Browsing by Author "Gindaba, Jiregna"

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    Water and nutrient relations of selected tree species of Ethiopia
    (Stellenbosch : University of Stellenbosch, 2004-12) Gindaba, Jiregna; Rozanov, Andrei Borisovich; Negash, Legesse; University of Stellenbosch. Faculty of Agrisciences. Dept. of Forest and Wood Science.
    ENGLISH ABSTRACT: This study was conducted to evaluate and compare the water and nutrient relations of three indigenous deciduous tree species, viz., Cordia africana Lam., Croton macrostachyus Del., Millettia ferruginea (Hochst.) Baker and two widely planted eucalypts, viz., Eucalyptus camaldulensis Dehnh and Eucalyptus globulus Labill from Ethiopia. The study was organized as glasshouse and field observations in Ethiopia. Owing to the lack of baseline studies on the water and nutrient relations of the deciduous tree species, the glasshouse experiments involved a wide range of water and nutrient applications. Seedlings were grown with the supply of various levels of water and nutrients during which gas exchange, water potential, relative water content, tissue nutrient content and biomass production were measured. The field observations were limited to the study of surface root distribution and leaf nutrient composition of mature trees and their effects on soil nutrient pool. The observations were made on isolated trees and mixed or pure stands of trees in Badessa area, Eastern Ethiopia. The field site was selected because of the availability of the study species and suitability of the trees for the study. In the glasshouse, increased water deficit significantly reduced predawn leaf water potential, relative water content, stomatal conductance, photosynthetic rate, whole plant water use efficiency, plant height, diameter, leaf area and biomass production. Both of the eucalypts did not grow faster than the deciduous species under well-watered conditions unlike under water stress conditions. C. macrostachyus and C. africana had higher transpiration rates and tissue nutrient accumulations than the other species. They also demonstrated higher biomass allocation to roots than all the other species to support the intensive water and nutrient uptake rate. Due to the ability to re-orient its leaves to avoid direct solar irradiance, M. ferruginea maintained higher tissue water potential and relative water content than all the other species under water stress regimes. The impact of imposed drought was quick and more damaging to the eucalypts compared to the deciduous tree species indicating that the eucalypts may not survive extreme drought conditions unlike the deciduous species that drop their leaves and may remain dormant for weeks. The current study gave new experimental proof that E. globulus was more vulnerable to drought than E. camaldulensis. Soil N stress resulted in an overall reduction of tissue N concentration, N:P ratio, photosynthetic rate, stomatal conductance and photosynthetic water use efficiency in all the species studied. Pants with high foliar nitrogen concentration had higher photosynthetic capacities indicating that N plays a key role in photosynthesis and growth of all the studied species. The current study showed that for all the tree species, more attention has to be given to soil N than to P as soil P had minor effects on the photosynthetic activities of plants of all species compared to N.The investigation on tissue nutrient composition confirmed that N:P ratio could be used to detect Plimitation in plants. However, N:P ratio could not distinguish between N-limitation and combined limitations of N and P. The study of isolated C. africana and C. macrostachyus trees on soils in Badessa, Eastern Ethiopia indicated improved soil N, P and K under tree canopies whereas no effects were observed on the other soil nutrients studied. Similar to glasshouse conditions, C. macrostachyus and C. africana produced extensive surface roots, interfering with crops grown in association. Due to their high nutrient cycling potential the net effect on soil was positive. Comparison of E. camaldulensis woodlot and a mixed stand composed of deciduous species indicated that the fine root biomass in the surface soil under E. camaldulensis was about three times that under the mixed stand. The fine root biomass of E. camaldulensis inside the stand and 10 meters away from the stand were comparable in the surface soils showing the presence of root competition with adjacent crops. Therefore, planting of E. camaldulensis in association or adjacent to croplands should be avoided. Nutrient and carbon pool of soil inside the mixed stand was generally higher than that of E. camaldulensis indicating that trees of the mixed stand recycled more nutrients to the soil.