Understanding dense eucalypt stands and the pros and cons of thinning

Figure 1. Dense regrowth of grey box (Eucalyptus microcarpa) on a grazing property in central Victoria. (Photo by Chris Jones)

Stands of dense woody regrowth are increasing in extent across Australia and around the world, and that raises many questions on how they should be managed. What’s their value and should we leave them alone or actively thin them?

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 (see further down, on valuing woody regrowth). 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 (see Figure 1). These stems grow more slowly than in natural systems due to competition for resources. And this competition also suppresses the understorey vegetation, which was the focus of our research.

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.

“Thinning is a viable option but it’s not a silver bullet. It will not produce desired outcomes for all sites.” 

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 biodiversity and what is the benefit of thinning? But perhaps more importantly, we need to ask whether thickets pose a problem that warrants major investment from government. At what scale would thickets need to be a dominant form to cause concern for those species, and communities, and at what scale is the treatment cost effective?

A dense stand of eucalypts in central Victoria. (Photo by Chris Jones)

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 (Jones et al., 2015). 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 published benchmarks of stem density and understorey vegetation cover (Gibbons et al., 2010).

We found that stands with stem density greater than benchmark levels suppress native understorey vegetation cover below its benchmark levels. Thinning stems can restore native understorey vegetation (richness and cover) in the short term, providing 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 result in producing a negative outcome for a native understorey. Land tenure and environmental factors also influence the response of stands to thinning treatments and should be considered before thinning is applied.

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.

A thinned eucalypt stand showing the recovery of thick understorey vegetation some 6 years after thinning. (Photo by Chris Jones)

So, to thin or not to thin? That is the question – but what’s the answer? Excessively dense stands (more than benchmark) 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 isn’t only determined by the expected ecological outcome at the scale of a site. It also depends on how much it costs. Many questions remain about the cost-effectiveness of thinning for managing dense stands, both because the uncertainties have not been resolved and the objectives aren’t clear.

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 conservation at larger scales.

Valuing dense woody regrowth

Woody regrowth down a grazed hillside in Spain. In Europe dense woody regrowth frequently follows land (farm) abandonment and is generally regarded negatively. (Photo by A Moran-Ordonez)

The ‘value’ of dense woody regrowth is typically determined by its cultural and ecological context and origin. In Europe, dense woody regrowth frequently follows land (farm) abandonment. Therefore it is generally regarded negatively, having replaced ‘cultural’ grassland meadows maintained for centuries by clearing and grazing. Similarly, woody encroachment into natural savannas and grasslands following changes to grazing or fire regimes can reduce the productive capacity of grazing land.

On the other hand, in the Neotropics, the structural and functional attributes of dense regrowth can be ecologically similar to forest that existed there prior to clearing. Dense woody regrowth in these situations represents a desirable state.

Elsewhere, there can be considerable nuance where dense stands are defined in comparison to benchmark or reference states that are thought to have existed prior to the post-industrial period of human impact.

Dense stands are generally considered ecologically undesirable compared to stands with benchmark density. In Australia and North America, these benchmarks typically represent vegetation states predating European arrival. In Australia for example, spontaneous woody regrowth is positively received on land no longer managed for production purposes while the plants are young but there is concern about the biodiversity and habitat value of these simplified stands as they age. Dense woody regrowth is considered problematic because it is presumed to retard or exclude desirable biodiversity and habitat values.

More info: Christopher Stuart Jones csjones@unimelb.edu.au 

References 

Gibbons P, SV Briggs, DY Murphy, DB Lindenmayer, C McElhinny & M Brookhouse (2010). Benchmark stem densities for forests and woodlands in south-eastern Australia under conditions of relatively little modification by humans since European settlement. Forest Ecology and Management 260, 2125-33.

Jones CS, DH Duncan, L Rumpff, FM Thomas, WK Morris & PA Vesk (2015). Empirically validating a dense woody regrowth ‘problem’ and thinning ‘solution’ for understory vegetation. Forest Ecology and Management doi:10.1016/j.foreco.2014.12.006