Farm Forestry New Zealand

Northland experience has identified eucalypt species that have grown well in our region, are relatively untroubled by insects, and offer high potential for quality timber production. The high quality attractive timber certainly has appeal.

Eucalypt species produce hardwood of varying densities, durability and strength. Timber colour also varies with species, from pale brown to rich red.

For simplicity, species can be grouped according to timber properties. The species identified as having the greatest potential for premium applications are the "medium density" eucalypts.

The "rule of thumb" is that for greater durability the species will be slower growing, have higher density and be less easily worked.

Species also vary in their siting requirements, in particular soil drainage requirements and wind exposure:

Species selection can also be based on economic factors that influence risk, log value and species productivity:

1. Sapwood width affects recovery of heartwood products from the log, which tend to hold significantly higher value than those containing sapwood.
2. Compression core is the wood in the centre of a eucalypt tree that has very low strength and durability resulting from compressive forces that cause micro-fractures in the wood and make it brittle. This can affect sawn recoveries from a log.
3. Biological risk is the assumed risk of a new incursion occurring that impacts on the health of the species, evaluated based on historical incursions of insect pests and fungal pathogens.

Nursery stock and planting

Eucalypts should be container grown and planted out when 15-25 cm tall, well hardened and with 6-8 pairs of leaves. The root system should not be constricted and excessively deformed in the container. The bottom quarter of the plug should be cut off in roottrainer stock to encourage new roots and reduce deformation in regenerating roots. Side-slot containers are the best for reducing root deformation, which can strangle the growing plant and increase the risk of windthrow in the first few years after planting. A sample of trees should be removed from the containers and the soil removed to expose the taproot. This should be checked for deformation resulting from poor pricking out procedures. Generally eucalypts are planted in early spring when the risk of hard frosts is over but soil moisture is still good. Winter planting is acceptable on slopes above where frost drains to.

Weeds are usually desiccated prior to planting by spot spraying with glyphosate. Eucalypts are sensitive to weed competition, particularly grasses. Containerised trees should not be "slit & stuff" planted. The soil should be sufficiently cultivated and clods of soil well broken up so that the plug does not get squashed and deformed when the soil is firmed back into place. Air cavities must not be present in the planting hole after the soil is tramped back in place. Trees should not be planted deep in winter-wet soils to avoid collar rot. Trees respond well to fertiliser tablets placed in the planting hole underneath the tree. If weed control is good 50g DAP can be spread immediately above the tree on sloping ground or around the tree on flat ground. Fertile soils should not be fertilised at all.

Eucalypt silviculture

Silviculture of eucalypts can be very different from radiata pine. If planted at a high stocking, eucalypts self prune (shed their lower branches), negating the need to prune for clearwood, which can offset the high costs of planting at a high stocking rate. Efficient methods are available for thinning high stockings of eucalypts, such as chemical injection, which kills the tree to the roots. Chainsaw thinning is useful for thinning eucalypts where it is desired for them to coppice from the stump and remain as an understory tree, for example within a continuous cover forest. Most eucalypt species coppice (re-grow) from the stump when felled. The roots stay alive and a new stem grows, providing a second tier of understory trees and an opportunity to regrow a poorly formed tree. Because their roots do not die when cut down, eucalypts are a desirable species for retaining soil in erodible hill country.

Eucalypts also require different processing methods than softwoods such as pine to overcome tension in the logs. The grower must determine what the optimum log diameter is for processing in order to design a regime that maximises profit. Key variables influencing tree diameters are rotation length and stocking. Traditionally larger diameter logs were favoured because of inherently lower tension stresses, but modern sawmills offer solutions to overcome the higher levels of stresses inherent in smaller diameter logs and produce straight boards cost-efficiently. Large logs take longer to grow and require a low final crop stocking, so the grower should be well informed on processing options when thinning eucalypts, because extending rotation length and reducing tree stockings to achieve large diameters carries with it risk of windthrow. Suggested regime is a final crop stocking of 300-400 stems per hectare and a 25-30 year rotation.