My previous post on dense stands briefly described what they are, why they occur, and their effects on tree growth and understorey vegetation. Much of my research on dense stands has been focused on the response of understorey vegetation to the density and age of the stand, so here is a post on some of these results.
The area I surveyed for this study was the Box-Ironbark woodlands and forests of central Victoria. These vegetation types have been extensively cleared but now contain many sites that would be considered overly dense stands. I surveyed 64 sites with dense tree stands and combined this with an additional study of 46 sites in a similar area. I surveyed a range of understorey and overstorey vegetation attributes – if you would like to know about my survey method just let me know.
So firstly it is important to consider what densities I am talking about, and how much higher these are than what they ‘should’ be. The expected (or desired) density of trees in a given vegetation type can be estimated from existing high quality vegetation of that type. This was done for a range of vegetation types, including those in my study area, by Gibbons et al. 2010. They estimated ranges of densities for different tree sizes that are expected in a high quality site.
I compared the densities recorded from my data with the ranges in the Gibbons et al. report. This showed that the observed densities far exceeded the expected densities for many sites, particularly in the smallest size class 5-20 cm DBH. This allowed me then to examine the response of understorey vegetation to stem densities above, within, and below the expected natural levels.
I looked at the understorey vegetation cover in sites with the different stem densities in the smallest and most abundant size class (5-20 cm DBH). This showed that when stem densities were above the expected natural levels, the cover of native and exotic understorey plants was much lower than when densities were below or within those levels, and much lower than the levels expected for these same vegetation types (according to EVC benchmarks).
So what does this mean for dense eucalypt stands? This study showed that tree densities within the study sites were in many cases much higher than those expected in a ‘natural’ stand of that vegetation type. This was most evident in the smallest size class (5-20 cm DBH). Stem densities that exceeded that of known ‘natural’ stands had less than a fifth of the native understorey vegetation cover of other stands and a third of exotic cover.
So, if high stem densities suppress the understorey vegetation, is this a very bad thing for ecosystem value or function? How long does it last for, i.e. do the stands self-thin and return to ‘natural’ levels that then allow the understorey to return? If management is to be considered, what options are available and what is the likely response?
I will continue with some of these questions, with a particular focus on ecological thinning in my next post on this topic.
Gibbons P. et al. (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, 215-33.