Encroachment, thickening, desertification, densification, woody weed invasion. There’s no shortage of names for the process. But until lately there’s been a shortage of views on how dense regeneration by woody plants affects the world’s ecosystems.
The dominant view can be encapsulated by the simple adage: grass is great and shrubs are $#@%.
In many places, this view is entirely appropriate. If your business is raising livestock, then grass is your paycheck, and unpalatable shrubs a liability. But somewhere along the way, this view has been elevated to a universal reality. The (otherwise laudatory) Millennium Ecosystem Assessment, for example, appears to view bush encroachment as being synonymous with degradation and ‘desertification’. Regardless of where it occurs, encroachment = desertification.
To an ecologist in southern Australia – a region that urgently needs more trees and shrubs to address environmental problems – it seems extraordinary to suggest that woody plants that regenerate in areas cleared by humans represent a form of ‘desertification’. It also makes little sense to equate the process of encroachment with the valuation of degradation. The same process may be valued very differently depending on our land use goals. Encroachment may be lauded on one side of the fence for providing certain outcomes (e.g. habitat for birds) and lambasted on the other side for other outcomes (e.g. loss of grass).
Fortunately, a new review demolishes this inappropriate view.
The review was conducted by a global team led by David Eldridge from the University of New South Wales. They reviewed 144 scientific papers from around the world, to determine how encroachment alters a wide range of ecosystem properties, from soil carbon and nutrients to species richness of vertebrates, plants and ants.
Their findings are highly informative. For most of the ecosystem properties that they examined, encroachment had opposing impacts in different places. For example, encroachment reduced plant species richness (i.e. diversity) in 25 reviewed papers, had no impact in another 22 studies, and increased richness in a further 21 studies. Overall, it had consistent impacts on only 10 of the 43 properties that were assessed. For the other 33 properties, encroachment had opposing impacts in different regions, and no generalizations could be made.
One of the properties that was consistently affected by encroachment was grass cover; grass usually declines under encroaching trees and shrubs. This is not too surprising, since shrubs and trees capture resources (water and nutrients) that might otherwise support grasses. Given this result, its also not surprising that encroachment is viewed negatively by people whose livelihoods rely on grass.
Encroachment sometimes increases, sometimes decreases and often has no impact on most ecosystem properties, including things like the cover of cryptogams (mosses and lichens), available soil nitrogen and soil moisture. The researchers concluded that it was not feasible to view encroachment as being synonymous with degradation. In their words:
Our work demonstrates that a single interpretation of shrub encroachment as a form of degradation is not possible, and that many outcomes ranging from desertification to ecosystem enhancement may occur. (Eldridge et al. 2011)
This review raises many fascinating questions. A key goal of the science of ecology is to develop generalizations that apply over many ecosystems and regions. (In this light, “encroachment = degradation” can be seen as a generalization gone bonkers). Now we are faced with the question; if encroachment promotes soil respiration as often as it reduces it, then what drives contrasting outcomes? How can we predict the outcome? Do impacts vary because of attributes of the encroaching plants or the environments being encroached, or both?
This new review provides a wonderful springboard for our research in central Victoria, on how encroachment after agricultural retirement affects ecosystem properties. It seems reasonable to predict that encroached sites will have less grass than open paddocks, but most other bets can be thrown out the window. We still have a lot to learn about how regenerating trees and shrubs influence ecosystem properties and the environmental services that we desire.
References
Eldridge DJ, Bowker MA, Maestre FT, Roger E, Reynolds JF & Whitford WG (2011). Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis. Ecology Letters 14(7), 709-722. DOI: 10.1111/j.1461-0248.2011.01630.x
Ian Lunt Ecology 
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