Mechanisms of bush encroachment and its inter-connection with rangeland degradation in semi-arid African ecosystems:a review

Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local woody species and/or incursion of woody species introduced from other ecosystems into semi-arid savannas and grassland ecosystems. Rangeland degradation due to bush encroachment causes several challenges, affecting the production of livestock and pastoral people livelihoods in most parts of Africa. Scientists have long been attempting to develop schematic and mathematical theories to explain the observed phenomenon of bush encroachment, and several theories were proposed and developed. The well-regarded theories include： （1） Waiter＇s two-layer model, （2） Moir＇s one-layer model, （3） state-and-transition theory, （4） equilibrium theory, （5） disequilibrium theory, and （6） non-equilibrium theory. Within those theories, the most frequently-indicated driving factors that explain bush encroachment include over-grazing, availability of soil nutrient and moisture, elevated CO2 levels, frequency and intensity of fire, spread of seeds of woody species by livestock and wild animals. It should be stressed that couplings and interactions among diverse driving factors are more often at work in determining the condition of bush encroachment. To summarize, the effort in managing semi-arid ecosystems needs critical knowledge to understand the cause-effect relationships of underlying factors through integrated approach. Therefore, future research on encroachment of woody plants should be multi-discipline oriented and multi-partnership involved.

Effect of Vachellia tortilis on understory vegetation,herbaceous biomass and soil nutrients along a grazing gradient in a semi-arid African savanna

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.

Effect of exclosure ages on woody plant structure,diversity and regeneration potential in the western Tigray region of Ethiopia

Exclosure is a method of rehabilitating degraded lands by protecting them from the interference of animals and from human encroachment, and is used to regenerate native vegetation as a way to reduce soil erosion, increase rain water infiltration and provide fodder and woody biomass in degraded grazing lands. Therefore, we studied woody plant structure, diversity and regeneration potentials in 5-and 10-year grazing exclosures in comparison with open grazed sites in a semi-arid environment. Data on species diversity, abundance, structure, basal area, frequency, density, and regeneration status were collected from 270 sample plots. Forty-one woody species representing 20 families were identified, with 18, 28 and 38 species found in open grazed areas, and in 5-and 10-year grazing exclosures, respectively. The 10-year grazing exclosures had a higher(P <0.05) species richness and plant densities compared to the 5-year grazing exclosures and the open grazed areas. The population structure and regeneration status of woody species in both grazing exclosures showed an inverted J-shape, indicating a healthy regeneration status, whereas hampered regeneration was observed in open grazed areas. The establishment of grazing exclosures had positive effects in restoring woody plant diversity and improving vegetation structure and regeneration potentials of degraded grazing lands.

Effect of Senna obtusifolia(L.)invasion on herbaceous vegetation and soil properties of rangelands in the western Tigray,northern Ethiopia

Introduction:Invasion of exotic plant species is a well-known threat to native ecosystems since it directly affects native plant communities by altering their composition and diversity.Moreover,exotic plant species displace native species through competition,changes in ecosystem processes,or allelopathic effects.Senna obtusifolia(L.)invasion has affected the growth and productivity of herbaceous vegetation in semi-arid regions of northern Ethiopia.Here,we investigated the species composition,species diversity,aboveground biomass,and basal cover of herbaceous vegetation,as well as soil properties of rangelands along three S.obtusifolia invasion levels.Methods:Herbaceous vegetation and soil properties were studied at two locations,Kafta Humera and Tsegede districts,in the western Tigray region of northern Ethiopia under three levels of S.obtusifolia invasion,i.e.,non-invaded,lightly invaded,and heavily invaded.Herbaceous plant species composition and their abundance were assessed using a 1-m^(2) quadrat during the flowering stage of most herbaceous species from mid-August to September 2015.Native species were classified into different functional groups and palatability classes,which can be useful in understanding mechanisms underlying the differential responses of native plants to invasion.The percentage of basal cover for S.obtusifolia and native species and that of bare ground were estimated in each quadrat.Similar to sampling of the herbaceous species,soil samples at a depth of 0–20 cm were taken for analyzing soil physical and chemical properties.Results:Highly palatable grasses and legumes based on the frequency of occurrence were 1.9 and 3.8 times higher in non-invaded sites than in lightly and heavily invaded sites,respectively.Furthermore,unpalatable plant species were 4.0 times higher in the heavily invaded sites than in the lightly invaded sites.Aboveground biomass,basal cover,and species diversity of the herbaceous vegetation were significantly(P<0.01)lower in the heavily invaded sites compared to the non-invaded sites.However,most soil properties were not affected by S.obtusifolia invasion compared with non-invaded sites in our study.Conclusions:Our findings contribute towards better understanding of how S.obtusifolia invasion affects the community structure and composition of native herbaceous vegetation in semi-arid rangelands in northern Ethiopia.Further long-term studies are required to understand the mechanisms of how S.obtusifolia invasion influences soil properties for better control and eradication of its invasion in the semi-arid rangelands of northern Ethiopia.

Tree species determine soil seed bank composition and its similarity with understory vegetation in a semi-arid African savanna

Introduction:The presence of larger trees in semi-arid African savannas creates sub-habitats,which influences on herbaceous plant communities grown under their canopies differently from opened areas.Knowledge of seed banks accumulated in the soils over time beneath larger trees could facilitate the recovery of plant communities that might disappear due to sustained heavy grazing,prolonged fire,or other anthropogenic factors in semi-arid African savannas.However,the impact of larger trees on soil seed bank composition and its similarity with plant communities grown under their canopies are less understood in semi-arid African savannas.Therefore,we studied the effect of leguminous and non-leguminous tree species and their canopies on soil seed bank(SSB)composition and its similarity with understory vegetation(USV)in a semi-arid savanna of Ethiopia.Methods:We selected 20 matured trees from 3 dominant tree species,representing one leguminous(Acacia robusta Burch)and 2 non-leguminous tree species(Ziziphus spina-Christi and Balanites aegyptiaca(L.)Del),found in isolation,a total of 60 trees for this study.Under each selected individual tree,the species composition of USV were recorded using 1-m2 quadrat in four directions(north,south,east,and west)under the inside and outside tree canopies during the flowering stage.Similarly,soil samples in a 1-m2 quadrat were also collected under the inside tree canopies and their corresponding outside canopies,in each individual tree,for the determination of SSB composition,using a seed emergence method.Then,the soil was thoroughly mixed after removal of all roots and plant fragments,and spread over sand in plastic pots to a depth of 20 mm.The pots were placed at random in a glasshouse,examined every 3 days,for the first 2 months,and thereafter weekly for 6 months.A total of 960 soil samples were used for the determination of SSB composition during this study.Results:A total of 64 species were emerged from the SSB samples,of which 27 were grasses(19 annual and 8 perennial grasses),35 annual forbs and 2 woody species.Acacia robusta had a higher seedling density in the SSB compared to other tree species,whereas Z.spina-Christi had higher species diversity in the SSB than other tree species.Moreover,seedling density and species diversity were higher under the inside canopies than outside tree canopies.The mean similarity in species composition between the SSB and USV was low.However,it was higher under the leguminous trees than non-leguminous trees,and under the inside tree canopies than outside canopies.Conclusions:We found that mature tree species maintained a higher SSB species diversity and abundance under their canopies than the surrounding opened areas.Therefore,conservation of mature dominant tree species is of paramount importance for ecological stability and possible restoration of degraded semi-arid savannas under the changing climate and global warming.