Water and nutrient relations of selected tree species of Ethiopia

[摘要] ENGLISH ABSTRACT: This study was conducted to evaluate and compare the water and nutrient relations of threeindigenous deciduous tree species, viz., Cordia africana Lam., Croton macrostachyus Del.,Millettia ferruginea (Hochst.) Baker and two widely planted eucalypts, viz., Eucalyptuscamaldulensis Dehnh and Eucalyptus globulus Labill from Ethiopia. The study was organized asglasshouse and field observations in Ethiopia. Owing to the lack of baseline studies on the waterand nutrient relations of the deciduous tree species, the glasshouse experiments involved a widerange of water and nutrient applications. Seedlings were grown with the supply of various levels ofwater and nutrients during which gas exchange, water potential, relative water content, tissuenutrient content and biomass production were measured. The field observations were limited to thestudy of surface root distribution and leaf nutrient composition of mature trees and their effects onsoil nutrient pool. The observations were made on isolated trees and mixed or pure stands of treesin Badessa area, Eastern Ethiopia. The field site was selected because of the availability of thestudy species and suitability of the trees for the study.In the glasshouse, increased water deficit significantly reduced predawn leaf waterpotential, relative water content, stomatal conductance, photosynthetic rate, whole plant water useefficiency, plant height, diameter, leaf area and biomass production. Both of the eucalypts did notgrow faster than the deciduous species under well-watered conditions unlike under water stressconditions. C. macrostachyus and C. africana had higher transpiration rates and tissue nutrientaccumulations than the other species. They also demonstrated higher biomass allocation to rootsthan all the other species to support the intensive water and nutrient uptake rate. Due to the abilityto re-orient its leaves to avoid direct solar irradiance, M. ferruginea maintained higher tissue waterpotential 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 comparedto the deciduous tree species indicating that the eucalypts may not survive extreme droughtconditions 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 droughtthan 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 speciesstudied. Pants with high foliar nitrogen concentration had higher photosynthetic capacitiesindicating that N plays a key role in photosynthesis and growth of all the studied species. Thecurrent study showed that for all the tree species, more attention has to be given to soil N than to Pas 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 Plimitationin plants. However, N:P ratio could not distinguish between N-limitation and combinedlimitations of N and P.The study of isolated C. africana and C. macrostachyus trees on soils in Badessa, EasternEthiopia indicated improved soil N, P and K under tree canopies whereas no effects were observedon 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 theirhigh nutrient cycling potential the net effect on soil was positive. Comparison of E. camaldulensiswoodlot and a mixed stand composed of deciduous species indicated that the fine root biomass inthe surface soil under E. camaldulensis was about three times that under the mixed stand. The fineroot biomass of E. camaldulensis inside the stand and 10 meters away from the stand werecomparable 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.

[发布机构] Stellenbosch University