<P> The closed forest types such as broadleaf forests and rainforests are usually not grazed owing to the closed structure precluding grass growth, and hence offering little opportunity for grazing . In contrast the open structure of savannas allows the growth of a herbaceous layer and are commonly used for grazing domestic livestock . As a result, much of the world's savannas have undergone change as a result of grazing by sheep, goats and cattle, ranging from changes in pasture composition to woody weed encroachment . </P> <P> The removal of grass by grazing affects the woody plant component of woodland systems in two major ways . Grasses compete with woody plants for water in the topsoil and removal by grazing reduces this competitive effect, potentially boosting tree growth . In addition to this effect, the removal of fuel reduces both the intensity and the frequency of fires which may control woody plant species . Grazing animals can have a more direct effect on woody plants by the browsing of palatable woody species . There is evidence that unpalatable woody plants have increased under grazing in savannas . Grazing also promotes the spread of weeds in savannas by the removal or reduction of the plants which would normally compete with potential weeds and hinder establishment . In addition to this, cattle and horses are implicated in the spread of the seeds of weed species such as Prickly Acacia (Acacia nilotica) and Stylo (Stylosanthes spp .). Alterations in savanna species composition brought about by grazing can alter ecosystem function, and are exacerbated by overgrazing and poor land management practices . </P> <P> Introduced grazing animals can also affect soil condition through physical compaction and break - up of the soil caused by the hooves of animals and through the erosion effects caused by the removal of protective plant cover . Such effects are most likely to occur on land subjected to repeated and heavy grazing . The effects of overstocking are often worst on soils of low fertility and in low rainfall areas below 500 mm, as most soil nutrients in these areas tend to be concentrated in the surface so any movement of soils can lead to severe degradation . Alteration in soil structure and nutrient levels affects the establishment, growth and survival of plant species and in turn can lead to a change in woodland structure and composition . </P> <P> Large areas of Australian and South American savannas have been cleared of trees, and this clearing is continuing today . For example, until recently 480,000 ha of savanna were cleared annually in Australia alone primarily to improve pasture production . Substantial savanna areas have been cleared of woody vegetation and much of the area that remains today is vegetation that has been disturbed by either clearing or thinning at some point in the past . </P>

What kinds of plants live in the savanna