Savanna (2)

Savanna
A savanna is a grassland ecosystem with scattered trees or shrubs. In savannas trees are small or widely spaced so that the canopy does not close . It is often believed that savannas are characterized by widely spaced, scattered trees, however in many savanna communities tree densities are higher and trees are more regularly spaced than in forest communities. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses. Savannas are also characterised by seasonal water availability, with the majority of rainfall being confined to one season of the year . Savannas can be associated with several types of biomes. Savannas are frequently seen as a transitional zone, occurring between forest and desert or prairie.
Although the term savanna is believed to have originally come from a Native American word describing " land which is without trees but with much grass either tall or short", by the late 1800s it was used to mean "land with both grass and trees". It now refers to land with grass and either scattered trees or an open canopy of trees.

Changes in fire management

Savannas are subject to regular fires and the ecosystem appears to be the result of human use of fire. For example Native Americans created subtropical savannas by periodic burning in some areas of the US southeastern coast where fire-resistant Longleaf Pine was the dominant species . Aboriginal burning appears to have been responsible for the widespread occurrence of savanna in tropical Australia and New Guinea and savannas in India are a creation of human fire use. The maquis shrub savannas of the Mediterranean region were likewise created and maintained by anthropogenic fire.
These fires are usually confined to the herbaceous layer and do little long term damage to mature trees. However, these fires do serve to either kill or suppress tree seedlings, thus preventing the establishment of a continuous tree canopy which would prevent further grass growth . Prior to European settlement aboriginal land use practices , including fire, influenced vegetation and may have maintained and modified savanna flora . It has been suggested by many authors that aboriginal burning created a structurally more open savanna landscape . Aboriginal burning certainly created a habitat mosaic that probably increased biodiversity and changed the structure of woodlands and geographic range of numerous woodland species. It has been suggested by many authors that with the removal or alteration of traditional burning regimes many savannas are being replaced by forest and shrub thickets with little herbaceous layer.
The consumption of herbage by introduced grazers in savanna woodlands has led to a reduction in the amount of fuel available for burning and resulted in fewer and cooler fires. The introduction of exotic pasture legumes has also led to a reduction in the need to burn to produce a flush of green growth because legumes retain high nutrient levels throughout the year, and because fires can have a negative impact on legume populations which causes a reluctance to burn.

Grazing and browsing animals

The closed forests 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.
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 and Stylo. These alterations in savanna species composition brought about by grazing can alter ecosystem function , and are exacerbated by overgrazing and poor land management practices.
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 soil 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.

Tree clearing

Large areas of savanna 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.
Clearing is carried out by the grazing industry in an attempt to increase the quality and quantity of feed available for stock and to improve the management of livestock. The removal of trees from savanna land removes the competition for water from the grasses present, and can lead to a two to fourfold increase in pasture production, as well as improving the quality of the feed available. Since stock carrying capacity is strongly correlated with herbage yield there can be major financial benefits from the removal of trees. The removal of trees also assists grazing management. For example in sheep grazing regions of dense tree and shrub cover harbours predators, leading to increased stock losses while woody plant cover hinders mustering in both sheep and cattle areas.
A number of techniques have been employed to clear or kill woody plants in savanna. Early pastoralists used felling and ringbarking, the removal of a ring of bark and sapwood, as a means of clearing land). In the 1950’s arboricides suitable for stem injection were developed. War- surplus heavy machinery was made available, and these were used for either pushing timber , or for pulling using a chain and ball strung between two machines . These two new methods of timber control, along with the introduction and widespread adoption of several new pasture grasses and legumes promoted a resurgence in tree clearing. The 1980’s also saw the release of soil-applied arboricides, notably tebuthiuron, that could be utilised without cutting and injecting each individual tree.
In many ways ‘ artificial ’ clearing, particularly pulling, mimics the effects of fire and, in savannas adapted to regeneration after fire as most Queensland savannas are, there is a similar response to that after fire. Tree clearing in many savanna communities, although causing a dramatic reduction in basal area and canopy cover, often leaves a high percentage of woody plants alive either as seedlings too small to be affected or as plants capable of re-sprouting from lignotubers and broken stumps. A population of woody plants equal to half or more of the original number often remains following pulling of eucalypt communities, even if all the trees over 5 metres are uprooted completely.

Exotic plant species

A number of exotic plants species have been introduced to the savannas around the world. Amongst the woody plant species are serious environmental weeds such as Prickly Acacia (Acacia nilotica), Rubbervine (Cryptostegia grandiflora), Mesquite (Prosopis spp.), Lantana (Lantana camara and L. montevidensis) and Prickly Pear (Opuntia spp.) A range of herbaceous species have also been introduced to these woodlands, either deliberately or accidentally including Rhodes grass and other Chloris species, Buffel grass (Cenchrus ciliaris), Giant rats tail grass (Sporobolus pyramidalis ) parthenium (Parthenium hysterophorous) and stylos (Stylosanthes spp.) and other legumes. These introductions have the potential to significantly alter the structure and composition of savannas worldwide , and have already done so in many areas through a numbers of processes including altering the fire regime, increasing grazing pressure , competing with native vegetation and occupying previously vacant ecological niches.

Climate change

There exists the possibility that human induced climate change in the form of the greenhouse effect may result in an alteration of the structure and function of savannas. Some authors have suggested that savannas and grasslands may become even more susceptible to woody plant encroachment as a result of greenhouse induced climate change. A recent case described involved a savanna increasing its range at the expense of forest in response to climate variation , and potential exists for similar rapid, dramatic shifts in vegetation distribution as a result of global climate change, particularly at ecotones such as savannas so often represent.