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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.
Topography
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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