As a side project to my PhD research, I have spent some considerable time over the last two and half years researching dense woody vegetation, or ‘thickets’. There has been recent interest in this area due to their increased occurrence in the landscape. This post gives a brief overview of dense stands.
What is a dense stand (thicket)?
A dense stand usually refers to patch of similar aged woody plants (often eucalypt trees) that are growing much more densely than would normally occur in an undisturbed ‘natural’ setting.
How do they arise?
There are four things usually required to create a dense stand:
- Space – cleared land free from heavy plant competition
- Source – a seed source such as a paddock tree or roadside vegetation
- Freedom – no/little grazing by insects or mammals
- Rain – sufficient rainfall to trigger a mass germination event
These are precisely the conditions that we see on agricultural pastures. Grazing animals will usually eat seedlings and so these stands won’t occur in heavily stocked pastures, but if the stock are removed or the numbers reduced and rainfall stimulates the mass recruitment of seedlings then these are ideal conditions for creation of dense stands. Some great examples of this were recently posted by Ian Lunt.
Dense stands are becoming more common in the landscape due to changes in land management practices. Some landholders are grazing less, some are fencing off parts of properties for environmental management (e.g. fencing off creeks), some are leaving and being replaced by ‘lifestyle’ farmers who are less focused on intensive agriculture.
What effects do they have? Are they bad?
The two primary reasons why land managers care about dense stands are: reduced tree growth, and suppressed understorey vegetation, both as a result of competition for resources.
Trees in dense stands grow much slower, have fewer large boughs, have smaller canopies, and produce fewer flowers later. Reduced tree growth is therefore an issue for the provision of resources by large trees, i.e. hollows, flower (nectar) production, and litter/logs.
Suppression of the understorey vegetation is a serious concern for provision of habitat, food, soil stability and structure, and amenity. If the understorey is suppressed for long enough, long-term detrimental effects can be caused by erosion of topsoil (not stabilised by plants) and loss of the soil seed bank (loss of recovery potential).
Naturally, for both of these issues with dense stands, there is a continuous scale of effects. ‘Some’ reduced tree or understorey growth from open stands might be ok, whereas ‘lots’ from very dense stands would be highly undesirable. Different environments will also respond differently to these stands. It is therefore very difficult to know how much is too much, how much is bad, and when we might want to do something about it. Studies that attempt to use good quality, relatively undisturbed, examples of vegetation to estimate what the vegetation ‘should’ be like in these systems (e.g. Phil Gibbons et al. in 2008 and 2010), can be useful for setting targets for management.
Do they self thin? Is there a need for management?
This is a great question and the answer comes down to time. Yes, dense stands do self thin, as competition for resources becomes so intense that some individuals die off. As some mortality occurs those resources are taken up by the remaining individuals. Eventually, this is likely to result in a more natural vegetation state, however this may take many decades or even centuries to occur. Management (such as thinning) is likely to be important if we want to achieve some desired state or attributes sooner.
In future posts on this topic I will present some of the research I have been doing with some people from the QAECO lab. Much of which focuses on the understorey suppression aspects of dense stands, and how it responds to management. Stay tuned…