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Know your site

In farming, it is common practice to talk about land classes. These are relatively homogenous areas of land that have similar capabilities, potential and management issues. Farmers might use terms such as the 'heavy wet flats', 'steep rocky slopes' or 'dry sandy ridges' to describe these areas. These descriptors highlight the factors the farmers consider critical in distinguishing land classes based on their current land uses.

Because different tree species respond differently to various site characteristics it is difficult to use the same attributes to assess site suitability or quality for a range of species or management options. For example, some species are sensitive to particular environments or events such as waterlogging, drought, frost or soil salinity. Sites are commonly assessed on the basis of their suitability for a particular species or management regime.

Alternatively, where decisions have not yet been made about species, factors likely to affect species performance and growth are simply acknowledged. For example, water logging and soil water reserves can affect the survival of some species and their performance. In this case soil texture, depth, location within the landscape and drainage will all be important in delineating sites for planting sensitive species. Likewise, if growing a species that is susceptible to frost, then low lying areas or open drainage lines might be considered unsuitable due to the concentration of cold air on clear nights.

Site related factors that are most often important in farm forestry include climate, soils and topography. Also important is the presence or absence of plants, animals, insects, diseases or other agents that can affect tree growth and development:

Climate factors
Within the broad climatic regions (temperate, subtropical, alpine, coastal etc) local climate factors of importance are rainfall total, distribution and intensity, the timing and severity of frosts, air temperatures, wind strength and humidity. It is important to appreciate that the impact of these on growth and development will depend upon other factors. For example, rainfall is important in as much as it replenishes soil water reserves therefore the soil type, drainage and topography are just as critical.

Although average climatic conditions are a useful guide, some thought should be given to their variability. Because trees are grown over many years, the risk of experiencing an extreme weather event during this time, such as severe drought, cyclonic winds or large hail may be high.

Soil factors
Although the physical and chemical properties of the surface soils, as commonly assessed for agricultural production, are important there are other factors that are likely to be overlooked by conventional soil analyses. For example, soil depth to an impeding layer or water table has been shown to be critical in influencing site potential for tree growth. Whereas soil fertility can often be corrected or enhanced it may be impossible or extremely expensive to enhance soil depth. For this reason it is worth examining soil structure to a depth of at least 1m and possibly more.

Other soil factors that may influence the choice of site preparation technique or management options include the presence of rock, risk of water logging, soil compaction, presence of non-wetting sands or other problems.

Anyone familiar with the Australian bush will have noticed that the species composition and performance of native vegetation commonly changes with aspect, slope, and elevation.

On the north facing slopes and ridges the forests are generally drier, trees do not grow as tall and the species are those more able to withstand dry conditions. On the south facing slopes the understorey is often more dense and the trees taller and straighter. Whilst this may be partly due to the fire history it does highlight the impact of topography on tree growth.

Where the intention is not to plant a whole slope it may be useful to concentrate plantings along drainage lines or on sheltered sites. It is possible that these trees will grow tall enough to shelter the dry exposed ridges.

Other site factors
It is worth considering the impact of existing animals and vegetation when assessing sites. The presence of vermin such as rabbits, hares, goats or pigs that cannot be eliminated may require a change in the species, planting arrangement or management. Similarly, weeds, pastures and other vegetation that are likely to remain on the site will compete with the young trees and affect their growth if not controlled.

In some cases there are also disease agents and insects present at the site. For example, the presence of soil pathogens like Phytophthora cinnamonmi or Armillaria may make some susceptible species entirely unsuitable. For those interested in growing Australian Red Cedar for timber the likelihood of tip borer reaching the plantation must be considered.

Site quality for timber production
Site quality is a useful concept in farm forestry where it is used to delineate areas of different productivity. The factors used to define site productivity will depend on what species are to be planted and the region where they will be grown. In commercial forestry it is common to define site quality on the basis of the anticipated height of the trees at a particular age.

Site quality for pine is generally based on the height of a typical plantation at age 20 years. Height, rather than volume or diameter growth, is the preferred method of assessing site quality because it is the measure least likely to be affected by other stand management. Thinning and pruning, for example, will affect the mean diameter and volume but has very little impact on height growth in most situations. Height growth, particularly in later years, does appear to vary with site factors including soil depth, water holding capacity, rainfall and humidity.

The original vegetation and site quality
Knowledge of what native vegetation was present at a site can give an indication of the limiting factors that may affect the growth of a range of species and even allude to their potential productivity. Of particularly interest is the presence, or even absence, of certain key species (indicator plants) that are sensitive to particular soil or environmental factors.

For example: there are a range of salt tolerant weeds that farmers use to identify the extent of a saline discharge area; native species like bracken fern are only found on well drained soils; and the likelihood of waterlogging may be evident from the presence of tolerant species be they weeds, shrubs or trees.

Whilst there are limitations to the use of indicator species within a region, or on a farm, it is possible to develop a local knowledge that can be extremely valuable.

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