Report: Trees for steep slopes
Sustainable Forest Solutions
Reviewed by Mike Marden, July 2018.
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Please note that the web report is regularly updated whereas the pdf download above is dated July 2018.
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In a nutshell
Poplar is not suitable for planting in exposed, drier hill country or where soils are thin. Markets for the timber require development, although demand exists for export logs. Poplar grows fast on good sites but does not perform well where conditions are unsuitable.
Poplar and willows are widely planted in pastoral hill country for erosion control. Poplar also has potential as a plantation forestry species and as silvo-pastoral plantings in the presence of stock.
If large areas of erodible hill country are to remain in pastoral production in New Zealand, there is potential for poplar agroforestry to create "a large, economic resource of pruned poplar sawlogs", an industry with "contrasting fibre and wood characteristics to those of pine" suitable for integration with pastoral farming as a viable crop that complements the radiata resource (Wilkinson, 2000). However, eighteen years after Wilkinson published his vision, the question still remains whether sufficient research has supported a future role for poplar whereby "high value products can be identified and grown".
Poplar is currently planted for erosion prevention in pastoral land with the primary purpose of retaining pastoral production. Wide spacing between trees does not cause significant shading of pasture (McIvor et al. 2011). Wilkinson (2000) recommended a stocking of 100 stems/ha for agroforestry in order to balance timber production with pastoral production. Where timber production is prioritised under an agroforestry regime, the stocking can be increased to 200 stems/ha (Wilkinson, 2000). Farming poplar for carbon also offers hill country livestock farmers an opportunity to reduce erosion risk at low cost, with the requirement being for 30% tree canopy cover, potentially fulfilled with as few as 40 space planted trees per hectare (Eyles, 2010).
Poplar root systems exhibit strong extensive growth and bind soil effectively, with root grafting between adjacent trees (McIvor et al. 2011). However, little data is available on how effective "spaced" poplar is in controlling erosion (McIvor et al. 2011). Knowles (2006) suggested that "the low stockings often employed for poplar pole planting may make them less effective for erosion control than commonly thought", while a 1992 study showed that mature spaced trees, where well maintained, "reduced soil erosion by about 60% to 70%, while poorly maintained poplar plantings had minimal effect." (Hocking, 2006a). Root mass is proportional to tree diameter and high planting densities are required for younger poplar trees to develop a structural root network sufficient to control erosion (McIvor et al. 2011). Once trees are mature, close-tree planting will reduce erosion by 90%, compared with 70% for spaced-tree planting on erodible pastoral land (McIvor et al. 2011). Although conservation poplar plantings do not ensure erosion will be prevented, with spaced plantings of poplar, production losses attributable to landslides could potentially be significantly reduced once the trees reach diameters greater than 30cm (McIvor et al. 2011). Thus, wide spaced trees would take longer to achieve high levels of soil binding than where closer planted; and where erosion potential is severe, a closed canopy tree cover is recommended (McIvor et al. 2011).
Poplar is easily established in the presence of stock by using large poles and protecting these with plastic sleeves. However, this method is suitable only for establishing poplar in the presence of stock rather than establishment of high tree stockings. In the absence of stock, high stockings are achieved by using small stakes or forestry "wands" (Wilkinson, 2000). Rooted cuttings may play an increasingly important role in woodlot establishment, especially in drier sites (Wilkinson, 2000) where deep planting is required for trees to access permanent soil moisture and establish successfully (Hunter and McIvor, 2008). Two to three years are required from establishment until livestock can be re-introduced (Wilkinson, 2000) and regular releasing from grass competition for at least the first year is required.
Poplar requires at least moderate soil moisture (Wilkinson, 2000). Large areas of New Zealand’s hill country, including much of the East coast of both islands is not suitable for stabilising with poplar and willow because of inadequate soil moisture during summer (Van Kraayenoord and Hathaway, 1986). Poplar only grows in low rainfall areas where the water table is high (Wilkinson, 2000), so on hillsides in summer-dry regions, planting should be restricted to channels, tunnel gullies and seepage areas (Wilkinson, 2000).
Poplars prefer and grow best in fertile, moist and friable soils (Van Kraayenoord and Hathaway, 1986; Hunter and McIvor, 2008). Because poplar prefers deep soils, growth rates tend to decrease going up the slope (McIvor et al. 2011). Poplars are not happy on exposed eroding hillsides (Knowles, 2006), preferring alluvial terraces and moist valley bottoms (Wilkinson, 2000). Desiccating winds are damaging (Van Kraayenoord and Hathaway, 1986) and trees become deformed and stunted where planted on exposed upper slopes and ridges (Wilkinson, 2000). Some poplar cultivars such as black poplar hybrids cope with some wind exposure, while balsam poplar hybrids offer improved possum resistance but where exposed to wind have poor form (Wilkinson, 2000).
Poplar can be grown to 800 m altitude without frost damage (Van Kraayenoord and Hathaway, 1986). In their deciduous state poplar can tolerate heavy frosts, but some cultivars may suffer frost damage to the growing shoot caused by late spring and summer frosts.
Poplar is a fast growing tree, but site adaptability for the range of poplar cultivars grown in New Zealand is not yet fully understood (Wilkinson, 2000). The Forest Research Institute modelled growth rates for pruned trees and found that on good sites poplar can produce over 400m3/ha volume production at 200 stems/ha on a 20 year rotation and over 500m3/ha on a 30 year rotation. This reduces to 128m3/ha for a 20 year rotation on a poor site and 161m3/ha for a 30 year rotation on a poor site (Wilkinson, 2000). This data clearly shows that site is important for volume production, which increased by a factor of 3 from a poor site to a good site.
In order to produce knot-free timber pruning is essential (Wilkinson, 2000). Pruning for timber requires an initial form prune before year two and for clearwood requires interventions from year two to four, with lifts every two years until the lifts reach between six and eight metres (Hunter and McIvor, 2008), best achieved by retaining 50% of the height of the tree as green crown (Wilkinson, 2000). The aim is to restrict the knotty core to a central diameter of 150 mm over pruned branch stubs, but epicormic shoots that follow pruning can be an issue (Wilkinson, 2000). Poplar prolifically produces epicormic shoots when pruned, which need subsequent removal. Pruning is best undertaken in Autumn rather than spring to minimise epicormic shoots (M. Hunter, pers. comm).
Although poplar has the potential to become a serious weed species where breeding populations are established, very few populations of poplar in New Zealand are established and self-perpetuating (i.e. naturalised), likely because the widely distributed clones are either unisexual or non-breeding hybrids, limiting spread by seed (Wilkinson, 2000). Breeding programmes thus focus on producing male cultivars (Wilkinson, 2000).
Because of poplar's rapid growth rate, timber suitable for a range of products can be produced in relatively short rotations (McIvor, 2010).
Poplar wood has low to medium density, an even pale white colour, indistinct growth rings and a fine texture (Wilkinson, 2000). The wood has an attractive lustre where clear coated (Williams et al. 1986). The heartwood can be difficult to distinguish from the sapwood and the wood is odourless (Wilkinson, 2000). The light colour of the timber is appreciated by the appearance market but low surface hardness detracts from use in furniture making, despite the wood's attractive appearance (Wilkinson, 2000).
Poplar wood can be used for solid timber applications, chip and paper pulp (Hunter and McIvor, 2008). Applications include decorative veneers, plywood, construction, furniture and wood-based composites (McIvor, 2010). Globally, emerging applications for poplar include engineered wood composites, chemical extracts and bio-energy (McIvor, 2011). Specialised markets exist in Asia, where poplar is an accepted timber species (Wilkinson, 2000).
High quality pruned logs have excellent potential for sliced and peeled veneer (Williams et al., 1986).
Poplar has good strength properties in relation to density (Hunter and McIvor, 2008; McIvor, 2010). The New Zealand breeding programme has aimed for a basic wood density of at least 360 kg/m3 (Wilkinson, 2000). Although this is lower than for radiata pine, there remains potential for selecting higher density clones for sawn timber production (Wilkinson, 2000). Within-cultivar density varies little between region, site and position in the tree (Wilkinson, 2000), suggesting that very even strength properties can be expected, possibly an advantage when characterising strength properties for use in structural products. Although the product would be inferior to and in most cases be in direct competition with radiata pine (Williams et al., 1986), structural products requiring good appearance properties such as exposed rafters, posts and beams along with laminated structural products may hold some market potential.
Poplar wood has low natural durability. Poplar is not listed for structural applications in NZS 3602:2003 Timber and Wood-based Products for Use in Building, so durability performance would need to be demonstrated before structural products could gain acceptance in the New Zealand construction market. Although boron penetration has been found to be satisfactory for protection against insect attack in protected interior situations (Wilkinson, 2000), protection to the H1.2 hazard class required for structural applications would require demonstration of durability performance equivalent to H1.2 treated radiata (T. Singh, pers. comm). The H1.2 hazard class applies to timber used in situations protected from the weather, where there is also a risk of moisture content conducive to decay (NZS 3640:2003 C6.1). Pressure treatment of poplar with boron has provided unsatisfactory penetration results (T. Singh pers. comm) but two months of boron diffusion provided boron retention well above the requirements of the H1.2 specification for both 25 mm and 50 mm thick boards (Williams et al. 1986).
In-grade testing of poplar has shown that working stresses were similar to medium-density radiata pine visually graded to No. 1 framing (Wilkinson, 2000), provided adjustments were made for density; and critical joints had additional fixing (Wilkinson, 2000). Therefore, some evidence-based research would be required before introducing poplar into the building code as an acceptable solution for structural applications.
Poplar pulps have excellent papermaking qualities suitable for fine paper production (Williams et al., 1986) and pulps with "high bulk, moderate strength properties, and excellent optical properties" can be produced from poplar (Richardson and Jones as cited in Wilkinson, 2000), offering an excellent addition to a softwood base of radiata pine (Williams et al., 1986). However, studies and deployment in New Zealand to date have been limited.
Pressure treatment of dry sawn timber with CCA salts has been shown to provide variable penetration (Williams et al. 1986). However, poplar appears to have not been tested using contemporary CCA treatment methods such as steam pre-treatment under pressure to condition green timber for treatment. Nevertheless, CCA preservative treatment of roundwood was found to be satisfactory (Williams et al. 1986) with Wilkinson (2000) reporting that "CCA-treated poplar has been widely used for fence battens and gates". However, poplar fencing may not hold nails and staples as well as radiata pine (Williams et al. 1986).
Poplar saws easily (Williams et al. 1986) but sawn timber recoveries are lower than for radiata pine, with the main timber defect being knots (Wilkinson, 2000). Tension wood is present which may cause wooliness and collapse in sawn boards (Williams et al., 1986). Kiln drying of poplar timber from green can be achieved in two to three days without degrade (Wilkinson, 2000). Machining properties are inferior to radiata pine (Wilkinson, 2000). Surface coatings are easily applied and poplar takes an even stain (Wilkinson, 2000). Some poplar cultivars have been evaluated in New Zealand for wood density, sawing, machining and drying properties (Wilkinson, 2000).
There is an export market for the logs, which may attract a slight premium over radiata pine (Hunter and McIvor, 2008) and "there is a small but growing market for poplar timber." (Eyles, 2010).
Disclaimer: The opinions and information provided in this report have been provided in good faith and on the basis that every endeavour has been made to be accurate and not misleading and to exercise reasonable care, skill and judgement in providing such opinions and information. The Author and NZFFA will not be responsible if information is inaccurate or not up to date, nor will we be responsible if you use or rely on the information in any way.