| dc.description.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. | en |
