Report: Trees for steep slopes
Sustainable Forest Solutions
Reviewed by Mike Marden, July 2018
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In a nutshellCypress has the potential to be a profitable and useful species for plantation forestry on erodible steeplands. It is well suited to longer rotations, is productive and produces a high-value timber. Cypress is reasonably tolerant of wind and lower fertility soils but understanding its limitations is important. Wet soils should be avoided, along with extremely low fertility. Siting of cypress requires careful species/clone selection, therefore an intimate understanding of the individual species site requirements. Cypress is well suited to either clearfell or continuous cover forestry as either pruned or unpruned regimes.
Macrocarpa and cypress have been planted for shelter, shade and aesthetic values for well over 100 years throughout New Zealand, primarily because of easy establishment and good growth rates (Satchell, 2017). Because of the highly regarded wood properties and domestic demand for the timber, macrocarpa has emerged as a viable plantation forestry option for growers. However, siting requires care, as does management for high quality timber.
Cypress is reasonably site demanding, with a range of environmental factors that regulate productivity, including level of wind exposure, salt laden winds, soil moisture, depth and fertility (Satchell, 2017).
Tantrum (2005; 2010) suggested that good drainage is necessary for a successful cypress crop. High soil moisture combined with high soil fertility together encourage vigorous foliage growth while discouraging root development, thereby inducing toppling (Brown, 2005c). Cypress planted on fertile ex-farm sites subject to periodic high wind and heavy rain events are particularly vulnerable to toppling (Brown 2005c). However, toppling is rare in low fertility, dry, exposed sites (Brown, 2005c), suggesting the combination of fertility and high soil moisture are to be avoided. Wide spacing of trees could also increase the risk of toppling on fertile ex-pastoral sites (Fitzsimons, 2014).
For fast growth rates macrocarpa requires more soil fertility than radiata pine (Hocking, 2006) but significantly less than redwood (D. Tantrum, pers. comm). Tantrum (2010) suggested that both reasonable soil fertility and reasonable rainfall are required for good cypress growth. However, cypress has proven to be very resilient to climatic extremes and although slower growing, can tolerate both low rainfall and low fertility sites (Satchell 2017). Cypress species can be matched to most steepland hill country sites in the North Island and are well suited to the soils of erodible hill country, provided these are not skeletal and there is some fertility (D. Tantrum, pers. comm).
Resilience to wind exposure is highly dependant on clone or species (Satchell, 2017). Macrocarpa has "excellent tolerance to extreme salt wind, even salt spray" (Hocking, 2006b), while Leyland cypress can grow in adverse conditions and tolerates both exposure and also dry sites (Brown, 2005b). Lusitanica cypress, on the other hand, is not tolerant of exposure to wind, especially salt laden winds (Milne, 2006a). Himalayan cypress (C. torulosa) is extremely tolerant of dry conditions and wind exposure and although it is slower growing than other cypress species, produces better results on bony exposed ridgetops (D. Tantrum, pers. comm).
Nootka cypress hybrids (e.g. Leylands, Ovensii) are more adaptable to low soil fertility than macrocarpa and lusitanica (Milne, 2006b; Satchell, 2017) and are likely to have greater tolerance to cool and wet conditions (Milne, 2006b). One particular Nootka hybrid clone, Oven's cypress ("Ovensii"), has proven to be healthy, vigorous and adaptable to site and grows well throughout most of the country (Milne, 2006a; Satchell, 2017). Some limitations have been reported, in particular it's intolerance of hot, dry or exposed sites and salt laden winds (Milne, 2006a). Observations suggest that Ovens cypress does grow well on exposed inland North Island sites (S. Rapley, pers. comm), even Wairarapa where strong hot winds are common (N. Cullen, pers. comm), but not in South Otago where winds are very strong but also cold (N. Cullen, pers. comm).
Long rotation lengths are required for a cypress clearwood regime in order to produce high volumes of the more valuable clear heartwood, and extended rotations of macrocarpa offer the potential to produce very high returns (Laurie, 2013).
Production of high value pruned buttlogs on 36-40 year rotations involves:
- Early pruning that minimises the defect core; along with
- severe early thinning down to 300-400 stems per hectare (Trost, 2005).
Such intensive management, it is argued, improves economic value by optimising recovery of clear heartwood available for processing (Trost, 2005). However, for highest returns, the pruned buttlog regime would take into account the tradeoff between headlog quality and buttlog volume and although the aim is for large diameter trees that produce large volumes of clear heartwood:
- Reducing tree stockings to low numbers in order to produce large diameter trees carries with it a risk of windthrow resulting from a reduced basal area (J. Moore, pers. comm).
- Trees respond to early thinning by producing large branches and knots, which also cause degrade and timber defects in the headlog that severely limits value (Thomsen, 2011).
Thomsen (2011) therefore suggested that competition needs to be retained for long enough to restrict branch size in the headlog. Delayed thinning, in contrast to severe early thinning, produces deeper crowns and less tapered trees (J. Moore, pers. comm). However, radial increment is slowed in order to achieve height growth, finer branching and better quality headlogs. This intervention point, if delayed for too long, would result in excessive height to diameter ratios, which carries with it an increased risk of windthrow, together with a higher risk of bark encased knots forming in the wood from dead branches (J. Moore, pers. comm).
Macrocarpa and cypress have good tolerance to shade (Hocking, 2006b), suggesting that live branches are retained well, even under high stocking levels. Macrocarpa also "tends to produce larger numbers of branches and larger branches than other cypresses which makes for more expensive pruning" (Hocking, 2006). Therefore controlling level of branching with tree stocking would appear to be a sensible strategy to improve unpruned log value and reduce management costs. However, balancing the need to maximise productivity and fully occupy the site with the need to thin down to an appropriate stocking that maximises production of clear heartwood i.e. logs with sufficient diameters, while at the same time mitigating the risk of windthrow, involves a strategy of gradually but progressively removing a proportion of the basal area over time (J. Moore, pers comm.). Thinning severity needs to be carefully managed.
Younger, smaller diameter pruned trees have a high percentage of sapwood and will only yield low volumes of clear heartwood (Satchell, 2013a). However, if the DOS is kept to a minimum, high recoveries of clearwood that includes sapwood is possible in rotations as short as 20 years (Satchell, 2013a). That said, level of market demand for clearwood that includes sapwood has not been resolved. Nevertheless, silvicultural operations that produce a very small DOS considerably improve recovery of clearwood regardless of rotation length, especially the higher value clear heart (Laurie, 2013; Satchell, 2013a). This requires pruning little and often (Tantrum, 2010), which is costly but justified by the value of the crop.
Intensive management and impeccable timing of silvicultural interventions may not be a suitable regime for steeplands. Where intensive management isn't practiced, rotation lengths of 50-60 years may offer best clearfell value recoveries (Trost, 2005), because "Big trees yield a higher percentage of clear timber" (Tantrum, 2010).
Although individual trees are generally slower growing than radiata pine, because cypress has to some degree fairly consistent wood properties from pith to bark (Scion, 2014) and its shade tolerance offers high basal areas in short rotations provided stocking is high, young unpruned trees with small diameters can potentially produce good volumes and returns, provided sawn recoveries of appearance grades were achievable from small diameter logs (Satchell, 2017). However, because the durability of inner heartwood tends to be lower than for outer heartwood (Dungey et al. 2014) and the percentage of sapwood is high in smaller diameter logs, the regime is contingent on market demand for sawn product that can include sapwood.
Small diameter sawlogs, because knots are small and green, are still able to be utilised for high value products and "a small unpruned tree yields better timber with green knots than one that has been pruned" (Tantrum, 2010). This is because "small diameter pruned trees produce very little clear timber and what is not clear usually has pruning scars which make it unsuitable for dressing grade" (Tantrum, 2006). Silvicultural management for short rotations would aim only to minimise bark encased knots and maximise volume production of logs above a minimum utilisable diameter. The trade-off is between knot size and log diameter. Dead knots only occur when stands are thinned too late and after branches have died, with a subsequent growth response (such as from thinning). Although unpruned cypress would likely require some thinning to ensure log diameters are sufficient for processing, in a clearfell setting returns as annualised volume production depend on only two factors, rotation length and stocking. The customised regime simply requires either a predetermined rotation length that determines final crop stocking, or a predetermined final crop stocking based on required diameters, that sets rotation length. This takes advantage of the ability for cypress to produce large basal areas in a relatively short time and offers a simple management regime that either culminates in clearfell, or develops into selective harvesting under a permanent canopy.
Thinning of highly stocked stands of trees with dense crowns can be problematic (M. Dean, pers. comm). Ringbarking offers a cost efficient method for thinning cypress (Brown, 2005c; Satchell 2018) and is practiced commercially for cypress that is planted at high stockings to both give high selection ratios and also to control branching (Growing cypresses for timber, Some Example Regimes).
The simplicity of an unpruned regime has appeal for hill country plantations because of the low levels of silvicultural intervention required and the versatility in terms of rotation length (D. Tantrum, pers. comm).
Macrocarpa often produces heavily fluted buttlogs. Both genetic and environmental influences appear to contribute to fluting (Hocking, 2006).
Cypress canker disease is regarded as the biggest constraint to success with cypress as a plantation species (Satchell, 2017). Tantrum (2010) suggested that cypress canker is caused by stress − "too wet, too dry, too exposed, over pruning, mineral deficiency or any other reason causing stress to the trees". Some attribute the cause of canker to mineral deficiencies in the soil (A. Laurie, pers. comm). Satchell (2017) found that genotypes resistant to canker are available and the focus should be on breeding for resistance.
Macrocarpa performs best in cooler regions because of the risk of canker (Milne, 2006a). Hocking (2006b) claimed that problems with cypress canker may be due to misplacement and mismanagement, with the worst canker normally reserved to exposed north facing slopes. However, after comparing the same clones across a range of sites, Satchell (2017) concluded that canker-tolerant clones are resilient across all sites, offering flexibility in terms of matching clone to site. However, clonal (cutting-grown) trees are slower to establish and are more expensive (Tantrum, 2010). Growers of less resilient seedlines and clones might best follow Hocking's (2006b) advice that although "sometimes things work better than expected on less than ideal sites", macrocarpa is best planted on cool sites such as Southern slopes".
Thomsen (2011) suggested that lusitanica cypress has the best potential for timber production on medium to good quality sites. However, because there is so much genetic variability in the seedling planting stock currently available in New Zealand, planting numbers need to be high for sufficient selection choices (Thomsen, 2011). On moist sites, young trees are vulnerable to toppling, which can be addressed to some degree by tip pruning, i.e. cutting 50% off large branches annually (Thomsen, 2011). However, cypress is vulnerable to toppling at all ages and simply avoiding planting the species on moist sites appears to be the best solution to avoid toppling later on.
Leyland cypress clones have performed well in most areas, but have proven to be susceptible to cypress canker on some North Island sites (Milne, 2006b). Leylands are slower growing than macrocarpa and lusitanica and tend to be more heavily branched, but with straighter stems (Low, 2006).
Cupressus torulosa (Himalayan cypress) has a high quality scented light-brown timber, good narrow pyramidal form, excellent health and tolerates very dry and windy sites, but is slower growing than macrocarpa and lusitanica (Milne 2006a). Thomsen (2011) suggested C. torulosa is a good option for medium to poor quality sites because it is tolerant of dry, cold and windy conditions. The timber is highly scented and with a golden colour similar to macrocarpa (Satchell, 2010b).
Lawsons cypress Chamyaecyparis lawsoniana appears to grow best in cooler sites that are also fertile (Milne, 2006a).
Propagation is by either seed or cuttings and stock is either provided bare-rooted or containerised. Bare rooted cypress survive well if planted in the winter and are usually lower cost than container-grown trees. Seedlings are highly variable in both form and growth and are often planted at high stockings, whereas improved clonal stock offers good form but on average has slower growth rates than seedlings (D. Tantrum, pers. comm). Scion have a breeding programme well underway for cypress that aims to provide growers with improved clonal and seedling stock (H. Dungey, pers. comm).
Cypress are cold hardy (Milne, 2006a) and can be planted in sites too frosty for establishment of radiata pine (M. Self, pers. comm).
Snow damage to young trees can cause severe stem deformation, in particular to lusitanica and macrocarpa, whereas Arizona cypress (C. arizonica) and the nootkatensis hybrids (Leylands and Ovensii) can withstand heavy snowfalls (Wardle, 2012). As mature trees, lusitanica suffers from little snow damage whereas macrocarpa can suffer extensive branch breakage and stem deformation (A. Gordon, pers. comm). Lawson cypress and the Nootka cypress hybrids (the Leylands and Ovensii), because of their thin horizontal or downwards pointing branches, shed snow and withstand heavy snow events (Cairns, 2012).
Stock and deer can cause extensive damage by stripping bark from trees (Purey-Cust, 2011). Possums can strip bark from the growing shoot which can result in poorly formed trees (M. Parker, pers. comm) and kaka is an emerging threat.
Very high returns can be expected from well managed and mature stands of macrocarpa, provided there are available markets that are focussed on high-end uses for the timber (Laurie, 2013). Domestic demand for high quality cypress timber is limited however, and log prices could be vulnerable if supply begins to exceed demand (Laurie, 2013). However, if this were to occur, export markets for sawn timber would likely be available to Asia and export markets are currently available for logs (A. Laurie, pers. comm).
Cypress timber is used for higher-value appearance applications such as doors, joinery and kitchens and those requiring stability such as boat building (Low, 2006). Strength is similar to radiata pine, but stiffness is lower (Low, 2006). In particular, younger lusitanica lacks stiffness and strength, but this increases with tree age (Low, 2006).
Pruned cypress logs suitable for producing clear heartwood attain the best market price (Dungey et al. 2014). Heavily branched logs and those with dead limbs have limited market opportunities available (Trost, 2005).
Cypress logs saw well and are very stable, and store indefinitely if only heartwood is required (Tantrum, 2006).
Market preference is for timber that is free of knots or with small tight knots for appearance applications (Trost, 2005), with both heartwood and sapwood suitable for panelling and indoor furniture, and heartwood suitable for outdoor furniture and external cladding. (Dungey et al. 2014). The "magic length for clears is 2.1 metres" (Tantrum, 2006).
Cypress sapwood holds reasonable levels of durability and accelerated decay structural framing tests undertaken by Scion demonstrated durability to be greater than Douglas fir sapwood (Hedley, 2005), which itself is more durable than radiata heartwood (Hedley et al. 2009).
Kiln drying of cypress cannot be practiced from green because the timber cracks and warps (Low, 2006). Air drying is standard practice, followed by low temperature kiln drying (Tantrum, 2006).
Macrocarpa remains the local industry cypress species "timber standard and market preference" (Hocking, 2006b), perhaps because it has a long history of use. Macrocarpa and lusitanica yield a rich honey coloured heartwood, whereas Leyland cypress, Ovensii and Lawsons cypress yield a paler timber. Tantrum (2006) suggested that although darker timber has traditionally been more popular, "some people argue that lighter timber is better because it can always be darkened".
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.