Farm Forestry New ZealandThe "Kawa" variety poplar was bred in New Zealand in 1986 and is a hybrid between Populus yunnanensis (Chinese poplar) and P. deltoides (black poplar). Kawa has good form for timber production and is a vigorous variety well suited to Northland's subtropical climate, holding its foliage well into winter and flushing fairly early in spring with no risk of frost damage.
Health: Very healthy, not damaged by poplar rust and fairly resistant to possum browsing. Pruning wounds are a risk factor for entry of silverleaf fungus which can kill the tree. Low susceptibility to blackheart.
Timber: Poplar has a pale whitish coloured timber with a very attractive sheen or lustre. Poplar is low density, very soft and of low natural durability. Although for its density it has good mechanical properties, it does not meet the density and strength requirements for structural applications under the building code. Air-dry density is 440 kg/m3 (lower than for radiata pine) and stiffness (modulus of elasticity) is below 6 MPa, the threshold for structural radiata. However, because density varies so little within and between trees, this suggests potential for customised structural grades and applications that take advantage of the cultivar's consistent strength and stiffness. Because the heartwood is perishable poplar would require H1.2 boron treatment for interior structural applications. For exterior structural applications CCA treatment to H3.2 would be required, along with evidence that treatment methods achieve penetration and retention requirements as per NZS 3640.
Because the wood is odourless, potential applications for poplar wood may include fruit and vegetable packaging crates. Finishing grades of timber are suitable for appearance applications such as internal joinery and panelling, but low surface hardness tends to detract from using poplar for furniture. Research into hardening technologies would open market opportunities for high-value appearance applications.
High quality pruned logs have excellent potential for sliced and peeled veneer and poplar is suitable for paper pulp.
Sawn CCA treated fence posts have proven to be a successful product in Hawkes Bay but growers would need to be confident with potential products and applications before a plantation forest industry were to become viable. The export log market is available, however, with demand for logs from Asian countries.
| Species | Density, dry (kg/m3) | Bending strength, MoR (MPa) | Stiffness, MoE (GPa) | Hardness, Janka (kN) |
|---|---|---|---|---|
| Poplar, Kawa | 440 | 56.9 | 5.4 | 2.3 |
Siting: Requires good soil fertility and moisture, but tolerates poor soil drainage. Requires adequate moisture through the summer months, therefore adequate soil depth. Adaptable to reasonably strong winds but not coastal conditions and salt spray. A good site is important for volume production.
Steep slopes: Often used for stabilising steep pastoral slopes to retain grazing. Rough bark takes some years to form and although the bark on younger trees appears rough it is thin and susceptible to stock browsing. Stumps and roots remain alive and coppice after harvesting. Wide spaced poplar would take longer to achieve high levels of soil binding than where closer planted. Where erosion potential is high, a closed canopy tree cover is recommended.
Species characteristics: A fast growing hardwood with a soft, low-density timber. Suitable for lower slopes and alluvial soils with some fertility.
Recommended regime
- Production forestry, Plant 90cm forestry wands or rooted cuttings at 600 stems per hectare. Prune to 10 m and thin to 300 stems per hectare by year 15. Harvest at 25-35 years old.
- Timberbelts (pastoral), Poles, not less than 4 m apart in single rows.
- Conservation/timber plantings (pastoral), Poles at 100-200 stems per hectare, approximately 8 m x 8 m, pruned to 10m. Log volumes of 500 m3/ha can be expected by year 30, with approximately 40% of the total log volume being pruned logs.
Planting and establishment: Forestry wands require pre-plant spot spraying with glyphosate and one release in late spring. If glyphosate is used for releasing it must not come into contact with green stem. Larger poles require plastic sleeves to protect them from stock browsing. Smaller poles and wands require exclusion of stock for 2-3 years, until the trees are sufficiently well established and the bark thick enough to overcome browsing temptation.
Silviculture: Poplar prolifically produces epicormic shoots when pruned. Pruning should be undertaken in late summer to reduce the incidence of epicormic shoots. Epicormic shoots tend to arise with lower tree stockings and those that do arise need to be removed
Key message: A good species for soil stabilisation in pastoral settings. Markets for the wood are under-developed and research is required to generate market opportunities for the timber.
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