Dryland River Red Gum Forest Silviculture

Note Number: AG0903
Published: October 2001
Updated: June 2009

This Agriculture Note provides silvicultural guidelines for landholders with dryland River Red Gum ( Eucalyptus camaldulensis) forests that are being regenerated for commercial and biodiversity values. The riverine Red Gum forests have different silviculture requirements as they are more reliant on flooding episodes for regeneration.

Introduction

Managing Red Gum for timber is an art and a science. The natural tendency of this species is to grow short and stout with many heavy branches. A tree that is to be grown for timber production needs to be of good form – tall, straight, branchless bole and large diameter. For Red Gum to achieve this, the following silvicultural information may be of use.

Managing dryland Red Gum forest sustainably

Before any silvicultural activities take place in a private native forest, a discussion with the local government authority is needed to determine if a planning permit and management plan are required under the native vegetation retention regulations. With any harvesting activities in a forest, a Timber Harvesting Plan must be submitted to the relevant local government, stating harvest details and addressing issues relating to environmental and landscape values. All forest harvesting activities must comply with the Code of Practice for Timber Production (2007).

Site preparation

To encourage successful natural regeneration, it is vital to prepare the site well. It is worth noting that in a Red Gum forest, seeds do not germinate under the canopy of another mature tree and hence there is no point in preparing the ground directly beneath a retained tree.

Seeds disperse at least one tree height away from the seed tree and usually drop on the south or south-easterly side of the tree. Therefore site preparation should be done only on this side of the tree, outside the reaches of the retained tree's crown.

All site preparation should be completed prior to August to give the site time to settle before seeding occurs.

There are several site preparation methods available to promote a successful natural or artificial regeneration.

Mechanical disturbance

The first option is mechanical soil disturbance. This is achieved through the use of offset discs that penetrate the soil surface about 6-10 inches (150-250mm). If native pasture is present, disturbance should be limited to the use of a harrow rather than discs. Soil disturbance enhances the initial seedling vigour and hence lowers early mortality rates.

The disturbance should be done after the autumn break when there is some surface soil moisture available. This allows time for the native ground flora (herbs and grasses) to flower and seed over the spring and early summer period.

Burning

Another method is to have a cool burn over the site to be regenerated. Ash provides a very receptive seed bed which is ideal for natural regeneration. The development of seedlings on ash beds and cultivated surfaces is much more rapid than on hard bare surfaces or grassed sites.

Cool patchy burns carried out at different times of the year are beneficial for a variety of species: in particular the native ground flora typically found in a dryland Red Gum forest. However it must be noted that Red Gums are fire sensitive. This means that although low intensity fires do not kill the trees, severe cambial damage can result and significantly affect growth rates.

Spraying

Spraying alone is not an effective site preparation method. It is best used in conjunction with mechanical disturbance and/or burning. The use of a non-residual herbicide is useful for improved pasture sites, as this type of pasture does not die back as the seedlings develop.

Native pasture (both grasses and herbs) does not compete with the Red Gum regeneration and will die back naturally as the regeneration establishes itself on the site. Therefore there is no need to spray native pasture.

Seedling establishment

A successful regeneration is imperative to achieve ecologically sustainable forest management objectives. Commercial utilisation of a forest is not complete until a successful regeneration has been achieved.

Flowering

Dryland Red Gum forests tend to flower from late spring to the middle of summer. The intensity of flowering is spasmodic and can vary greatly from one year to the next. It can be up to 5-7 years between good years.

A good year may be expected after significantly wet seasonal conditions have prevailed.

A heavy flower crop is vital in assuring a future seed source and successful natural regeneration. About 45% of the flowers fail to mature. The flowers that do mature then have to be sufficient in number to compensate for the high loss of seeds due to insects.

Seed

Red Gum seed matures six months after flowering finishes. If Red Gum seed is being collected it is a good idea to mark the heavy flowering trees in December, so that as the flowers die off, it is known which trees to look for the seed on.

Seed collection should take place around July before natural seed fall occurs in spring. Natural seed fall occurs nine months after the end of flowering. It is greatest in spring and summer when the northerly winds begin to blow.

Germination and seedling development

Red Gum seeds germinate well in moist soil and in high temperatures (around 35ºC). Most germination occurs over spring to late summer. Newly germinated Red Gums are frost tender however their quick root and shoot development will make the seedlings less susceptible to harsh environments.

The stocking (number of trees per hectare) of the site is crucial for good stand development. The tendency for one shoot of a Red Gum seedling to become dominant is very weak. This means when open grown, it tends to grow as a short, heavily-branched tree. Close initial spacing is required to produce a timber tree as it forces the seedling to reach for the light and grow up rather than out.

An ideal stocking for River Red Gums is 50,000 stems per hectare – in a 2 x 2m quadrat, there should be 20 seedlings. The minimum stocking required for sawlog is 10,000 stems per hectare. If the stocking is less than this the trees will grow with poor form and will be unsuitable for sawlog.

Thinning and harvesting

Competition is the key to growing a tall, straight, branchless red gum. Thinning the stand too early, too heavily or too quickly encourages branching in the retained trees. The essence of thinning a Red Gum stand is to thin a little and often.

First thinning

To maximise the growth of a clear long bole, thinning should take place between 12 and 15 years after stand establishment. At this age dominant and co-dominant trees are clearly distinguishable from the suppressed stems. On good sites, suppressed stems tend to persist and have to be manually removed. On poorer sites, the stand naturally begins to dead top or self-thin.

The first thinning should involve the removal of the suppressed stems. This may be up to 50% of the total stand. It is important to note that dryland Red Gum forests differ in their growth rates and it may be necessary to hold off on the first thinning until 20 years of age: it is dependent on the site and environmental factors. As a rule of thumb, the first thinning is not commenced until the stand has begun to self-thin (dead top).

Thinning should be carried out during the summer months. The stumps of the thinned trees are less likely to coppice at this time of year. Red gum does coppice quite vigorously and herbicide may also be required to control it.

Subsequent thinning

Light thinning should take place about every 10 years after the first thinning is completed. Top height and bole length of the trees is paramount to produce good sawlog. To achieve this, competition is vital for the first 20 years of growth.

Thinning should be from below which means assessing the crown vigour, the bole length and the straightness of each tree and removing the poorest of them all. Dominant crowns should not overlap the co-dominants and only when the gaps between the crowns fill up or overlap, should the stems be removed.

In a dryland Red Gum forest, it is aimed to get height and bole growth for the first 50 years. The final crop contributes the majority of diameter growth when the stand is aged between 50 and 100 years old. By this stage all thinnings are completed and the stand basal area should be around 10 square metres per hectare.

Harvesting of the final crop

The cutting system used for harvesting the final crop is somewhat dependent on the size of the stand. How the site is to be regenerated needs to be taken into consideration. If natural seed fall is to be relied upon, seed trees need to be retained. If hand seeding is planned, the seed can be collected from: a) the trees that are being harvested; or b) from neighbouring red gum stands. If this is the case then a clear falling system or a group selection system can be chosen. If the group selection system is used then there must be a sufficient gap opened up to ensure a successful regeneration.

Pests and diseases

Red Gums are prone to insect attack. The Gum Leaf Skeletonizer moth larvae can completely defoliate the crown of the trees. However, if there is no repeat attack, the trees usually recover. Tree growth is directly related to crown vigour; hence growth rate is affected by defoliation.

Defoliation can also be caused by psyllids (lerps). As with the Gum Leaf Skeletonizer, the growth rate of the infected trees will be slowed however the trees will probably recover if there is no repeat attack.

Mistletoe is common in many dryland Red Gum forests however it seldom causes tree death. Once again, the growth rate of the tree can be affected depending on how large the mistletoe infection is.

Acknowledgments

I would like to thank Roger Edwards, Forest Officer, Department of Sustainability & Environment, Cavendish for much of the technical assistance in writing this note.

This note was originally authored by Virginia Forrest and published in October 2001, and reviewed by Philippa Noble in June 2009.

Further information

Refer to Farm Forestry Notes that cover different aspects of private native forest management at the DPI Forestry Website:

www.dpi.vic.gov.au/forestry

References

• Dexter, B.D. (1967) Silviculture of the river red gum forests of central Murray flood plain, Melbourne : Research Branch, Forests Commission of Victoria.
• Dexter, B.D. (1968) Flooding and Regeneration of river red gum, Eucalyptus camaldulensis, Dehn, Melbourne : Presented to the Commonwealth Forestry Conference.
• Edwards, R. (unpublished) Dryland River Red Gum (E. camaldulensis) Silviculture Notes.
• Incoll, W.D. (1981) Effects of thinning and initial stocking on growth of Eucalyptus camaldulensis Unpublished report by the research branch of the Forests Commission Victoria.
• McKinnell, F.H., Hopkins , E.R. &. Fox, J.E.D (Eds.) (1991) Forest Management in Australia , Chapter 18, NSW: Surrey Beatty & Sons Pty Ltd.
• Victorian Conservation Trust (Fact sheet) River Red Gum Woodlands of Western Victoria.