The Effects of Prosopis julifora (DC) Hyne and Acacia tortilis (Forsk) Trees on Understorey Plant Species and Soil Properties on Njemps Flats, Baringo District, Kenya

Abstract

The effects of Prosopis juliflora (exotic species) and Acacia tortilis (indigenous species) trees on soil properties and understorey herbaceous plant species production were investigated on the Njemps flats, Baringo district, Kenya. The objective was to compare the effects of an invasive and indigenous tree species on soil physical and chemical properties, the occurrence and production of the understorey plants under their canopies relative to the adjacent open areas. Five mature P. juliflora and A. tortllis trees of similar canopy size and structure, without shrubs or termite mounds under their canopies were systematically selected. Samples of soil and herbaceous plant species were obtained at 1m, 2m and 3m from the tree trunk within the canopy and at 4m, 5m and 6m from the edge of the tree canopy. Soil samples were collected 0-5cm, 15-20cm and 40-45 cm depths at the above-mentioned points along four cardinal directions of the tree trunk. Soil samples under each experimental unit, from all the three distances and depths, were composited into a single sample before carrying out the analyses. Standing biomass, frequency and cover of understorey plant species were significantly (p<O.05) higher in the open area, than under the tree canopies. Biomass was 3 and 5 times higher in open areas than under A torlilis and P. juliflora canopies, respectively. Cover for herbaceous plant species was 63% under P. juliflora, 82% under A tortilis and 90% in open areas. All forbs occurred under the shade indicating that they are more adapted to the microenvironment found under the shade than grasses. Soils under the tree canopies had significantly (P<O.05) higher organic carbon and total nitrogen than those in adjacent open areas. Organic carbon and total nitrogen concentration in soils under P. ju/iflora were 13% and 45% higher than in the open areas, respectively; and 25% and 153% higher under A. torti/is than in the open areas respectively. Soils under A. toriilis had significantly (P<O.05) higher organic carbon and total nitrogen than soils from under P. juliflora. Soils were slightly more acidic under the tree canopies than in the open areas. Bulk soil density was significantly (P<O.05) higher in the open area than under the canopies, suggesting that tree canopies protect the soil from compaction. The results suggest that A. tortitis trees are more beneficial to soil physical and chemical properties than P. juliflora trees. Therefore, the common practice of clearing woody trees indiscriminately for crop cultivation or to improve grassland for livestock production should be reconsidered. The practice also removes beneficial effects of trees such as A. tortilis, on soils, such as the provision of shade for grazing animals, habitats for birds and wildlife, and as source of protein in the dry season when the grasses are in short supply. Based on the result of this study, P. juliflora tree species should not be encouraged to grow in rangelands as it inhibits the development of herbaceous plants species under its canopy. In areas where the tree is already established, research should be conducted to determine the best methods of eradicating them.