Why don't grazed trees and shrubs indicate that reducing grazing pressure will lead to forest regeneration?

Australia's rangelands, which is where most human-induced forest regeneration (HIR) projects are located, are mostly shrublands, grasslands and open woodlands because their arid and semi-arid climates provide too little water to support more dense, forest vegetation. Forests tend to occur in areas where water is concentrated by run-off, like along gully’s and watercourses, and are most common in higher rainfall areas. 

Although the rangelands include areas that have been degraded through their use for grazing by domestic stock, as well as by introduced and native herbivores like goats, rabbits and kangaroos, the extent of forests in Australia’s rangelands today is not that different to when grazing was introduced, in most places more than a century ago. If anything, there is probably more forest, and more shrubby cover, in the rangelands now than at Federation. Grazing has certainly caused change, especially in the cover and composition of ground layer vegetation, like grasses and other herbs, as well as through degradation of the soil surface. But there is no evidence that grazing has removed forests over extensive areas of Australia’s rangelands.

Bulldozers and tree killing by ring-barking or poison, not cattle or sheep, are the primary way that land use for grazing has reduced Australia’s forest cover. And those changes have mostly occurred in higher rainfall regions. Rangeland vegetation is naturally open enough in most places to allow grazing without clearing. This is the first reason that many HIR projects will not deliver the forest regeneration they are being credited for, even if they include vegetation showing signs of grazing. The country they are managing is simply not capable of supporting permanent forest. Releasing small plants from grazing pressure will only increase mortality from other causes, especially drought. It generally won’t convert naturally open country to forest.  

As well as being generally dry, rangeland climates are also highly variable.This variability can drive changes in the amount of woody cover in rangelands over decades or even longer time frames. Woody cover increases following runs of good wet years, and is reduced when fire or droughts kill trees. HIR projects tend to be focussed on managing mulga, the dominant tree species in the rangelands. Mulga is palatable to stock, so smaller plants that are within reach of stock often show signs of grazing. Mulga can grow rapidly after rain, and is one of the toughest trees known to science, but it still dies in droughts. Importantly, the history of grazing in mulga regions shows that mulga regeneration occurs even when livestock and feral animals eat it, albeit at a slower rate.

This is the second reason why grazed shrubs do not indicate that reducing grazing pressure is necessary for forest regeneration. A century of experience with mulga in grazed systems has shown that tree cover has generally been maintained or increased under grazing. Under normal land management conditions, regeneration does occur, even though it may be slower. This means that the difference reduced grazing pressure can make is far less than the full-scale forest regeneration HIR projects are being credited for achieving.

Scientific trials with complete grazing exclusion, which is only feasible at small scales and is much more intensive than HIR projects can hope to achieve, do show increases in small plant density when grazing is removed, but they also show high mortality among larger trees. It is simply not safe to assume that signs of grazing impacting on young mulga mean that reducing grazing pressure will result in forests developing where they have not grown previously. Increases in the growth of small plants and herbs that may follow grazing removal will threaten the survival of carbon-dense older trees.

For a more detailed explanation of the ecology and environmental history at play here, please read Professor Don Butler's short article for a lay audience or Professor Rod Fensham's literature review. Professors Butler and Fensham are two of Australia's leading ecologists. Prior to his role at the ANU, Professor Butler served as the Chief Scientist of the Queensland Land Restoration Fund and led the Ecosystem Survey and Mapping program at the Queensland Herbarium. Professor Fensham is a professor in the School of the Environment at the University of Queensland. He is an academic specialist on the impact of grazing in the Australian rangelands and was commissioned by the Clean Energy Regulator to undertake this literature review, which upon receiving, decided not to accept. Professor Fensham's report contradicts a key basis for crediting under the HIR method.