THE EFFECT OF CONIFEROUS TREE SPECIES ON GRASSLAND HERBACEOUS VEGETATION AND SOIL CHEMICAL CHARACTERISTICS IN GORO-GUTU DISTRICT, EASTERN ETHIOPIA

Abstract

The study was conducted at Keramile exclosure of Goro-gutu district, Eastern Ethiopia, to determine the effects of coniferous tree species on distribution, species diversity, ground cover and biomass yield of the herbaceous vegetation; and soil chemical nutrients of grassland. Three tree species, Podocarpus falcatus and Juniperus procera from indigenous and Cupressus lusitanica from exotic tree species were used. Twenty large trees from each tree species, a total of 60 trees were selected purposively and 480 samples (four quadrats under and outside canopy in four directions for each tree) of herbaceous vegetation and soil samples were collected. Our results showed that tree species, canopy cover and their interactions had significant (P<0.05) effect on grassland herbaceous plant distribution, diversity, aboveground cover and biomass yield. Herbaceous species diversity, ground cover and biomass yield were significantly (P<0.05) higher under P. falcatus compared to J. procera and C. lusitanica trees. The overall result shows that under canopy cover strongly decreased species diversity (P<0.009), biomass yield (P<0.0001) and ground cover (P<0.0001) of the herbaceous vegetation. However, there was no significant difference in species diversity between under P. falcatus canopy and outside canopy. Tree species and canopy cover had also significantly (P<0.05) influenced soil pH, cation exchange capacity (CEC),electrical conductivity (EC) organic matter (OM), total nitrogen (TN), available phosphorous (av. P), available potassium (av. K) sodium (Na) and organic carbon (OC) contents, in which soil pH, TN, Na, OM and OC contents were significantly higher under P. falcatus compared to J. procera and C. lusitanica. Except the lower soil pH and av. K under canopies of J. procera and P. falcatus respectively, both had almost comparable in their soil chemical constituents when compared to the outside canopies. However, C. lusitanica had lower, OM, OC, TN and av. K under canopy when compared to the outside canopy. Soil OM, pH, OC, av. K, TN, and CEC were significantly higher in the outside canopy than under canopy, whereas EC and Na were higher under canopies. No significant difference was observed between under canopy and the outside canopy in av. P, Ca and Mg contents. Soil pH, OM, OC, TN and CE Ccontents were significantly affected by the interactions of tree species and canopy cover. Generally, tree species, canopy cover and their interaction had negative impact on species diversity, ground cover and biomass yield of herbaceous vegetation, and soil chemical nutrients in Keramile exclosure, Goro-gutu district, eastern Ethiopia. However, among the three tree species studied, P. falcatus tree species enhanced herbaceous diversity and soil chemical nutrients with relatively small impact on herbaceous ground cover, compared to the outside canopy in the study area.