Growth Strain & Veneer Testing

Growth-strain is the single most important reason why most eucalypts grown in plantations are chipped for the pulp and paper industry rather than converted into solid wood products.

Growth-strain manifests itself during solid wood processing by log splitting and log deformation. Measuring growth-strain is challenging, but we have developed a novel methodology to assess growth-strain quickly and reliably in young trees by splitting. This test simulates solid wood processing rather than small scale internal stress measurements.

Growth Strain project - now complete

The ‘Minimising Growth Strain in Durable Eucalypts to Transform Processing’ project, funded by MPI’s Sustainable Farming Fund, was completed in 2019.

Initially a three-year project, the work was extended by a further year so that further screening work could be undertaken.

The objectives of this project were to:

screen the NZDFI breeding populations at young age for growth-strain and growth
establish clonal propagation protocols from cuttings
establish breeding population and propagation stock of superior trees/families
demonstrate the potential of peeled veneers of a NZDFI species for LVL
draw from national and international expertise
promote NZDFI species within New Zealand.

The project was embedded in a larger effort aiming to establish a sustainable durable timber resource in New Zealand. The outcomes of this project achieved key requirements towards this goal. Superior genotypes with lower growth-strain have been identified and are now commercially available. The potential and the main obstacles (i.e. gluing) of durable eucalypts for LVL production have been demonstrated.

Dr Nick Davies successfully completed his PhD based on this work.

Final reports

Homegrown eucalypts tested for veneer

Part of the SFF project 'Minimising growth-strain in eucalypts to transform processing' was a peeling/LVL trial to test the suitability of NZDFI eucalypts for LVL production. These eucalypts have a higher stiffness and density than radiata pine. They are also naturally durable.

On the other hand, they are more challenging to process. Logs often split making peeling into veneers difficult. Splitting is thought to be associated with internal growth-strain in the trees. Ultimately our aim is to select superior trees for breeding with low growth strain properties.

First durable eucalypt peeling trial

In 2016 the project team felled some 30-year-old E globoidea trees on a Manawatu farm. A range of rigorous tests were carried out as the logs went through the process of being peeled to form veneer sheets. This work was done with the assistance of Nelson Pine Industries Ltd, and is summarised in two reports, available here:

Eucalyptus globoidea peeling/LVL trial - Part 1

Eucalyptus globoidea peeling/LVL trial - Part 2

Second peeling trial at Nelson Pine Industries

A second peeling trial at Nelson Pine Industries (NPI) in Richmond involved rotary peeling of 15-year-old E. bosistoana and E quadrangulata logs. Logs were peeled, and the veneers returned to the School of Forestry for analysis.

Results were encouraging, indicating for example that the E. bosistoana veneer would out-perform radiata in terms of stiffness as predicted. In future the use of spindle lathes for the peeling process could greatly enhance veneer recovery from small logs. Some spindle lathes peel logs to leave a core, which could be marketed as a ground durable post. Other lathes can handle smaller logs, but there would be no bi-product of a post from the core of each log.

The next phase of research will look at suitable gluing techniques for the eucalyptus veneers.

Read the full report of the peeling trials here.