loading karri thinnings
Loading karri thinnings -
Photo © Parks and Wildlife

Silviculture is the management or cultivation of forest trees.

It is the science and practice of managing forests and woodlands to meet the wide range of needs of and benefits to landowners and the community, now and for the future. It considers the ecology, establishment, growth, health, use and quality of forests.

How a forest grows depends on the characteristics of the tree species, and how they are affected by the soil and climate.

Silviculturists consider this, the biological diversity, ecosystem health, and soil and water resources, to plan for and deliver a range of forest outcomes, such as recreation, protecting wildlife, and timber production.

Many of our State forests have multiple uses which can seem to be competing. Silviculture is part of a system that aims to provide a balance between these uses, while still ensuring that our forests are managed sustainably.

Competition

Every tree in a forest is competing for water, nutrients and light. This competition between trees is measured in terms of the density of trees in a particular stand.

  • Density affects the growth rate of individual trees, as well as the stand as a whole, and other components in the forest such as understorey plants, tree health and water availability.
  • Density of the overstorey in forests is expressed as basal area (BA) in units of square metres per hectare (m²/ha). This is the total cross sectional area of all the tree trunks, in square meters, in 1 hectare of forest.
  • Density can also be expressed as ‘stems per hectare'. The diameter of a tree trunk is measured at ‘breast height' (1.3 metres above ground).
    • When the stand density is low and the forest canopy is relatively open, trees do not compete vigorously with each other for water and nutrients, and grow relatively freely.
    • When stand density is high, the trees compete vigorously with each other and can become stressed.

The point at which trees begin to compete for resources and the stand's growth slows is called ‘critical density'. This will vary according to soil type and rainfall in the forest.

  • At lower densities, individual trees may grow more quickly, however each hectare will produce a lower volume of wood.
  • At higher densities, the total volume of growth is higher, but the growth of individual trees is lower due to competition from other trees.

Silvicultural methods

Silvicultural methods are designed to create stands of trees of a similar age (clearfelling) or uneven-aged stands (by selecting group and individual trees).

  • For example, the jarrah forest is naturally a mosaic of different stand types, so the main method of silviculture used is the selective harvesting of group and single trees to keep its uneven-aged stand structure.
  • Due to the large size of karri trees and their intolerance to competition during regeneration, karri forests are managed using even-aged stands.

Silvicultural methods emphasise the retention of some older trees and patches of forest to provide older elements of the forest as a seed source, wildlife habitat, aesthetics, structural complexity and for other purposes.

When a tree is removed from a stand in a forest, it is done to:

  • improve the health and growth of the remaining trees (thinning—no regeneration required)
  • reduce competition to allow the regeneration to develop into saplings (gap creation/clearfell—open up the overstorey)
  • reduce competition to allow seedlings to establish and develop into ground coppice (in jarrah only—shelterwood—partial removal of the overstorey).

Only one of these is done in any area at one time, and the area must be large enough to remove the effect of surrounding large trees, including minimising the chance of these trees damaging the new growth if they are felled in the future.

Thinning to promote growth

Thinning is the removal of smaller or poor quality trees to improve the growth and health of the remaining trees. Stands may be thinned a number of times to reduce competition on the retained trees, allowing them to grow larger, particularly in diameter.

  • Tree diameter is important in the production of sawlogs. Not only must the tree reach a minimum size before it produces a sawlog, but with a bigger diameter there is less waste in producing sawn timber and greater scope to tolerate any defects in the wood.
  • Potential crop trees are identified that are most likely to provide good quality sawlogs during the next harvest. The maximum number of crop trees is based on the number the stand can support before growth suppression occurs.
  • Thinning can be done by removing trees which are not suitable as final crop trees (culls/non-sawlog product), or by removing trees commercially which have already reached sawlog size or are suitable to use for another wood product.
image depicting regeneration after fire in a Karri forest
Karri forest regeneration after fire -
Photo © Parks and Wildlife

Gap creation/clearfell

Gap creation involves creating a gap in the overstorey so that the established regeneration trees have less competition.

  • Gaps are created by removing some trees which are mature or old, when their tree crowns are no longer able to expand to occupy the space made when individual trees are removed.
  • In jarrah forests, gaps are only created where there is an adequate regeneration pool—creating the gaps allows for the rapid growth of existing regeneration.
  • Trees which are of sawlog size are removed and sold, and some cull trees are also removed. Some potential crop trees and habitat trees are retained in gaps.

Shelterwood regeneration establishment (jarrah forest)

Where the forest is mature or old and the regeneration pool inadequate, we use the shelterwood method to encourage regeneration.

  • Shelterwood harvesting is carried out when there are not enough trees at ground coppice or sapling stage. The aim is to reduce competition from the overstorey, but to preserve enough forest cover to provide a seed source, as well as maintain a forest environment for aesthetic, water and wildlife purposes.
  • Understorey trees and plants also compete with the tree seedlings so a proportion is temporarily removed (either by fire or by machines) to increase the tree seedlings' chances of establishment and survival. We strike a balance between maintaining the diversity of understorey plants and the requirements of tree seedlings, allowing them to compete effectively with other understorey plants.
  • Fire also plays a role in silviculture for fuel reduction (to reduce the chance of future bushfires), to stimulate seed germination, create a suitable seed bed and temporarily remove understorey competition so that seedlings can develop into ground coppice.
  • Sometimes, fire is also used before harvest to make it easier to identify seedling coppice and ground coppice, or for improved access and safety.
Numbat in a hollow tree R Kay
Numbat in a tree hollow -
Photo © Rebecca Kay

Habitat or legacy elements

Legacy elements provide diversity of habitat for some species. They are structures that are long lived or characteristic of mature forests and include mature trees, mature second storey species and large woody debris.

All silvicultural methods used in the forest include retaining elements of the previous 'old forest' in the new stand to provide habitat for species which require 'old forest' structures, such as hollows. The main types of legacy elements retained are:

  • large, primary habitat trees likely to contain hollows that animals can use immediately
  • smaller, secondary habitat trees that will develop hollows in the future when the primary habitat trees die and fall over
  • habitat logs from dead and fallen trees on the forest floor that provide homes for fungi, insects, small mammals and reptiles, and substrate for some plants
  • mature second storey plant species which provide structure and a food resource.

Silvicultural methods are outlined in the:

  • Silviculture Guideline for Karri Forest (2014)
  • Silviculture Guideline for Jarrah Forest (2014)
  • Silviculture Guideline for Wandoo Forest (2014)

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