Forest types and vegetation patterns in New South Wales

Forest Types

Rarely when you visit a native forest will you see only one kind (species) of tree. If you stop and look around, you will see a large number of different species of trees growing close together in a small area.

Particular plants, including trees, grow in particular places because the environmental factors favour them living in those places. For example, if you go up onto the sandstone plateaux north of Sydney, to Terrey Hills, Frenchs Forest or around Hornsby, you will find a rather low, open kind of forest made up of scribbly gum, red bloodwood and white stringbark trees and many smaller plants. The same kind of forest can be found south of Sydney and on the lower parts of the Blue Mountains to the west of Sydney. This particular group of trees, including the other plants that often grow with them, are suited to the environmental factors found in these areas.

Over millions of years plants have evolved to fit into a particular environment. Ecologists usually recognise five types of environmental factors which combine to make up the environment of a particular place: climate, soil, topography, past history and living organisms. These factors interact in a complex manner, forming environments which favour the growth of certain plants yet are unsuitable for others.

Each particular environment supports a unique group of plants (vegetation type), each having a fairly consistent appearance and composition of species. Botanists refer to these types as associations and foresters refer to them as forest types. Forest types are an extremely useful reference for classifying forests for mapping and forest management purposes.

There are about 150 different forest types that occur naturally within New South Wales. These include rainforest, found in the north-east corner of the state; sclerophyll forests; mallee woodlands on the western plains; coastal swamps, and the alpine forest near Mt. Kosciusko.

Each forest type is easily distinguishable in a forest, though the boundaries between different types are rarely clearly distinguishable. Different forest types merge as the environmental factors of a particular area gradually change, resulting in the forest being made up of a new mosaic of vegetation that reflects the subtle changes of the environment itself.

Changing vegetation patterns

Let's take a closer look at changing vegetation patterns by having another look at the forests growing near Sydney. In the district surrounding Sydney the major environmental factors are:

• An annual rainfall ranging from about 1300 mm near the coast to less than 750 mm in the Windsor-Penrith area.
• Two main rock types - infertile Hawkesbury sandstone and moderately fertile Wianamatta shale.
• A variety of topographic situations.

The sandstone plateaux in this district support a low, open forest with the main tree species being scribbly gum and bloodwood, sometimes giving way to mallee or heath on shallow soil. Down in the sandstone gullies, where the soil is deeper and slightly more enriched with decomposing material from the vegetation growing above, the forest becomes taller and denser, dominated by the tree species Sydney peppermint and smooth-barked apple on the slopes and occasionally tall blackbutt trees in the moist soils of the gully basins.

In contrast, the forests growing on the shale soils in the higher rainfall parts of the district are characterised by blackbutt with Sydney blue gum and a few other tree species.

On more exposed shale ridges this forest type gives way to one dominated by grey ironbark and grey gum. As you move westward into the lower rainfall area, these tree species gradually disappear, and beyond Parramatta the shale soils support a more open forest of grey box, various ironbarks and forest red gum.

The basic forest type of this region is blackbutt, which is typically a tall, fairly dense forest of blackbutt, supporting a number of other eucalypts including tallowwood, red and white mahogany, bloodwood and often Sydney blue gum, together with an understorey of forest oak and, depending on the frequency of fire, a ground cover of either grasses or various shrubs and ferns.

Blackbutt is one of the most important native forest types in New South Wales, generally found on well drained and moderately fertile soils derived from shales, slates and granites and in areas having an annual rainfall of about 1250 mm. As the basic environmental factors vary within regions, the vegetation pattern changes.

Each forest type reflects the environmental factors as well as the principal tree species, usually one or two, of plants growing in an area which can also contain hundreds of different plant species.

Environmental factors which influence vegetation

The environmental factors of an area interact and affect the vegetation pattern of the area. There are five types of environmental factors, each consisting of a number of individual factors, which can be of particular importance in different areas. Factors of considerable significance in New South Wales are:

Climate

1. Temperature - extremes, annual and diurnal variation, minimum and maximums, frost occurrence
2. Moisture - precipitation, in the form of rain, snowfall, mist and fog, including quantity, distribution throughout the year, reliability, extremes, humidity
3. Light - including day length variation, shadows and shading
4. Wind - prevailing wind direction and strength, destructive storms and cyclones, pollutants, diseases carried by air.

Soil

1. Physical characteristics - depth, drainage, moisture retention, aeration, texture, structure
2. Chemical characteristics - nutrient content, availability, addition of nutrients.

Topography

Topography modifies the effects of climate and soil, for example by increasing or reducing exposure of vegetation to wind and sun or by producing more or less favourable soil moisture factors. This includes aspect (position).

Past History

1. Long term climatic changes
2. Past destruction, for example as a result of cyclone, fire or clearing
3. Introduction of new organisms, for example, lantana, prickly pear, rabbits, exotic fungi, diseases or insects.

Living Organisms

1. Local flora growing in the area
2. Local fauna - seed distribution and loss (through consumption), pollination, loss of plants (through consumption and damage)
3. Microorganisms - mycorrhizae, decay organisms, pathogens, nitrogen-cycle organisms
4. Humans - logging, clearing, burning, grazing, soil disturbance
5. Fire - destructive effects, beneficial effects.