Ecologically unsustainable anthropogenic activities such as agriculture, mining, and oil exploration, coupled with climate change and variability, facilitate plant species invasions. This is exacerbated by increases in the frequency and extent of natural disturbances such as droughts, hail storms, fire, insect outbreaks and disease in boreal forests which upset ecological and economic balances, perpetrating invasions (IPCC 2007). The Convention on Biodiversity (CBD) targeted ‘to achieve by 2010 a significant reduction of the current rate of biodiversity loss at the global, regional, and national level as a contribution to poverty alleviation, and to the benefit of all life on Earth’ (SCBD 2010). This facilitated development of policies to preserve biodiversity including elimination or control of alien invasive species. Most of the studies on plant species invasion ecology focus on these alien invasives, yet the control of exotic and native species by the same ecological processes may explain the positive correlation often found between exotic and native species richness (Lonsdale 1999).

Dichrostachys cinerea is a deciduous low thorny tree or shrub that produces bicoloured fragrant flowers. It is native to Africa and Asia, where it is common in the tropics of Africa, the Sahelian and Sudanian ecozones, and the South Arabian Peninsula (TTPC 2010). Both old and young Dichrostachys cinerea plants can produce many viable seeds annually which can survive for more than 5 years in the soil. This increases propagule pressure of the species. Its infructescence has a strong aroma, and this probably attracts animals to feed on the pods, facilitating seed dispersal. The species also forms impenetrable thickets that are difficult to eliminate due to high propagation capacity and propagule pressure of Dichrostachys cinerea. Increased grazing and trampling pressure around boreholes was shown to cause a species-poor zone in the immediate vicinity of boreholes, followed by thickets with Acacia spp. and Dichrostachys cinerea, transitional to tree savanna (Tolsma et al. 1987a).

Collection and grinding of Dichrostachys cinerea fruits have been suggested as a management tool to control its encroachment (Mlambo et al. 2004). Chemical ways are often costly, and most herbicides are dangerous and have residual effects. Manual control is labour intensive and also less effective since new seedlings and coppicing replace the mechanically removed Dichrostachys cinerea plants. The main aim of this study was to assess the effects of Dichrostachys cinerea on herbaceous species diversity, richness, basal cover, litter cover, plant vigour and top hamper, determining the major herbaceous characteristics of invaded sites and the extent to which invaded and uninvaded sites differ. An analysis of the invasive capacity of Dichrostachys cinerea on rangelands will contribute immensely to its management and control, hence achievement of the 2010 Convention on Biodiversity (CBD) target, which was also incorporated as a new target in the Millennium Development Goals (MDGs).

Herbaceous variables

Dichrostachys cinerea invasion has caused declines in herbaceous species richness. Competitive interactions are critical between woody plants and grass dominated layers in arid to semi-arid areas (Jacoby et al. 1982). Dichrostachys cinerea may be competing with the native herbaceous species for resources such as light, nutrient and moisture. Cynodon dactylon was one of the most abundant herbaceous species. It is very competitive, particularly in fertile soils, and only aggressive legumes are capable of forming an association with it (Harlan and de Wet 1969). This explains its occurrence in invaded sites. High litter deposition by a dominant plant species can also modify competition; suppress competing plants, and lower plant species richness (Xiong and Nilsson 1999). Dichrostachys cinerea may have reduced herbaceous species richness through litter deposition or due to its vigorous growth characteristic, extensive and dense root system that is important in propagation and recruitment as compared to native species. Uninvaded sites also recorded higher species diversity indices (Shannon Weiner and Simpson`s indices) than the invaded sites. Due to occurrence of certain species only unique to uninvaded sites in the absence of the suppressive nature of Dichrostachys cinerea on other species, uninvaded sites had higher species richness than the invaded sites, hence higher Shannon-Weiner index.

In a study by Tolsma et al. (1987b), there was less than 10 % ground cover by herbs and grasses in Dichrostachys cinerea thickets, with the most frequent species being annual herbs like Boerhavia diffusa and weedy species such as Acanthospermum hispidum, the latter of which was only found in invaded sites in the present study. Dense thickets such as those that are formed by Dichrostachys cinerea can result in a decrease in carrying capacity through loss of grass cover caused by replacement and by competition for limited resources. In studying an invasive species of the genus Prosopis, although there were certain grasses that were adapted to shade conditions, there were others which were shade intolerant and were thus inhibited by competition with species of genus Prosopis (Jacoby et al. 1982). Philodendron maximum grows better at 30 % shade although yields are reduced by half at 50% shade (Harty et al. 1983). In the present study Philodendron maximum was found in uninvaded sites which had lower canopy cover than the Dichrostachys cinerea invaded sites, hence less shading. The reduced abundance of herbaceous species in this study could also be due to the negative impacts of increased Dichrostachys cinerea canopy cover which may affect shade-intolerant species. Light deprivation may have affected basal cover, plant vigour and richness of herbaceous species noted in invaded sites as compared to uninvaded sites. The occurrence of Heteropogon contortus under the dense Dichrostachys cinerea thickets is also in consistency with findings that Heteropogon contortus tolerate light shading, often dominating the understorey of Eucalyptus woodlands in tropical and subtropical Australia (Bhatt et al. 2006). While standing litter may protect seedlings from desiccation and high temperatures, it may also reduce light availability (Käller 2003) hence affect growth and production of other herbaceous species resulting in the observed lower herbaceous basal cover of invaded sites than that of uninvaded sites. Rapid root suckering of Dichrostachys cinerea in propagation as well as high seed germination also make the specie a more successful invader with better plant vigour than the native herbaceous species.

In the present study there were significant differences in environmental variables between invaded and uninvaded sites. Invaded sites had higher soil N, K and P than uninvaded sites. Biological invaders change ecosystems as they differ from native species in resource acquisition and/or resource use efficiency. They may also alter the trophic structure of the area invaded, or the disturbance frequency and/or intensity (Vitousek 1990). The fluctuating resource availability theory states that plant species invasions are associated with increases in resources (Davis et al. 2000). Dichrostachys cinerea is a nitrogen fixing shrub. The effect it has on soil could have been translated into herbaceous species composition. The decline in species richness means that native species are displaced by Dichrostachys cinerea, resulting in a decrease in nutrient sequestration, hence increases in N, P and K observed in the invaded sites. Altitude also varied significantly among invaded sites. Dichrostachys cinerea can occur on a wide range of altitude.

Eragrostis spp., Hibiscus spp., Sida alba and Urochloa mozambicensis were found in the Dichrostachys cinerea invaded sites. These grasses are associated with disturbed land (Wild 1972). Disturbance is one of the major factors affecting species invasions (Davis et al. 2000). The Eragrostis spp. were positively correlated with erosion and negatively correlated with K. Eragrostis viscosa is an indicator of poor soil conditions such as is characteristic of eroded land (Wild 1972). Eragrostis chapelieri is also common on sandveld and it has poor forage value. Eroded soils are low in soil nutrients such as N, P and K. Erosion mostly affects the topsoil which provides the main nutrients needed by plants. Urochloa mozambicensis is usually found in wooded grassland and deciduous bushland, or on disturbed sites where the soil is fertile (Burt et al. 1980). This explains the positive correlation of the species with potassium in Dichrostachys cinerea invaded sites. Stylosanthes guianensis can extract P very efficiently from low P soils, but still responds to applications of P, as well as K, sulphur and calcium in soils with low levels of these nutrients (Chakraborty 2004). The species was positively correlated with K. Cynodon dactylon was one of the most abundant herbaceous species. It grows on a wide range of soils, but best in relatively fertile, well-drained soils. It was correlated with N. It is very competitive, particularly in fertile soils.

The herbaceous species that were negatively correlated with the measured environmental variables were Aristida spp., Philodendron maximum, Pogonarthria squarrosa, Digitaria penzii, Hyparrhenia filipendula, Hyperthelia dissoluta and Rhynchelytrum repens among other species. Philodendron maximum grows best in moist, well-drained soils just like Rhynchelytrum repens, although some of its varieties are tolerant of lower fertility and poorer drainage (Harty et al. 1983, Wild 1972). The species was found in uninvaded sites which had lower canopy cover than the Dichrostachys cinerea invaded sites, hence less shading. The correlation of Pogonarthria squarrosa with uninvaded sites is in consistency with the findings of Wild (1972) who report that the species is a normal constituent of grassland on sandy soils. The Aristida spp. are drought resistant perennial grasses that grow in poor, gravely soils and also on clay soils.

Dichrostachys cinerea is a native invasive woody shrub or tree. Here, it adversely affected native herbaceous species plant vigour, basal cover and species richness. Reduced litter cover and top hamper were also observed in invaded sites. These observations can be attributed to the fast growth, propagation and propagule pressure that characterize Dichrostachys cinerea, giving it a competitive advantage with respect to acquisition of light, nutrients and other resources. Therefore, these adverse effects on herbaceous species may, in the long term, reduce the carrying capacity of rangelands, making them even more susceptible to alien invaders. However, a long-term study may provide more information on the biology and invasive capacity of the species and on its impact on other vegetation and animal species. There is also need to find environmentally friendly and effective methods of controlling Dichrostachys cinerea inorder to prevent its spread, hence adverse effects on herbaceous species.