This chapter reviews ecological site-classification and mapping of the central Borana Plateau and aspects of environmental change induced by pastoral land use. A secondary objective is to highlight the use of native vegetation by pastoral households and livestock. An ecological map (at a scale of 1:500 000) is presented that integrates soils, vegetation and climate in defining six agro-ecological zones for a 26 600-km² area. The zones include subhumid (6.5 % of the region), upper semi-arid (22.4%), lower semi-arid (40.3%), arid (19.2%) and bottomlands (11.6%). Zones vary in resource dynamics and management. For example, while the subhumid zone has recently been stable in terms of the area of forests, the upper semi-arid zone has been vulnerable to grazing-induced bush encroachment. Roughly 16 species of woody plants have been implicated as encroachers in the southern rangelands. Traditional pastoral units of resource allocation (or madda) vary in extent of zonal diversity. Three of 29 madda contain six zones while 10 others contain three zones or less. By the mid-1980s about 40% of the study area had experienced significant bush encroachment while erosion, attributable to grazing pressure, affected 19% of the study area. These trends are coincident and most apparent in the upper semi-arid regions that have hilly relief and in the vicinity of permanent water development where pastoralists have become more sedentary. Less than 2% of the study area was under small-scale cultivation in 1986 following the 1983-84 drought and this was concentrated in bottomlands and upland sites in subhumid and upper semi-arid zones.
It is postulated that variation in the long-term grazing history of madda can be assessed using inventories of woody plant populations. It is hypothesised that heavy grazing of herbaceous plants by cattle increases woody cover by reducing fire risk. For example, sites currently enduring heavy grazing pressure could be identified by high densities of very young woody plants, while sites that had heavy grazing pressure in the past that was discontinued several decades ago could be identified from moderate to high densities of mature woody plants. It is contended that the Boran used to exploit the rangeland in terms of a sustainable patch dynamic, but that this has recently been compromised by high population growth. The patch dynamic may have consisted of the following stages: (1) pastoralists settled around a group of wells characterised by mixed savannah vegetation; (2) cattle depleted the local environment by over-utilising the grass layer and transporting nutrients to corrals at encampments; (3) woody plants invaded as competition from grasses was lessened and fire risks reduced; (4) negative change in vegetation for cattle encouraged pastoralists to abandon the area; (5) maturing stands of woody plants restricted access by herbivores and added nutrients to soils through leaf litter; and (6) the grass layer gradually recovered to an extent where fire risk increased and fires re-established mixed savannah vegetation by thinning stands of woody plants. It is at this stage that the site could be re-occupied by pastoralists; it has been speculated that this cycle could take 60 to 100 years to complete.
Comparative analyses of upland sites subjected to continuous grazing versus those protected for seven years suggest that perennial grasses are relatively resilient in terms of cover and productivity in response to grazing. The major effect of continuous grazing over the short-term appears to be encouragement of fortes that probably have a lower grazing value for cattle. Other studies indicate that it is difficult to generalise concerning interactions among woody and herbaceous vegetation due to effects of species, woody plant density and site. Tree removal experiments suggest that some woody species can reduce herbaceous production over 50%. Other research indicates that herbaceous cover can increase under canopies of other species of large trees. Local informants report that a moderate degree of woody encroachment can be beneficial to the diversity of the grazing system overall, but once encroachment becomes advanced it usually has negative consequences for pastoralism. During the 1980s the Boran noted that they were less able to burn the rangelands than in earlier times because of Government policy prohibiting burning; and the need to use standing herbage as forage, not fuel, because of high numbers of cattle. The Boran commonly offer the view that range trend is declining as a result of high stocking rates. Higher densities of people also reportedly preclude the freer movements of households that characterised previous generations.
Studies of the population ecology of two encroaching woody species (Acacia brevispica and A. drepanolobium) indicate that: (1) large quantities of seeds are produced during the warm, dry season (i.e. November through March); (2) seed pools are largely at the soil surface; (3) despite substantial seed production, recruitment of seedlings is very low probably due, in large measure, to parasitism and predation on seeds by insects; and (4) seeds vary in terms of ecological cues required for germination. Acacia brevispica seeds may be stimulated to germinate by fire. Seeds of both species germinate more in response to additional moisture and shallow planting depths (<3 cm); neither species appears to rely on maintaining a large seed pool in the soil. Seedling establishment is probably thus an episodic phenomenon dependent on coincident factors related to grazing pressure, insect populations and rainfall in the current year of seed production.
Analysis of livestock food habits in the upper semi-arid zone indicates that cattle and camels are exclusively grazers and browsers, respectively. Sheep (with 36% dietary browse) and goats (84% dietary browse) are mixed feeders. Only two of 29 browse species provided most of the browse forage overall and one (A. brevispica) is commonly regarded by range managers as an encroacher. Other important species that should receive development attention as forage resources include browses such as Euclea shiperi, Dichrostachys cinera, Rhus natalensis, Pappea capensis, Acacia etbaica, Grewia tembensis, G. bicolor, Ormocarpum mimosoides, Acacia tortilis, Balanites spp, Cadaba farinosa and Capparis tomentosa.
Browse species vary markedly in morphology and concentrations of tannins and other anti-nutritional factors for livestock. Goats, camels and sheep appeared to select against browses with higher levels of phenolic compounds especially during wet seasons when forage abundance and diversity were greatest. A variety of native grasses are also critical for the production system. Grasses important for calf feeding include Pennisetum, Chrysopogon, Cynodon and Cenchrus spp. Native plants have many uses as human food, medicine, construction materials and other cultural roles. A list of 114 plant species and their uses in the Borana household economy is provided. Few common species have no utility. Woody species that are increasing in the environment and apparently have low value for forage and household use include Acacia drepanolobium, Albizia amare, Acacia horrida and A. mellifera.
