Managing the tree layer
|Profile written by Gabriel Crowley|
|Last updated March 2010
Woody thickening in tropical grassy savannas is an increase in the density of resprouting tree saplings and shrubs. Woody thinning is when trees and shrubs decrease, creating a more open environment and often leaving dead trees standing for many years. There is much disagreement about how many trees and shrubs should be in a landscape. Many pastoralists prefer landscapes with fewer trees because this means more grass for grazing. However, as trees and shrubs provide wildlife habitat, many conservationists are more worried about woody thinning than they are about woody thickening. It is important to realise that landscapes are dynamic, that the density of tees and shrubs will change over time and from place to place. So there is no exact answer to how many trees or shrubs should be on any one patch of land, but there are situations where a change in tree and shrub density is a cause for concern.
Some landscapes are naturally heavily-treed, and others naturally open. Savanna landscapes, by definition, are dominated by the grass layer, but they may have few or many trees. Part of this variation is caused by the soil. Heavy clay soils tend to support fewer trees than do light sandy soils. Climatic conditions also control the density of woody plants. In northern Australia, seedlings germinate in the wet season, and natural thinning occurs as they struggle to survive the long dry season. So, new trees do not necessarily establish every year. Greatest recruitment is often after a particularly long wet season that enables the root system to get established before the onset of dry conditions. Much of today's thickened landscapes in Far North Queensland appear to have formed when two long wet seasons occurred back to back in the 1970s.
Fire is also an important factor. Seedlings are most likely to survive if they are not burnt in their first year. After that, most woodland trees have developed a strong underground root system or lignotuber, which helps them withstand the dry season. Subsequent fires may reduce them in height, but most plants with lignotubers can resprout again and again. Many shrub species also resprout and follow the same cycle. Others, like several wattle species, are killed by fire, but fire stimulates the germination of their soil-stored seed, producing a mass of seedlings that quickly turn the understorey into a shrubby thicket. These shrubs usually live for a few years then decline. Their periodic eruption is a natural occurrence and not irreversible ‘woody thickening’.
Interest in the density of trees and shrubs has increased in response to concerns over climate change caused by the release of carbon to the atmosphere. Woody plants are important for carbon storage, and their growth can therefore help offset carbon emissions from other sources. However, herbs, grasses, soil and woody plants all contribute to the total carbon store in an ecosystem, and increases in woody plants may sometimes be offset by decreases in the other carbon sources. Woody plant density is not a simple indicator of carbon stores.
Many northern landholders are concerned about woody thickening of their country. They feel that there are now more trees or shrubs than there were before. Pastoralists, in particular, may be concerned that thickening has reduced the carrying capacity of their land, so want to clear or burn to reduce the timber. Before undertaking vegetation thinning, it is important to consider the impacts on native plants and animals, especially on threatened species. This profile is designed to help you decide whether the trees and shrubs on your property need special management for wildlife conservation. While some parts of the landscape may be thickening, tree thinning caused by frequent and severe wildfire is a more significant problem across much of the Top End. In these areas, protection of the tree canopy, and especially of tree hollows, is essential for wildlife conservation.
The causes of woody thickening are complicated. Increasing carbon dioxide in the atmosphere may be helping the trees and shrubs to grow faster. Increases in annual rainfall that have occurred across north-western Australia in recent decades may also have accelerated tree growth rates. Tree cover also fluctuates over cycles of wet and dry conditions. Ironbark woodlands in central Queensland thinned out after many years of drought then recovered with the return of wet conditions.
However, two experiments from northern Australia demonstrate that fire management influences woody thickening in grazed landscapes, regardless of the effects of climate. The first experiment was on a well-drained landscape of black and red soils at Kidman Springs in the Victoria River District. There, tree cover is gradually increasing in grazed paddocks from which fire has been actively excluded. Adjacent paddocks with the same soils and grazing regime have been kept open by fires lit at any time in the dry season once every four to six years. Fires at any time of the year top-killed small trees and shrubs. However, seasonality of fire was mildly important, with later fires more effective at keeping the paddocks open than earlier fires.
Illustration: Gabriel Crowley
Fires lit at different times of the year have different effects on the suckering of Broad-leaved Paperbark. The later in the year fire is lit, the less time suckers have to grow before they are surrounded by grasses.
In all cases, though grazing continued, a healthy grass layer was maintained by not overgrazing. This is important because grasses are the main source of competition for woody plants. When they are actively growing, they compete with plants of all sizes for nutrients and water, and when they are dormant in the dry season, they physically suppress the growth of suckers and seedlings by denying them light and space.
Maintaining a healthy grass layer also seems to be the key to preventing woody thickening on seasonally flooded environments where sub-soil moisture is sufficient for woody plant growth all year round. In a study of grassy flats on Cape York Peninsula, the grass layer was found to have a significant impact on recruitment of Broad-leaved Paperbark. Burning or over-grazing removes this physical barrier, allowing small suckers to grow unimpeded until the next wet season rains restore the grass cover. The earlier in the dry season grass removal occurs, the taller the suckers will be by the end of the year. After a fire lit in June or July, small suckers burnt back to the rootstock can be close to a metre high and beyond the influence of the grass layer when it recovers. After an October or November fire, the resprouting suckers are unlikely to overtop the recovering grasses. Combined with the fact that late fire are more intense and do more damage to woody plants, this means that fires can either promote or prevent woody thickening, depending on when they are lit. Early fires thus shift the tree-grass balance in favour of trees and shrubs, and late fires shift it in favour of the grasses.
Photo: © Gabriel Crowley
Frequent early dry season burning and heavy grazing led to this once open grassy flat on Cape York Peninsula being invaded by Broad-leaved Paperbark
Burning at the very start of the wet season (storm-burning) has an intermediate effect. If both the fuel load and tree bark are damp, storm-burns tend not to scorch the canopy, but will knock back seedlings and suckers, without affecting mature trees and shrubs. Burning at storm-time also means the rapid recovery of grasses, which is enhanced by the nutrients released in the fire. So storm-burning promotes a park-like environment, with well-spaced trees and lush grass cover. Through much of south-eastern Queensland storm-burning is used by pastoralists because it ensures forage is available for most of the year.
The worst examples of woody thickening in Far North Queensland are in areas that have been repeatedly burnt in the early dry season, where cattle have then been allowed to graze out the resultant green-pick. This weakens the grasses, which are eventually shaded out altogether by the emerging trees. Areas whose names like Silver Plains and Pussy Cat Plains were inspired by their open grassy structure are now covered in paperbark thickets too dense to walk through. Such areas are now being restored using a combination of spelling from grazing and storm-burning.
Woody thickening appears to be more likely on floodplains and low-lying areas, where woody plants can draw on the high water table throughout the dry season. Thickening also tends to be less pronounced in the Northern Territory than it is in similar areas of Queensland that are more heavily grazed and less frequently burnt. This reinforces the importance of good land management practices to the maintenance of healthy vegetation structure.
In the Top End, the main woody thickening issues are the loss of grasslands to woodlands dominated by Broad-leaved Paperbark, and Rosewood thickening in grasslands of the Victoria River District. Over recent decades, several rainforest patches in the Top End have expanded into the adjacent open woodland. Though sometimes described as a form of vegetation thickening, this is really a change in vegetation community.
Another form of woody thickening of concern is the invasion of open areas by introduced prickle bushes (e.g. Prickly Acacia, Mimosa Bush, Parkinsonia and Mesquite) in localised areas of the Victoria River District, the Barkly Tablelands and other semi arid regions of the Northern Territory. If they are a problem in your selected area of interest, you will find discussions about these weeds elsewhere in the booklet.
Woody thickening can reduce the forage available for cattle grazing, but does not necessarily have a negative impact on wildlife. While tree thickening is a threat to Golden-shouldered Parrots in Far North Queensland, there are no documented cases of it threatening any species in the Northern Territory. However, changes in the pattern of tree and shrub density do affect the habitats available for native plants and animals, some for the better, others for the worse.
An increase in tree cover may shade out many ground dwelling plants, especially grasses, while other herbs may thrive where the grasses no longer dominate. Suppression of grasses can also lower fire risk, thus reducing fire frequency and intensity. This can have as much of an impact on wildlife as can the change in vegetation structure itself. Mistletoes and orchids may increase in abundance where there are more trees on which to grow, and flourish where there are few severe fires. Many animals prefer dense vegetation that provides protection from predators or ample food resources in the form of nectar, seeds or insects that live on leaves or under bark. Nectar, seeds and insects may also be most abundant in the absence of severe fire. Other animals prosper in an open landscape. Those that feed or nest on the ground, such as the Australian Bustard and Flock Bronzewing, prefer treeless landscapes that provide a good view of approaching predators, and where fire exposes seeds and insects on the ground.
Work in the Desert Uplands of central Queensland has identified a suite of birds favoured by thick vegetation, including Common Bronzewing, Australian Owlet-Nightjar, Little Friarbird and Rufous Whistler, and another that preferred more open areas including Crested Pigeon, Budgerigar and Yellow-throated Minor. Reptiles and mammals are likely to have a similar range of responses to vegetation structure. As some animals prefer open vegetation and others closed, the highest animal diversity will be in landscapes that have a range of densities.
For wildlife in the Top End, vegetation thinning is a more significant issue than is vegetation thickening, especially where grazing and burning practices have promoted the spread of Gamba, Mission, Guinea, Para or Aleman Grass. The high fuel loads of these robust introduced grasses lead to intense fires that damage the tree canopy, destroying the habitat of numerous species.
Even though vegetation thickening is not currently a major problem in the Northern Territory, this does not mean it will never become one. Experience in Queensland has shown that poor land management can convert open grassland into dense vegetation, sometimes seriously threatening wildlife. As with all threatening processes, prevention is better than cure. Adopting good management practices now could prevent irreversible damage to the vegetation and its wildlife and production values in the long term.
The above discussion demonstrates that fire is one of the most useful tools for the prevention of vegetation thickening. How fire should be used will depend on the type of country being managed. Early dry season fires may be effective at reducing vegetation thickening on well-drained country, but should be used with caution in seasonally flooded areas. Be aware that early dry season fires may promote thickening in these situations, especially if combined with heavy grazing. Some early burning will often be required for the creation of fire breaks. If so, it may be best to burn around the edges of these habitats when they are still wet to restrict the extent of the burns. Also, give the grass layer at least two to three years to recover before burning the same patches again.
Storm-burning can be used to maintain and restore healthy vegetation structure in seasonally flooded areas. Burning practices by pastoralists in south-eastern Queensland would suggest that better-drained country can also be effectively storm-burnt, but its impact on Rosewood thickening has yet to be tested.
When using storm-burns, these should be lit within a couple of days after the first wet season storm when the grass layer will recover rapidly and suppress woody plant growth. If storm-burns are lit at two to three year intervals to allow the build-up of an adequate fuel load, they top-kill woody plants up to 2 m high while leaving the canopy intact. However, fires should not be lit after the grass seed has germinated or the soil will be left bare through the wet season, and exposed to erosion. Again, over-grazing (with or without fire) should be avoided, as it can cause soil degradation and promote thickening.
Regular burning using the above practices can produce a "park-like" landscape with well-spaced trees and a grassy understorey. Suckers may persist in the grass layer, but their recruitment to the canopy will be retarded. Shrubs can be reduced by fires burnt in two consecutive years, and then kept at reasonable densities by burning small patches of country each year.
Follow the stocking rates recommendation for your land systems by your pastoral advisor to avoid over-grazing, and spell the country as often as possible in the early wet season to promote the regeneration of perennial grasses. Adjust your fire and grazing regime, or seek advice, if you see signs of woody thickening. Be equally aware of signs of vegetation thinning, such as numerous dead crowns or fallen trees. If you have introduced grasses on your property, manage these so that they do not convert diverse woodland into barren grassland, and do not let them invade adjacent ungrazed areas. Eliminate the unproductive introduced species, such as perennial Mission Grass, that increase fire hazard and fuel intense fires that may kill canopy trees.
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