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20 Jan2008

Overstory #200 - The art of farm silviculture

Written by Rowan Reid and Peter Stephen.

Introduction

Silviculture (from the Latin "silva" meaning wood) is simply the manipulation of forests, and the trees within them, for wood production. The potential to direct tree and forest growth to enhance value makes silviculture the most powerful tool of the farm forester whatever their interests. For example, a tree that might otherwise only be of value for firewood can be turned into high value veneer or saw1og, or, a regrowth native forest dominated by one tree species can be thinned to promote greater biodiversity in the understorey.

A "silvicultural regime" is a series of management interventions that are imposed on trees, or forests, over time, from establishment through to harvest. The tools used include the choice of initial spacing, layout and establishment method and the type and timing of thinning, pruning, fire, grazing, harvesting or other interventions. Choosing to "let nature take its course" is, in a way, a silvicu1tura1 decision. Unfortunately, leaving a forest to its own devices is rarely the most appropriate means of providing the mix of economic products and environmental services farmers commonly seek. This article reviews how trees grow and highlights some of the more universal principles of silviculture that farmers can use to redirect forest growth in order to satisfy their own goals.

Tree growth and wood production

Growth of the above ground part of a tree essentially occurs in two ways. The most obvious growth occurs on the leading shoots of the main stem and branches making the tree taller and the branches longer. The second is the cambium growth. The cambium is a thin layer of cells found just below the bark that produces the growth that increases trunk diameter and branch thickness. The wood produced as a result of the elongation of the stems is quite different to that of the cambium. In most species, growth of the leading tips produces a soft corky wood called the "pith" that can be often seen in the centre of a log. In almost all cases the pith is considered a defect in sawn timber.

The cambium growth produces bark cells on the outside and wood cells on the inside. The young bark cells form the conductive "phloem" which carries sugars from the leaves down the stem feeding the cambium and, eventually, the roots. On the inside of the cambium the recently formed wood cells hollow out to allow for the movement of water up the stem. This is called the sapwood.

In the temperate regions cambium growth generally produces one growth ring each year. The ring is evident because the large thinned walled cells produced in spring (called earlywood) are lighter in colour than the small thick walled cells produced later in the growing season (called latewood). As the tree grows new sapwood, part of the inner sapwood is "retired". During this transition of sapwood to heartwood a number of chemicals (mostly lignin) are deposited in the wood cells often giving the wood attractive colours and increased durability.

Four aspects of silviculture

Forest growth is largely determined by how the mix of plants respond to the soil and climate in which they are growing. Silvicultural design and intervention allows us to direct this growth to where it has the most economic, environmental or aesthetic value. There are four aspects of silviculture that farmers need to consider:

1. The genetic composition of the forest

In many situations the farmer has an opportunity to choose the species mix and the genetic origins of any plants that will be added to the site. Active intervention may also be used to reduce, or increase, the prevalence of those species already present. These genetic choices may be based on the known performance of species or varieties, an inclination towards indigenous or introduced genetic material, or a preference for greater or lesser genetic diversity.

Although tree breeding has been able to demonstrate improvements in growth and productivity across a range of forestry and horticultural species, farmers should be cautious about tending towards a very narrow genetic base in their plantings. The extreme example is a clonal forest (such as is common in poplars) where there is no genetic difference between the individual trees. Because of the complexity of the interaction between the genetics and the environment, and the long time periods involved in forestry, maintaining a broad genetic base is often a sensible risk management strategy. Where there are particular selections (either species or varieties) that appear preferable, the farmer could choose to incorporate a number of these into a mixed planting rather than select just one.

Any decision regarding the genetic composition of a forest needs to be made carefully as the introduction of new genetic material may irrevocably change the genetic composition within the existing vegetation (through hybridisation or interbreeding) or be very difficult to remove in the future (weediness). Another concern is that native genotypes lost from a site may be expensive or impossible to recover.

2. Modification of the physical environment

Soil preparation, weed control, fertilisation, fire, irrigation and the provision of shelter are examples of ways in which farmers may be able to change the quality of the site for forest growth. The choice of technique will largely depend on what factors are currently limiting site quality, the costs involved, the availability of appropriate equipment and the anticipated impact. The impact of site modification on growth can be dramatic where treatments are able to address the factors most limiting growth. This is most clearly demonstrated by the effect of controlling weed competition in young plantations or the impact of deep ripping on soils that have pronounced shallow hardpans. However, as the forest develops the factors that limit growth will change. The use of intensive site preparation and fertilisation to promote early growth may simply mean the plantation reaches the growth limits set by other resources more quickly with no real long-term advantage. Rather than simply follow the recommended "best practice" for site preparation farmers must be clear about their purpose for trying to modify site conditions. In many cases the financial costs of intensive site preparation and its impact on other values such as the risk of erosion or leaching of nutrients and chemicals, may be unjustified. Farmers may also be able to use their own equipment or methods to achieve a similar outcome at a much lower cost.

These first two aspects of silviculture, genetic selection and site modification, are difficult to generalise for different sites and scenarios. Farmers are encouraged to explore their options locally and experiment themselves in an attempt to identify the most appropriate genetic material and site modification techniques for their particular circumstances.

3. Management of the competition between trees

Once established, the individual plants on a site will both support and compete with each other. As the trees grow up together what began as welcome shelter from the elements may very soon turn into competition for limited resources. The species composition, spatial arrangement and the impact of natural, accidental or managed interventions will determine how these interactions play out over time. Being able to manage the positive and negative interactions that occur between the individuals within a forest is possibly the most important aspect of silviculture.

Competition can have both a positive and negative effect on tree growth and wood quality. In young plantations, a dense forest encourages rapid tree growth by suppressing weeds and providing shelter from strong winds. However, as the trees grow they begin to compete for light and moisture thereby slowing each other's growth.

There is a great difference in the degree to which competition effects different species. Tolerant trees, such as pines can form dense narrow canopies that allow forests to reach high levels of competition before individual tree growth is suppressed. Many of the hardwoods, including the eucalypts and teak, are much less tolerant.

4. Treatment of individual trees

The final aspect of silviculture is the potential to manipulate growth, form and productivity of individual trees by direct treatment. Pruning the canopy or roots, coppicing, the application of chemicals and hormones, and other interventions can, if carefully timed, change the pattern of growth thereby improving the value of a tree for its intended use.

Managing individual trees in mixed species plantations

Many farmers are managing mixed species or mixed age forests for a range of products and services of which timber production is only one. In this case it may be viable for the farmers to manage individual trees within their forest rather than the plantation as a whole. The emphasis, again, must be on high quality to ensure that the trees are viable to harvest. Promising trees, located in areas with easy access for harvesting, could be pruned. At the time of each pruning it is important to release the tree from any competition. The aim is to ensure that the canopy of the selected tree is able to develop freely.

First, any tree that is overtopping the selected tree, or affecting its canopy shape, should be culled. Then the farmer can make a judgement as to whether any other trees need to be removed in order to reduce competition. A relatively simple method for assessing competition is the use of a spacing measurement. If, for example, the farmer wanted to reduce the competition in their forest down to 10 m2/ha, the spacing between trees of a similar diameter would need to be about 30 times the diameter. For example, if the trees were all around 15 cm in diameter then culling any tree within 4.5 m (l5 cm x 30 = 4.5 m) would reduce the basal area to less than 10 m2/ha. As the tree grows it will require more space. In a mixed forest where the trees are of differing size the same technique can be used to determine if a large tree adjacent to the selected tree should be culled. If it.is located within 30 times its own diameter then it might be assumed to be competing with the selected tree.

Silviculture for multiple values

Balancing timber production with other values is possible, such as land degradation control, agricultural production or biodiversity, especially where the emphasis is on the production on high value pruned trees. Understorey native species can be grown between the widely spaced pruned trees thereby improving the provision of environmental services.

Alternatively, farmers might choose to graze pastures between widely spaced trees or use the area for stock shade and shelter. This has the added advantage of reducing the fire hazard and stock may also benefit from access to fresh fodder from the prunings and thinnings. There is a risk of stock damaging the trees when they are young by rubbing or chewing on the bark.

 

Original source

This edition of The Overstory was adapted with the kind permission of the authors from the original:

Reid, R. and P. Stephen. 2001. The Farmer's Forest -- Multipurpose Forestry for Australian Farmers. RIRCD Publication No. R01/33. Melbourne, Australia.

Copies of this publication can be purchased from:

Australian Master TreeGrower Program Department of Forestry The Institute of Land & Food Resources The University of Melbourne Victoria 3010, Australia Tel: 61 3 8344 5011; Fax: 61 3 9349 4172 E-mail: rfr@unimelb.edu.au

About the authors

Rowan Reid (B. For. Sci., M. For. Sci.) is a Senior Lecturer in Agroforestry and Farm Forestry at the University of Melbourne and the developer of the Australian Master TreeGrower Program (MTG). More than 35 MTG programs have been conducted across Australia involving more than 750 farmers. In 2000 the program was awarded the $10000 Eureka Prize for excellence in environmental education. Rowan is also a tree grower himself and has recently made furniture out of ten year old eucalypt trees he planted and managed on his Otway Ranges farm. He can be reached at: Rowan Reid, Senior Lecturer, Agroforestry & Farm Forestry, School of Resource Management, The Faculty of Land & Food Resources, The University of Melbourne, Victoria 3010, Australia; Tel: 61 3 8344 5011; Fax: 61 3 9349 4172.

Peter Stephen was a Research Fellow with the School of Resource Management at the University of Melbourne, Australia at the time of writing. He has been involved with farm forestry extension throughout Australia for the past decade of which the last four years have seen a wonderful association with and coordination of the Australian Master TreeGrower Program. Peter has also worked overseas in a number of countries, but principally in India on farm forestry extension, community forestry and rural development and is presently involved in establishing a community forest management project in Australia. He is currently in Thailand working on community forestry projects with RECOFTC.

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