The spatial pattern and abundance of herbaceous vegetation in semi-arid savannas are dictated by a complex and dynamic interaction between trees and grasses. Scattered trees alter the composition and spatial distribut...The spatial pattern and abundance of herbaceous vegetation in semi-arid savannas are dictated by a complex and dynamic interaction between trees and grasses. Scattered trees alter the composition and spatial distribution of herbaceous vegetation under their canopies. Therefore, we studied the effect of Vachellia tortilis on herbaceous vegetation composition, biomass and basal area, and soil nutrients on sites with varying grazing intensities in the central rift valley of Ethiopia. Data were collected on species composition, cover and biomass of herbs and grasses, and soil moisture and nutrient contents under light,medium, and heavy grazing pressures, both under the inside and outside of V. tortilis canopies. Species richness was similar in both locations but decreased with increased grazing. Only the overall biomass and herb cover were significantly greater under the canopy than outside, and overall biomass showed significant unchanging decline with increased grazing. However, vegetation cover was significantly greater on moderately grazed sites compared to low and heavily grazed sites. All soil variables were significantly higher under V. tortilis canopies than outside.Our findings suggest that V. tortilis has more effect on composition and diversity of herbaceous vegetation than on species richness, and that V. tortilis promotes the herbaceous layer biomass by reducing soil moisture loss and increasing soil fertility under the inside than outside the canopies. Therefore, we suggest that management practices should be directed on reducing pressure on V. tortilis by regulating grazing. Low to moderate grazing levels(i.e., a stocking rate less than 39.6 TLU ha-1yr-1) seems to be tolerable to ensure sustainable conservation of the species in the study area in particular and in semi-arid savannas in general.展开更多
By stepwise adding of reducer N2H4.H2O, cuprous oxide （Cu2O） nanoparticles （NPs） with adjustable structures were synthe- sized. The features of Cu2O NPs were characterized by XRD, TEM and UV-Vis absorption spectra...By stepwise adding of reducer N2H4.H2O, cuprous oxide （Cu2O） nanoparticles （NPs） with adjustable structures were synthe- sized. The features of Cu2O NPs were characterized by XRD, TEM and UV-Vis absorption spectra. When the reducer was added into the reactant system at one time, the sizes of the Cu2O NPs are in the range of 120-140 nm. Most Cu2O NPs are sol- id spheres. As the reducer was divided into two equal parts and stepwisely added, almost all the NPs are hollow spheres with good size （150-170 nm） distribution and dispersity. But when the reducer was divided into three or four equal parts and stepwisely added, the NPs are hollow spheres, core-shell structures or solid spheres, and the sizes distribution of the products is deteriorated. The effect of sodium hydrate （NaOH） was also probed. Addition of NaOH speeded up the nucleation and growth processes of Cu2O NPs. With the alkalinity increase, the shells of the hollow spheres become compact and the thicknesses of the shells increase, but the size distribution of the NPs is deteriorated. The absorption spectra of the CuzO NPs are tunable. With the shell thicknesses increase, the absorption peaks have red shifts. An inside-outside growth model of Cu2O NPs was proposed to explain the results. The Cu2O single crystalline grains grow not only in the reactant solution, but also inside of the hollow nanospheres. The new Cu2O nanocrystallines can not only aggregate onto the shells of the nano hollow spheres, but also inside and outside of the hollow spheres, which leads to increasing the shell thicknesses of the hollow spheres, forming core-shell structures or small solid spheres of Cu2O NPs, respectively.展开更多
基金Wageningen University for scholarship opportunity of Temesgen Yadeta(Funding no:WUSRS-1205-10)
文摘The spatial pattern and abundance of herbaceous vegetation in semi-arid savannas are dictated by a complex and dynamic interaction between trees and grasses. Scattered trees alter the composition and spatial distribution of herbaceous vegetation under their canopies. Therefore, we studied the effect of Vachellia tortilis on herbaceous vegetation composition, biomass and basal area, and soil nutrients on sites with varying grazing intensities in the central rift valley of Ethiopia. Data were collected on species composition, cover and biomass of herbs and grasses, and soil moisture and nutrient contents under light,medium, and heavy grazing pressures, both under the inside and outside of V. tortilis canopies. Species richness was similar in both locations but decreased with increased grazing. Only the overall biomass and herb cover were significantly greater under the canopy than outside, and overall biomass showed significant unchanging decline with increased grazing. However, vegetation cover was significantly greater on moderately grazed sites compared to low and heavily grazed sites. All soil variables were significantly higher under V. tortilis canopies than outside.Our findings suggest that V. tortilis has more effect on composition and diversity of herbaceous vegetation than on species richness, and that V. tortilis promotes the herbaceous layer biomass by reducing soil moisture loss and increasing soil fertility under the inside than outside the canopies. Therefore, we suggest that management practices should be directed on reducing pressure on V. tortilis by regulating grazing. Low to moderate grazing levels(i.e., a stocking rate less than 39.6 TLU ha-1yr-1) seems to be tolerable to ensure sustainable conservation of the species in the study area in particular and in semi-arid savannas in general.
基金supported by the National Natural Science Foundation of China(Grant Nos.41172110 and 61107090)Shandong Provincial Natural Science Foundation(Grant No.ZR2011BZ007)
文摘By stepwise adding of reducer N2H4.H2O, cuprous oxide （Cu2O） nanoparticles （NPs） with adjustable structures were synthe- sized. The features of Cu2O NPs were characterized by XRD, TEM and UV-Vis absorption spectra. When the reducer was added into the reactant system at one time, the sizes of the Cu2O NPs are in the range of 120-140 nm. Most Cu2O NPs are sol- id spheres. As the reducer was divided into two equal parts and stepwisely added, almost all the NPs are hollow spheres with good size （150-170 nm） distribution and dispersity. But when the reducer was divided into three or four equal parts and stepwisely added, the NPs are hollow spheres, core-shell structures or solid spheres, and the sizes distribution of the products is deteriorated. The effect of sodium hydrate （NaOH） was also probed. Addition of NaOH speeded up the nucleation and growth processes of Cu2O NPs. With the alkalinity increase, the shells of the hollow spheres become compact and the thicknesses of the shells increase, but the size distribution of the NPs is deteriorated. The absorption spectra of the CuzO NPs are tunable. With the shell thicknesses increase, the absorption peaks have red shifts. An inside-outside growth model of Cu2O NPs was proposed to explain the results. The Cu2O single crystalline grains grow not only in the reactant solution, but also inside of the hollow nanospheres. The new Cu2O nanocrystallines can not only aggregate onto the shells of the nano hollow spheres, but also inside and outside of the hollow spheres, which leads to increasing the shell thicknesses of the hollow spheres, forming core-shell structures or small solid spheres of Cu2O NPs, respectively.
