Vegetation Classification

Structural classification of vegetation systems

Botanists have long used the Linnaean system of classification, giving names to individual species by using their genus and their specific name (binomial nomenclature), such as Eucalyptus regnans (Mountain Ash).

A hierarchy of taxonomic relationships is established with a species (e.g., E. regnans) belonging to a genus (Eucalyptus), which belongs to a family (Myrtaceae), which belongs to an order, etc.

This is well and good, but poses a problem in Australia in trying to understand something about entire plant communities: if you focus on the botanical names of individual species (for more than 20,000 'higher' plants alone), you are unlikely--unless you are a dedicated botanist--to be able to have the time or brain power to identify anything more than a few of the key species in a limited number of ecosystems.

Moreover, a focus on species literally risks not being able to 'see the forest for the trees', as a species-approach to understanding ecosystems may miss the vital point about ecosystems: that they are more than just the sum of their individual species.

Furthermore, even highly detailed and extensive knowledge about Australian species is likely to leave you ignorant about species in other parts of the world, and therefore unable to make meaningful comparisons about ecosystems here compared to elsewhere.

In order to deal with these limitations to the Linnaean system, a method of classifying vegetation communities based upon easily recognizable features of the communities as a whole has been developed.

It is known as the structural classification of vegetation systems.

The ecologist Ray Specht developed a system using this approach which has become widely used both in Australia and overseas.

It is based on three elements:

·  First, identify the lifeform of the tallest stratum (or layer) of plants in an area (e.g., trees, shrubs, hummock grasses or other herbs).

·  Second, determine the height and/or type of vegetation in the tallest stratum (e.g., ‘trees over 30m’, or ‘shrubs, non-sclerophyllous’, etc.).

·  Third, determine the projective foliage cover of the tallest stratum. This is the percentage of area which is covered by that foliage (leaves).

For example, in a tropical rainforest (Tall Closed Forest), the projective foliage cover will normally be about 90-100%, while in a mallee community (Tall Open Shrubland), this coverage is usually less than 10%.

Table 1 below gives a version of Specht's system.

Table 1: Structural Classification of Vegetation

Projective Foliage Cover of the Tallest Stratum

Life form of tallest stratum / 70-100% / 50-70% / 30-50% / 10-30% / <10%
Trees >30m / Tall Closed Forest / Tall Forest / Tall Open Forest / Tall Woodland / (N/A)
Trees 10-30m / Closed Forest / Forest / Open Forest / Woodland / Open Woodland
Trees <10m / Low Closed Forest / Low Forest / Low Open Forest / Low Woodland / Low Open Woodland
Shrubs >2m / Closed Scrub / Scrub / Open Scrub / Tall Shrubland / Tall Open Shrubland
Shrubs (S) 0.25-2m / Closed Heathland / Heathland / Open Heathland / Shrubland / Open Shrubland
Shrubs (NS) 0.25-2m / (N/A) / (N/A) / Low Shrubland / Low Shrubland / Low Open Shrubland
Shrubs (S) <0.25m / (N/A) / (N/A) / (N/A) / Dwarf Open Heathland / Dwarf Open Heathland
Shrubs (NS) <0.25m / (N/A) / (N/A) / (N/A) / Dwarf Shrubland / Dwarf Open Shrubland
Hummock grasses / (N/A) / (N/A) / (N/A) / Hummock Grassland / Open Hummock Grassland
Tussock grasses / Closed Grassland / Grassland / Grassland / Open Grassland / Very Open Grassland
Sedges / Closed Sedgeland / Sedgeland / Sedgeland / Open Sedgeland / Very Open Sedgeland
Herbs (forbs) / Closed Herbland / Herbland / Herbland / Open Herbland / Very Open Herbland
Ferns / Closed Fernland / Fernland / Fernland / (N/A) / (N/A)

Based on Specht, cited in Recher, H., Lunney, D. & Dunn, I., 1986, A Natural Legacy: Ecology in Australia, p. 106

(S)—sclerophyllous (NS)—non-sclerophyllous (N/A)—not applicable (doesn’t occur naturally)

Specht’s system identifies about 50 categories of vegetation systems in Australia.

Many of these are of very limited occurrence, for example, Closed Fernland. Other vegetation systems such as Open Forest, Woodland, Hummock Grassland and various Shrublands are widespread.

While at first glance this system may seem complicated, you only have to be able to recognize trees, shrubs, hummock grasses (spinifex), tussock grasses (e.g., Mitchell Grass), forbs ('flowers', herbs that aren't grasses), sedges (grass-like in appearance, but not true grasses) and ferns.

With this done, you can make a quick classification of a plant community into one of the following:

·  forest/woodland

·  scrub/shrubland/heathland

·  grassland/sedgeland

·  herbland, or

·  fernland.

Then you can assess more detail, such as projective foliage cover, height of the tallest stratum, and (for shrub communities only) sclerophyllous/non-sclerophyllous.

The term 'sclerophyll' means 'leather-leafed'. Plants with this leaf characteristic have relatively 'hard' leaves--quite different in texture from the softer leaves of many Northern Hemisphere plants such as maples (the emblem on the national flag of Canada), elms, and other broad-leafed species that enjoy predictably humid climates.

Sclerophylly is a mechanism to help reduce water loss from evapo-transpiration from the leaf-surface.

With these in mind, you can classify a plant community without knowing anything about the species present (in terms of their binomial names).

You can do this not only in Australia, but in most other parts of the world as well.

And since structural features are related to climatic and other environmental conditions (e.g., heathlands are often an indicator of extremely infertile soils), then you can start to build up a broader picture of the nature of the ecosystem you are viewing.

Here are two examples of how to use the table to classify a plant community.

In this view near Kata Tjuta in the N.T., we can see the life-form of the tallest stratum is small trees: trees not shrubs, because these have a single stem (trunk or bole), rather than the multiple stems or branches close to the ground that characterize shrubs. [These trees are in fact Desert Oak, a member of the Casuarina genus.]
The height of these trees is less than 10 m.
The projective foliage cover is (in the area where the trees are most prominent) in the 10-30% range.
Therefore we can (by checking with Table 1 above) classify this community as Low Woodland.
It is only a small patch of Low Woodland because slope/soil changes and fire regimes have created a mosaic of small patches of different communities in this region.
In this photo (near Alice Springs), the life-form of the tallest stratum in the foreground is shrubs (this is despite the couple of small trees on the left in the mid-distance: the odd exception to the 'tallest stratum' is usually ignored). [These shrubs are known as 'umbrella bushes' because kangaroos and other animals use them for shade on hot days.]
The shrubs are less than 2 m in height, but are more than 0.25 m in height.
They are sclerophyllous (you'll have to take my word for that, but most Australian shrubs are), and have a projective foliage cover of 50-70%.
Therefore this community would be classified as Heathland.
You might again note how on the slopes in the distance, the plant community changes radically due to different soils and fire regimes.
Try identifying the type of vegetation community based on what you can see in the photo below. What you can see in this close-up view is what you see over an extended area in this community.
Try to classify it yourself, and then check your result with the answer below.

Tussock Grassland

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