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 continually updated whereas this pdf version is dated July 2018.
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In a nutshellStringybark eucalypts have a history of success in New Zealand and offer excellent erosion control in steepland sites while productively producing valuable and durable hardwood timber. These species tend to be site-limited in terms of wind exposure, soil conditions and frost which needs to be understood to ensure successful establishment and yields. A breeding programme would benefit growers.
Eucalyptus are hardwood species but with very variable properties between species. Some species have soft, non-durable timber while others yield very hard, very durable timber.
Selecting eucalypt species that grow fast, adapt to a wide range of conditions, are well formed and have good timber properties is considered to be challenging (Gea & Shelbourne, 2006).
Eucalypt species provide nectar and pollen for honeybees and native birdlife (Millen, 2010) and "eucalypts are favoured trees for native birds with tui and bellbirds often looking for insects under loose bark or feeding off nectar from the flowers" (Orchard, 2010). Eucalypts in flower attract insects and insectivorous birds such as fantails (Orchard, 2010).
Durable eucalypt species, because of their ground-durable and extensive root systems that coppice, may be a good choice for combatting erosion (Millen, 2010). Eucalypts are normally coppiced in spring or summer (Brown, 2008). Millen (2010) considered eucalypt species to be well adapted to continuous cover forestry.
Eucalypts are divided into sub-genera or "families", each with general differences and distinguishing features in terms of siting and wood properties.
Monocalyptus species include the stringybark and ash groups of eucalypts. Stringybark and ash group species have proven to grow well in erodible hill country (Gea and Shelbourne, 2006). There is growing evidence that the stringybark group of eucalypts are well adapted to hill country, but site specialisation remains an issue and careful siting is required (Gea & Shelbourne 2006). Stringybarks "handle wind okay" (Satchell, 2006) but on windy hill country have a tendency to fork (Gea & Shelbourne, 2006). Forking of trees can be managed by corrective form pruning (Satchell, 2006) but such intensive management may not be appropriate for steeplands, especially where remotely situated. A high tree stocking is recommended to overcome forking by allowing for selection of straight stems, even from unimproved seedlines (Satchell, 2006). Satchell (2006) also suggested that forking could be easily overcome by breeding of improved stock, in agreement with Gea & Shelbourne's (2006) view that good potential gains can be made after only one generation of breeding.
Stringybark and ash group eucalypts are both in the Monocalypt subgenus, so share the same ecological attributes and tolerances to environmental factors. These are (Florence, 1996):
- tolerance of low fertility soils;
- limited tolerance of prolonged dry periods;
- sensitivity to antagonistic and pathogenic soil organisms.
In their natural environment, Monocalyptus species occupy upper slopes and are replaced by Symphyomyrtus species towards the lower slopes (Florence, 1996). Lower slopes are more likely to have poor soil drainage with a soil water-air balance conducive to root disease (Florence, 1996). In general terms, Monocalyptus species don't like poor soil drainage, with similar requirements to radiata pine (Satchell, 2006). Ash and stringybark eucalypt species have also proven to be highly resistant to pests in New Zealand (Hocking, 2006c; Satchell, 2008; Satchell, 2016) but do require free draining soils (Hocking, 2006c). Ash eucalypts are amongst the more commonly grown eucalypt species in New Zealand but prefer a cooler wetter climate to stringybarks (Hocking, 2010a).
Stringybarks grow at about 90% of the rate of ash eucalypts and depending on species can grow successfully throughout New Zealand (McConnochie & Nicholas, n.d.). Growth rates and site adaptability vary considerably between species, with some species well adapted to cooler areas and others better adapted to warmer conditions (Gordon, 2013). Stringybarks have similar soil requirements to radiata pine and prefer moderate soil fertility with reasonable drainage (Satchell, 2013b).
The stringybark group exhibit high wood density, high strength and stiffness, hardness, and have good natural durability in ground contact (McConnochie & Nicholas, n.d.). Stringybark eucalypts are also well known for ease of sawing and good appearance timber characteristics (Gea & Shelbourne, 2006). Gea and Shelbourne (2006) concluded that stringybark species "show good prospects for sawn timber if form were improved or managed". Current options for managing form include an initial high stocking of trees for high selection ratios, or form pruning (Satchell, 2008).
Ash group eucalypts (Eucalyptus delegatensis, E. fastigata, E. obliqua and E. regnans) perform well when planted on windy, erodible hill country (Gea & Shelbourne, 2006). Both Eucalyptus regnans and E. fastigata have been subject to breeding programmes and seed is available with improved form and growth rates (Dungey et al. 2014).
Symphyomyrtus species are better adapted to heavier, wetter, deeper and more fertile soils of lower slopes (Florence, 1996). However, Gea and Shelbourne (2006) found one Symphyomyrtus species, Eucalyptus cladocalyx (sugar gum), to be well adapted to erodible hill country. E. cladocalyx is a very drought-resistant species that yields a very durable, high quality timber (Hocking, 2010b). Hocking (2010b) described the form as relatively poor, but improved seedlines from Australian origin are being grown in New Zealand and showing good form (B. McNeil, pers. comm). The Australian work also indicates E. cladocalyx has good levels of natural durability and high levels of heartwood (Hocking, 2010b). Other attributes for the species include easy establishment, vigorous early growth, tolerance of very dry, windy conditions and even saline winds, with few insect pests (Hocking, 2010b). The species coppices freely and produces summer nectar for bees (Hocking, 2010b). Negatives include a very thin crown and crown shyness which suggests low volume production per hectare, and the species is somewhat frost tender (Hocking, 2010b). Young E. cladocalyx are also very prone to toppling when exposed to a combination of strong winds and high soil moisture resulting from heavy rain (B. McNeil, pers. comm). Hocking (2010b) suggested that E. cladocalyx could be a good choice for soil conservation with its extensive, durable root system along with being a species suitable for hot dry north facing slopes. Eucalyptus cladocalyx produces high sawn recoveries of high density and high durability heartwood which behaves well during sawing, is light in colour, stable in service and very hard (Hocking, 2010b).
Eucalyptus nitens has proven to be an extremely fast growing and cold-hardy forest species in cooler regions of New Zealand (Satchell, 2015). The species is also very tolerant of wind and can be planted on exposed hill country sites (Gea & Shelbourne, 2006). E. nitens is less tolerant of drought than E. obliqua (Woodley, 2012) and readily succumbs to an excessively high water table (P. Milne, pers. comm). Eucalyptus nitens has been subject to a breeding programme and seed is available with improved form and growth rates (Dungey et al. 2014). The timber has greater strength and stiffness than radiata pine and can produce high quality LVL (Gaunt et al., 2003). However, processing into solid wood appearance products remains problematic because of drying degrade (Satchell, 2015).
Eucalyptus bosistoana and E. quadrangulata are two durable species being trialled in dryland areas by the Marlborough Research Company and promoted by Vineyard Timbers. These species have a preference for lowland sites with good soil moisture and are both very susceptible to a range of insect pests (B. McNeil, pers. comm). E. bosistoana and E. quadrangulata tend to have poor form and require regular form pruning (N. Pollock pers. comm), along with low heartwood content (D. Satchell, unpublished). However a genetic improvement programme is underway.
A third subgenus (now classified as a separate genus) Corymbia, share adaptions and attributes with both Monocalyptus and Symphyomyrtus (Florence, 1996). Corymbia species are well adapted to low soil fertility, can be highly tolerant of dry environments and are also highly resistant to unfavourable soil organisms (Florence, 1996). Corymbia maculata (spotted gum), a durable species producing a high quality timber well known in Australia for its durability and strength, has proven to grow well in erodible hill country (Gea and Shelbourne, 2006). Corymbia maculata is relatively frost tender but may be suitable for planting on slopes throughout warmer areas of the North Island (Gordon et al., n.d.). Corymbia maculata yields a valuable, heavy, hard, durable, strong and decorative timber, but if heartwood is targeted may require relatively long rotations on account of its wide sapwood band (Satchell, 2015c).
Lastly, Eucalyptus microcorys (tallowwood) is renowned as "an excellent timber species" (Hocking 2006c) with a highly ground-durable timber. Tallowwood is the only species in a unique subgenus and has proven to perform well only on warmer sites in New Zealand (Gordon, 2007), with considerable promise for producing high volumes of extremely durable timber in warmer, more sheltered areas of the North Island (A. Gordon, pers. comm). Eucalyptus microcorys is "remarkably free of insect pests and diseases in New Zealand" (Hocking, 2006c) but is highly susceptible to snow damage, which causes toppling and snapping of stems even once trees are five to six metres tall (Cairns, 2012). Strong winds also cause stem breakages.
High quality plantation-grown timber has been available in the New Zealand market for some time and fetches good prices (P. Davies-Colley, pers. comm).
One of the key issues in terms of market development for eucalyptus timber in New Zealand is the lack of consistent supply of timber into the market, primarily because "instability in supply affects consumer demand and pricing arrangements" (Trost, 2005). Until there is a sufficient resource being grown, markets will not stabilise.
Issues with grade recoveries from the plantation resource have largely been overcome in New Zealand. Sawn recoveries from eucalypt are improved by sawmilling as soon as possible after harvesting (Trost, 2005; Satchell 2015), because logs tend to split at the ends over time, reducing grade recoveries. Therefore co-ordinating the rate of harvesting with sawmill demand is critical to ensure grade recoveries and value are optimised, with careful transport scheduling required along with removal of logs from the harvesting site as soon as practicable (Trost, 2005). Sawmills usually have sprinklers in place or coat the ends of logs with wax emulsion end grain sealer to limit further splitting (J. Fairweather, pers. comm).
Younger, smaller logs have greater levels of internal stresses than older, larger logs and "mills require logs with a small end diameter of at least 300 mm." (Trost, 2005)
Monocalypt eucalypts are renowned for ease of sawmilling, with minimal growth stresses and compression wood, easy drying and processing and with good wood properties (Hocking, 2010a). The ash group eucalypts are more difficult to saw and process than the stringybarks and the timbers are generally inferior, with none rated as durable (Hocking, 2010a). Eucalyptus fastigata is regarded as the best milling ash eucalypt producing good quality, blond timber (Hocking, 2010a), but somewhat lacking in surface hardness.
Uses for ash eucalypt are limited to furniture, joinery and sliced veneer because hardness is not sufficient for applications such as flooring that require hard wearing properties (Davies-Colley, 2006). However, anecdotes suggest that hardness need not limit these species from flooring applications (Tantrum, 2006), although value would likely reduce as softness increases (Satchell, 2015b).
Eucalyptus regnans and E. delegatensis can be milled with minimal degrade, provided the drying process is slow (Tantrum, 2006; Satchell, 2010).
Stringybark species, on the other hand, "produce a premiere hardwood timber" (Satchell, 2013b). The timber dries readily with little shrinkage or distortion, with excellent machining properties, yielding a medium-density honey-blonde timber that is stable in service and with very good surface hardness, suitable for applications such as flooring (Satchell 2013b). Stringybark timber also has good durability for applications such as untreated decking and outdoor furniture (Satchell, 2013b). The sapwood band is resistant to lyctus borer and is very narrow at only one to two centimetres, an important but often overlooked quality of these species (Satchell, 2013b). An additional bonus is that little colour change occurs between heartwod and sapwood, so for internal applications sapwod can be included (Satchell, 2013b).
A sawmilling and market value study demonstrated the commercial potential for solid timber production from short rotation Eucalyptus nitens, (Satchell, 2015). However, drying degrade remains an issue for some trees even where slow drying is undertaken to reduce degrade (Satchell, 2015).
There is also a potentially high demand for ground durable eucalypt hardwood in New Zealand, in particular for posts and poles used in vineyards, kiwifruit orchards, organic farms and general farm use (Palmer, 2013b). A minimum of 500,000 posts a year are needed in South Island vineyards alone (Palmer, 2013b). Power poles and cross-arms for power poles, railway sleepers and construction timber for rail bridges and wharfs where strength and durability are required, offer market opportunity for ground durable eucalypt produced in New Zealand (Palmer, 2013b).
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.