To Thin or Not to Thin

By Chris Jones

Stands of dense woody regrowth are increasing in extent across Australia and around the world. The effect of dense stands and thinning on tree growth is well understood but the impacts on the understorey are not.

Dense woody regrowth commonly pops up on cleared land where there has been some change in land use, usually a reduction in grazing pressure. In some places, this regrowth is considered a bad outcome.

In parts of Europe, for example, the grasslands that may have been grazed for centuries are considered valuable for biodiversity in their own right. In Australia, on the other hand, woody regrowth is often considered a good outcome for biodiversity as it represents a transition back to the pre-cleared vegetation state.

However, it is common for these regrowth stands to be much denser than undisturbed forest. They are often structurally simplistic with a high density of similar-sized stems. These stems grow more slowly than in natural systems due to competition for resources, and this competition also suppresses the understorey vegetation.

In Victoria there is an increasing call for management of dense eucalypt stands on both private and public land. The most commonly cited management option is thinning – cutting down a proportion of stems and applying herbicide to prevent regrowth. The theory is that the release from competition should make the remaining stems grow faster, larger and broader, as well allowing the recovery of understorey vegetation.

Self-thinning does occur in these systems and, given enough time, dense stands are generally expected to improve in quality. However, this is far slower than with active intervention. At the most basic level, the questions for managers then are:

How bad is a dense stand for understorey biodiversity and what is the benefit of thinning?
Do thickets pose a problem that warrants major investment from government?

We sought answers to these questions using data from two separate field projects conducted in box-ironbark woodlands and forests in central Victoria, where we evaluated the vegetation structure of dense regrowth stands of eucalypts, and the effect of thinning management. In order to determine what density of stems and cover of understorey “should” be expected in natural systems, we evaluated our results in relation to benchmarks of stem density (from existing research literature) and understorey vegetation cover.

We found that stands with stem density greater than benchmark levels suppress native understorey vegetation cover. Thinning stems can restore native understorey vegetation (richness and cover) in the short term as long as the soil seedbank has not been removed and there is no excessive grazing. This is the desired outcome from thinning, but the catch is that both native and exotic species can recover following thinning.

In places that were weedy prior to the dense stand forming, or are adjacent to highly weedy areas, thinning could produce a negative outcome for a native understorey.

Land tenure and environmental factors also influence the response of stands to thinning. For example, sites on freehold land, which typically indicates a history of dryland agriculture, had lower species richness and fewer native shrub counts than Crown land. Crown land, on the other hand, was less likely to have been grazed, cleared or otherwise intensively managed.

The increase in cover and richness of exotic species and decrease in native shrub counts at sites adjacent to a road reflects a land use history of disturbance without intensive grazing, but one still prone to exotic species invasion.

So, to thin or not to thin? Excessively dense stands are bad news for understorey vegetation. Given that dense stands tend to stay dense for a very long time without intervention, management may be a valuable activity. Thinning is a viable option but it’s not a silver bullet. It will not produce desired outcomes for all sites.

Of course, whether a land manager should thin or not also depends on how much it costs. Many questions remain about the cost-effectiveness of thinning for managing dense stands. With limited resources to manage conservation problems, being confident that thinning can improve habitat characteristics is not enough to justify a campaign of publicly funded thinning of dense woody regrowth. A better understanding of how and when dense woody regrowth develops, and how it is distributed spatially, would help to consider the merit of thinning proposals alongside other options to improve biodiversity.

Chris Jones is a member of the Environmental Decisions Group. He is based at the University of Melbourne.

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