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
Requires a nurse crop such as manuka if planted on exposed sites. Slow growing and requires management. Suitable for selective harvesting only. Black beech may be the best species for exposed eroding North Island slopes.
A number of Southern beech species are indigenous to New Zealand. These are all evergreen broadleaved hardwoods and include silver beech, red beech, hard beech, black beech and mountain beech. Leaf shape is the major characteristic used for distinguishing these species.
Beech is widespread throughout cooler areas of New Zealand and constitutes over half of the remaining native timber resource in the country (Smale et al. 2012). The New Zealand beeches "produce high quality functional and decorative timbers" and "substantial areas of manageable beech forests remain in freehold and Maori tenure" (Smale et al. 2012). The red and silver beech resource is estimated as being able to sustainably produce 200,000 m3 of sawlogs per annum (Smale et al. 2012). Historical constraints to developing this resource included low native softwood timber prices, cheap imported timber and opposition to harvesting native trees (Orwin as cited in Smale et al. 2012). Log volumes harvested in 2011 were just over 10,000 m3 (Ministry of Agriculture and Forestry as cited in Smale et al. 2012).
The beeches are amongst the fastest-growing of all planted native trees and "present good prospects for plantations" (Pardy et al., as cited in Smale et al. 2012). Beech can be managed by either continuous cover with selection, or a uniform system (Barton, 2005). However, there is evidence that clearfelling, even where undertaken in coupes, is not effective unless the coupes are small and retain side shelter (J. Wardle, pers. comm). Once released into the open, trees are subject to sun-scald in the bark which brings in rot and dessication of the crown (J. Wardle, pers. comm). Beech are considerably more difficult native species to manage in plantations compared with succession species like totara and rewarewa (J. Wardle, pers. comm). Selective harvesting while retaining a canopy offers the most reliable option for managing beech plantations (J. Wardle, pers. comm).
Form is generally similar for the beech species, with open growing trees having short trunks, horizontal branches and spreading crowns, whereas trees in denser stands have longer trunks, more upright branches and shallower crowns (Smale et al. 2012).
Beech tends to have a shallow root system but root grafting occurs in all species and may improve tree and slope stability (Smale et al. 2012).
Seeds are relatively heavy with small wings, so are not bird-dispersed and fall within the vicinity of parent trees, limiting the advance of existing stands (Smale et al. 2012). Although beech seedlings regenerate prolifically in existing forests, they are very vulnerable to drought in the first year of establishment (Smale et al. 2012). Mycorrhizal associations offer improved growth rates for beech seedlings and are considered to be an important association for establishment of beech (Smale et al. 2012).
Seedlings are easily raised in nurseries either as containerised or bare-rooted two-year-old stock (Smale et al. 2012).
Poor survival of planted stock is usually attributed to smothering by grass, rabbit and hare damage and drought during the first summer, especially where seedlings were planted into mineral soils (Smale et al. 2012). In cold climates there is also a risk of unseasonal frosts and winter desiccation damaging or killing seedlings. Desiccation is caused by very cold soil preventing roots from absorbing moisture, combined with desiccating winds that dry the plants (Smale et al. 2012). Neither desiccation nor frosting tends to affect larger saplings or trees, even where open-grown (Smale et al. 2012).
Drought can weaken or kill beech trees, predisposing them to attack by pathogens (Hosking & Hutcheson; Hosking & Kershaw, as cited in Smale et al. 2012).
Open-grown beech has improved survival prospects if a fast-growing nurse species such as manuka is first established (Smale et al. 2012). Manuka appears to have a shared mycorrhizal association with beech and beech may benefit from such an association beyond amelioration of above and below ground microclimate (Smale et al. 2012). By using a nurse species, side shading shelters crop trees on exposed sites, improves early height growth, reduces branching and decreases incidence of multiple leaders (Smale et al. 2012). Manuka has good potential as a nurse crop for beech because although it may overtop beech in the early establishment years, beech emerges through the manuka and outcompetes the manuka (J Wardle, pers. comm).
Regular releasing from competing vegetation is necessary for up to five years after planting of beech seedlings and the addition of fertiliser is necessary on grossly infertile sites (Smale et al. 2012).
Planting beech at high stockings such as 2 m x 2 m offers relatively early canopy closure and associated improved form and smaller branch size, but reduced diameter growth. Lower planting stockings require more form pruning to ensure single straight stems and more pruning to ensure branch-free stems (Smale et al. 2012). In general terms, unless the grower is willing to put in considerable effort into form pruning, low initial stockings are not likely to succeed and open growing of trees should be avoided (J. Wardle, pers. comm).
New Zealand beech species don't tend to coppice, although this can occasionally occur with silver beech (J. Wardle, pers. comm).
In beech forests natural regeneration is at very high densities, which produces good form and branch control, but at the expense of fast growth. Up to 100 years is required for dense stands to self-thin. Silvicultural intervention has the potential to greatly increase yields of merchantable sawn timber and shorten rotation lengths, in particular multiple light thinnings (Smale et al. 2012). Height growth is maximised where stems are sheltered by adjacent trees but are far enough away from them to reduce competition (Smale et al. 2012). Diameter increments in plantations can be between 8 mm and 9 mm per annum, with similar growth rates between species, but best growth is attained in fairly open stands in mild climates and with fertile soils (Smale et al. 2012).
With their stronger apical dominance, red beech and black beech can be thinned earlier and heavier than silver beech (Smale et al. 2012). Silvicultural systems aimed at producing high-quality timber can achieve 45 cm DBH sawlogs on rotations of 50-55 years, provided stands are slowly but progressively thinned down to 450 stems per hectare, beginning from twelve years of age, with pruning of all crop trees practiced (Wardle, 2005). Current recommendations for mixed-age stands are for 600-800 stems per hectare and 500 stems per hectare for even-aged stands (J. Wardle, pers. comm). Average harvest age for well managed black beech is 60 years with a range of 40-80 years (J. Wardle, pers. comm). With mixed age selective systems there is significant economic advantage in terms of harvesting each tree to maximise returns, which means that where growth rates vary according to topographic variation, the time taken to reach optimum economic value will vary accordingly (J. Wardle, pers. comm).
Without careful form pruning, beech planted at wide spacing will have poor form, short boles and large branches (Smale et al. 2012). Even at stem densities of 2 m x 2 m, beech will require form pruning of multiple leaders and branch pruning to ensure a branch-free single lower trunk develops (Smale et al. 2012). Pruning can result in production of epicormic shoots in open stands of planted beech (Smale et al. 2012).
Beech should only be planted in suitable climates. For example mortality of red beech seedlings can be high in warmer lowland regions, with better survival in cooler climates (Smale et al. 2012). Where possible local provenances should be used to ensure trees are well adapted to site (Wilcox & Ledgard, as cited in Smale et al. 2012).
Provenance trials of beech species showed that:
- red beech is fairly uniform as a species but silver beech is genetically variable;
- black beech is faster growing than mountain beech and on a par with red beech; and
- a third form of this species from well-drained lowland sites in the South Island, with seedlings intermediate between mountain beech and black beech, was found to be among the fastest growing of all the New Zealand beeches (Wilcox & Ledgard, as cited in Smale et al. 2012).
This third form offers reasonably fast growth rates and reasonably high density timber that is faster to dry than red beech (J. Wardle, pers. comm). Indeed black beech could be a stable hybrid between red beech and mountain beech (J. Wardle, pers. comm). Black beech may be the best option for exposed eroding hill country in the North Island because it tolerates poorer soils, is a lowland species, is a moderate competitor and is present in the lower North Island on exposed ridges with hard dry soils (J. Wardle, pers. comm).
Silver beech is a cold hardy species tolerant of heavy frost and snowfall (Smale et al. 2012). Silver beech is the most tolerant species of shade, so trees can grow up beneath existing canopy trees (Smale et al. 2012). Silver beech has weaker apical dominance than the other beech species and requires side shade for good form (Smale et al. 2012). Silver beech is also slower growing than the other beech species, prefers higher rainfalls and is less tolerant of infertile or poorly drained soils than mountain beech (Smale et al. 2012).
These species tolerate cold and dry conditions better than the other indigenous beech species. Rainfall can be as low as 750 mm per year for black beech, which is a lowland species that can achieve good growth rates (Smale et al. 2012). Mountain beech grows at higher altitudes in fairly dry conditions and is the least tolerant of shade, but is very tolerant of frost (Smale et al. 2012).
Black beech is usually found in conditions between mountain and red beech (J. Wardle, pers. comm).
Red beech, along with black beech, are the fastest growing of the beech species and red beech attains the largest size (Smale et al. 2012).
Red beech is intermediate in terms of shade tolerance and is less tolerant of frost than silver beech and Mountain beech (Smale et al. 2012).
Red beech prefers lower altitude fertile sites (J. Wardle, pers. comm).
Hard beech is a lowland species more suited to warmer climates than the other beech species (Smale et al. 2012). Hard beech is intermediate in terms of shade tolerance, tolerates poorer and drier soils than red beech, but is less tolerant of frost (Smale et al. 2012). Hard beech grows on hard infertile soils and being very resistant to wind exposure, is often found on bony ridgetops (J. Wardle, pers. comm).
Hard beech produces an attractive and hard timber, but is difficult to saw (rapid blunting of blades) and dry (case hardening) (J. Wardle, pers. comm).
Beech species are not a preferred food source for either possums nor deer (Smale et al. 2012).
All five beech species are attacked by pinhole borer (Platypus spp.). Although sawn wood and logs can be attacked, as soon as the surface of the wood dries it becomes less attractive (Smale et al. 2012). Trees that are damaged or under stress from drought or competition are more likely to be attacked and fast growing trees may be more susceptible. Older trees may be less vulnerable to attack (Smale et al. 2012). Management of pinhole borer is by reducing quantity of brood-rearing wood (Smale et al. 2012).
Pinhole borer is less likely to be a problem in managed plantations compared with natural stands because the point of infestations such as collapsing older trees is not likely to be present and therefore the innoculum potential would remain low (J. Wardle, pers. comm).
Platypus is also a vector for the Sporothrix fungus, a pathogen that discolours wood or kills wood tissue leading to a core of dead pathological heartwood, potentially killig trees and prompting a new outbreak of pinhole borer (Smale et al. 2012).
Saplings "can be attacked by by larvae of puriri moth (Aenetus virescens) in the North Island and larvae of the kanuka longhorn beetle (Ochrocydus huttoni) in both islands" (Smale et al. 2012).
Many native insects also feed on the canopies of beech and can partially defoliate large areas of Mountain beech (Smale et al. 2012).
The New Zealand beech species yield medium density hardwood timber with a fine, even texture and straight grain (Smale et al. 2012). Properties are excellent for sawing, machining, turning and nailing (Smale et al. 2012).
Beech wood is suitable for applications where strength, stability and decorative appearance are required. The timber is currently being finished into veneer, furniture, dimensional timber and decking products (Smale et al. 2012). Its fine even texture makes beech an excellent furniture timber and it is suitable for decorative veneers (Smale et al. 2012).
Neither heartwood nor sapwood is subject to attack to Anobium borer (Smale et al. 2012). Silver beech sapwood is more susceptible to decay than sapwood of the other beech species (J. Wardle, pers. comm).
Silver beech is easy to dry and black beech is also moderately easy to dry (Smale et al. 2012). The presence of tension wood makes hard beech and red beech more difficult to dry, particularly in larger sections (Smale et al. 2012).
Black beech is strong, stable and durable (Smale et al. 2012). Heartwood of black beech dries to a straw colour and the sapwood whitish-brown (Smale et al. 2012). It is used for furniture, exposed floors and interior joinery and tool handles (Smale et al. 2012).
Red beech heartwood is light red to medium red-brown in colour and the sapwood light brown to white (Smale et al. 2012). Heartwood is strong and durable and the wood has exceptional dimensional stability (Smale et al. 2012). Red beech timber is close grained and has a lustre and sheen that tends to improve with age, along with fine turning properties and achieves a very smooth finish (Smale et al. 2012). Red beech is used for furniture, exposed flooring, stair treads and decorative interior finishing, along with exterior decking and pergolas (Smale et al. 2012).
Silver beech has a pinkish to red overtone and is an attractive furniture timber with a deep lustre (Smale et al. 2012). Although lighter and more easily worked than the other beech species, it is also less durable (Smale et al. 2012).
Hard beech is light yellow-brown, durable dense and strong (Smale et al. 2012). It is the most difficult of the beech species to saw and season (Smale et al. 2012).
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.