2.03), P (t = 5.91), Mg (t = 3.25) and Ca (t = 2.04) were significantly higher inside the floodplain (P < 0.05), and bulk density outside the floodplain (t = -8.76, P < 0.05). Soil properties influenced the distribution of trees in the study area. The emergence, survival and growth of Acacia tortilis and Prosopis juliflora species were significantly higher inside the floodplain (P ˂ 0.01). Seedlings emergence, survival and growth were significantly high in heavily defoliated and cultivate plots, on bare soil and in cow dung (P ˂ 0.05). Seedlings of Prosopis juliflora and Acacia tortilis did not survive in perennial grass on the soil surface while the latter did not survive under Prosopis juliflora canopy. In the different environmental conditions, Prosopis juliflora was more successful than Acacia tortilis. In conclusion, the results show that changes in flooding regime of River Tana due to impoundments upstream compounded by the impacts of climate variability, changes in soil properties due to reduced silt loading and anthropogenic disturbances can modify the flood plain ecosystem. These affect the plant species dynamics and create favourable conditions for establishment of invasive plant species. The outcomes have serious negative implications on the value of the floodplain wetlands as prime areas for agriculture, livestock grazing and wildlife conservation. Human activities , which appear to facilitate proliferation of invasive woody species in the fragile floodplain ecosystem, should be regulated to conserve indigenous plant communities and control invasion. Key words: Plant composition, soil properties, anthropogenic disturbances, invasive Prosopis juliflora, Acacia tortilis, seedling emergence, biomass and survival; Tana River Flood plain.