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
Douglas fir is a cool climate species with considerable potential for wilding spread. It doesn't like exposure to high winds and prefers reasonable soils. It may not be the best choice for erosion-prone North Island hill country, but may be a good option for steeplands in the Marlborough and Tasman regions of the Northern South Island.
Douglas-fir, Pseudotsuga menziesii, is native to western North America, growing in humid summer-dry regions with mild climates, from sea level up to elevations of 800 metres in British Columbia (Miller & Knowles, 1994).
Douglas-fir is one of the most valuable and commercially important timber species in western North America (Miller & Knowles, 1994).
In New Zealand Douglas fir ranks as the second most important plantation softwood after radiata pine (Miller & Knowles, 1994) with a total area of approximately 105,000 ha (Ministry for Primary Industries, as cited in Kimberley et al. 2017). Although it eventually becomes a very tall and large-diameter tree, volume growth is relatively slow for the first thirty years (Miller & Knowles, 1994). At higher altitudes where there is a greater risk of wind and snow damage, Douglas-fir performs better than radiata pine in terms of yields, but profitability models for higher quality sites show radiata pine to be the better performer because of the shorter rotation required for the same volumes (Miller & Knowles, 1994) and possibly a log price premium that is marginal. However, exceptional growth rates have been recorded for stands in higher rainfall South Island high country, equivalent to the best radiata mean annual increments (Miller & Knowles, 1994).
Douglas-fir remains a relatively healthy species in New Zealand, although Swiss needle-cast Phaeocryptopus gaeumannii, a disease that arrived in New Zealand in 1959, is considered to be the greatest threat to its health (Miller & Knowles, 1994). Swiss needle cast significantly reduces growth in Douglas-fir, especially where stands are older or unthinned (Miller & Knowles, 1994). Carefully siting the species to favourable sites and avoiding high stocking rates are options for managing the disease (Miller & Knowles, 1994). Damage levels are also lower in coastal provenances so appropriate genetic material should be deployed (Miller & Knowles, 1994).
Douglas-fir prefers moderately high rainfall and moist, free draining un-compacted soils (Miller & Knowles, 1994). On sites where rainfall is less than 1000 mm Douglas-fir prefers shadier, southerly sapects (Miller & Knowles, 1994). Although altitudinal limits are well above 800 m, exposure is the primary limitation to good growth. Douglas-fir is resistant to snow damage (Miller & Knowles, 1994). Douglas fir does not grow well in warmer, more humid northern areas of the North Island (Miller & Knowles, 1994). Thomsen (2011) suggested that Douglas-fir is a good choice for lower altitude Hawkes Bay, provided the site is not stressed for soil moisture and a suitable seedline is selected. Douglas fir prefers a cooler climate, doesn't like hot summers and is subject to disease in the North Island where conditions are warm and humid (M. Dean, pers. comm). The "sweet spot" for Douglas fir may be the Nelson region, where conditions are cool enough for good tree health and warm enough for producing adequate wood density (M. Dean, pers. comm). High wind exposure, such as experienced in many parts of Wairarapa, Canterbury and the southern South Island, is not favoured.
Douglas-fir is considered to be more wind-firm than radiata pine, but "exposure is considered to be the main limitation to its satisfactory growth in New Zealand" (Miller & Knowles, 1994). Strong winds can cause leader loss and crown deformation, but recovery tends to be rapid with little stem deformity resulting (Miller & Knowles, 1994). Stem deformation can also occur on fertile ex-pasture sites (Miller & Knowles, 1994). Exposure damage can be reduced by planting at a high stocking so that establishing trees provide mutual shelter to each other (Miller & Knowles, 1994). Root grafting occurs and improves both tree and soil stability (M. Dean, pers. comm). Where the site has high exposure to winds, radiata shelterbelts are used in New Zealand to both improve form and stiffness for the more valuable Douglas-fir (De La Mare and Hitchings, 2007). P. radiata x P. attenuata shelterbelts are also showing good potential for sheltering the Douglas fir crop in exposed areas of the lower South Island (M. Dean, pers. comm).
Failure of Douglas-fir plantings can result from late spring or summer frosts at the time when seedlings are flushing (Miller & Knowles, 1994). Well conditioned seedlings with moist roots are required for successful establishment and the planting site needs to be free from weed competition (Miller & Knowles, 1994). Seedlings inoculated with suitable mycorrhizae are required for establishing Douglas-fir on new sites.
Douglas-fir is slower growing than radiata pine for the first few years and very sensitive to weed competition, so releasing is recommended for two to three years after planting (Miller & Knowles, 1994). Tolerance to herbicides is lower than for radiata pine and pre-plant preparation should to be to a very high level (Miller & Knowles, 1994). High survival rates should be aimed for, to ensure even branch suppression because stand gaps result in excessively large branches (Miller & Knowles, 1994). Shaded branches tend to be suppressed quickly and tend to leave sound knots within the stem (Miller & Knowles, 1994). However, Douglas-fir is still subject to black knots and these occur for approximately 8 years after branches become moribund, which is not a problem where these are less than 25mm (M. Dean, pers. comm).
Provenance trials started in 1955 showed that coastal Californian and Oregon provenances were generally more vigorous than those from Washington (Miller & Knowles, 1994). A breeding programme that started in 1970 resulted in a grafted seed orchard being established in 1989, followed by selection of more parent trees from coastal fog-belt provenances in North America (Miller & Knowles, 1994). Improved seed is commercially available but the species requires a continued genetic improvement programme (Miller & Knowles, 1994). Currently, industry routinely plants improved progeny, with seed from first and second generation seed orchards for some provenances (M. Dean, pers. comm). A breeding programme continues to make improvements and is looking to introduce the best families from over 300 tested families of North American imported progeny trialled in NZ conditions (M. Dean, pers. comm).
Douglas-fir is primarily grown for framing timber in New Zealand and is considered to be more dimensionally stable and consistently stiffer than radiata pine throughout much of the South Island (De La Mare and Hitchings, 2007). Douglas-fir is also "highly sought after for use in exposed interior posts and beams because of the species’ good stability and low incidence of twist" (Kimberley et al. 2017).
Conservative silvicultural regimes are often practiced with Douglas-fir to ensure production of high stiffness wood, including planting in sheltered locations, thinning lightly and holding high residual stockings (De La Mare and Hitchings, 2007).
Douglas fir will carry a higher basal area per hectare than radiata pine, and although it is considered to be a reasonably shade tolerant species it is difficult to grow under a canopy, so is not suitable for selective harvesting (M. Dean, pers. comm). One conservative method for continuous cover forestry suggested by MacLaren et al. (2006) is to harvest coupe sizes of about 0.25 ha, and harvesting half of the stand when the oldest trees were 60 years old, with half of the stand already 30 years old at that stage.
Tree weeds that are a problem in young Douglas fir stands in the South Island include radiata pine regeneration and sycamore (M. Dean, pers. comm).
Branch size is the most important factor influencing wood quality for structural applications (Miller & Knowles, 1994; Kimberley et al. 2017). A high initial planting stocking of 1600 stems per hectare is recommended in order to limit branch size in the second log (Miller & Knowles, 1994). On dry sheltered sites branching tends to be finer, allowing for a reduction in initial stocking (Miller & Knowles, 1994). Miller & Knowles (1988) also suggested that larch could be interplanted with Douglas-fir as an expendable self-thinning component to reduce the branch size of Douglas-fir. However hybrid larch tends to be more vigorous than Douglas fir for the first 8-10 years so care would be required (M. Dean, pers. comm).
The second most important factor influencing stiffness is wood density (Miller & Knowles, 1988; Whiteside et al. 1976 as cited in Kimberley et al. 2017). Average basic density of sawn timber is generally lower than for radiata pine, reported as averaging 400 kg/m3 for 50-60 year old trees by Miller & Knowles (1994) and 427 kg/m3 by Kimberley et al. (2017) for outerwood from 40 year old trees averaged across the country.
Density increases gradually with age from about the seventh growth ring from the pith until about thirty years of age, then stabilises (Kimberley et al. 2017). This density increase with age, at approximately 50 kg/m3, is significantly less than for radiata pine (Miller & Knowles, 1994; Kimberley et al. 2017) at 110 kg/m3 from rings 1 to 30 (Kimberley et al. 2015 as cited in Kimberley et al. 2017). There is also a density decrease from north to south, approximately 65 kg/m3 between the central North Island and South Otago (Miller & Knowles, 1994). Kimberley et al. (2017) described this latitudinal effect as a negative relationship between wood density and air temperature, particularly winter air temperature. Cooler, higher elevation sites have lower wood density than warmer sites in the North Island because warmer climatic conditions allow for a longer latewood growth period, meaning the proportion of higher density latewood is greater (Kantavichai et al. 2010b as cited in Kimberley et al. 2017).
Harris (1978, as cited in Kimberley et al. 2017) recommended that average wood density should be above 400 kg/m3, which is achieved only in the North Island and upper South Island. Improved genetics have led to North Island Douglas fir having a density averaging 462 kg/m3 for trees planted after 1970 (Kimberley et al. 2017), whereas density for lower South Island Douglas-fir averages approximately 100 kg/m3 less.
Higher fertility soils also tend to produce lower density wood (Kimberley et al. 2017).
Ring widths in New Zealand grown Douglas-fir are between 3 mm and 6 mm (Miller & Knowles, 1994). Although the effect of ring-width on density is very small, strength does increase slightly with narrowing ring width (Miller & Knowles, 1994). Although increasing ring width can be related to decreasing stand density, stand density has little influence over outer wood density, which is related to number of growth rings (Kimberley et al. 2017). The wood surrounding the pith is lower density than the outer wood. Therefore, as the proportion of basal area comprising this low density juvenile wood increases, then average wood density of the stem decreases. For cooler southern regions where density might be marginal, in order to achieve a satisfactory density without an extended rotation, the percentage of the total basal area comprising juvenile wood might need to be controlled by using a very high initial stocking, especially in more fertile soils, followed by progressive thinning that ensures good growth rates are achieved throughout the rest of the rotation. Longer rotations for larger diameters with a greater proportion of mature wood would still require a silvicultural regime that minimises branch index (larger branches). Breeding trees for greater wood densities offers another option for growers in cooler southern regions to achieve satisfactory wood densities (M. Dean, pers. comm).
For warmer sites, Harris's recommendation (1978, as cited in Kimberley et al. 2017) of growing Douglas fir as rapidly as possible on a short rotation could possibly be followed, even on higher-fertility sites, provided the regime produced a small branch index, because density is not the critical factor determining value.
Strength properties are broadly consistent outside of the central corewood area (juvenile wood), with this being the dominant factor affecting characteristic strength for Douglas-fir (Miller & Knowles, 1994). Miller & Knowles (1994) stated that "wood density and strength do not decrease near the pith, allowing framing timber to be sawn from much smaller logs including thinnings". Indeed, the price differential between log diameters of less than 14 cm and greater than 300 mm is only $15-20 per m3 for export logs (M. Dean, pers. comm).
Miller & Knowles (1994) recommended to thin to waste before the stand reaches twenty years old, to a stocking of between 300 and 600 stems per hectare, with no further thinning and harvest at about 45 years old. Early thinning ensures diameter growth is maintained and a healthy green crown is encouraged (Miller & Knowles, 1994). Traditional regimes involved rotation lengths of between 60 and 80 years and delayed thinning of trees only once between 30 and 40 years old, which with high land values now results in prohibitive growing costs (Miller & Knowles, 1994).
Current practice is to thin to 500-600 stems at about 15 m mean top height where thinning is to waste (M. Dean, pers. comm). Where terrain and tree form permit production thinning, a waste thinning is undertaken at 10-12 m mean top height to remove poorly formed trees leaving a stocking of 1000-1200 sph, with the first production thinning at 18-20 m mean top height to halve stocking to 500 stems per hectare, then a second production thin at 25-27 m mean top height to 350 stems per hectare (M. Dean, pers. comm).
Because of "its tolerance of wind, snow and low winter temperatures Douglas-fir is often the best species for moister areas of high country" (Miller & Knowles, 1994). However, Douglas-fir is considered to be a species capable of vigorous wilding spread, in particular "cooler, inland hill and high country areas, where the surrounding vegetation cover and grazing levels can be light" (Ledgard, 2007a). Douglas-fir is also "more shade tolerant than the common pines and will invade shrublands" (Ledgard, 2007). Although Douglas -fir is less palatable to sheep than pine (Miller & Knowles, 1994), well grazed and improved pastures such as found in fertilised, developed farmland pose little threat for spread of wilding Douglas-fir (Ledgard, 2007a). Where there is a risk of wilding spread, despite the site having potential for good Douglas-fir growth, the species should not be planted (Ledgard, 2007a). Species that are less prone to spread such as ponderosa pine and radiata pine are a better choice (Purey-Cust, 2008). Viable seed production occurs from 12 years of age so to removal of wildings before this age short-circuits continued spread (Miller & Knowles, 1994).
The potential for wilding spread in southern North Island hill country should be reduced where there is good rainfall (M. Dean, pers. comm) and in summer-moist areas it doesn't tend to produce cones (C. Low, pers. comm).
High longitudinal shrinkage near the pith, spiral grain and compression wood, features of radiata pine, are absent in Douglas-fir (Miller & Knowles, 1994). Shrinkage is slightly higher than for radiata pine but Douglas-fir is more stable than radiata pine in response to humidity change (Miller & Knowles, 1994).
Douglas-fir is less suitable than other species for appearance timber, because of difficulties in machining and finishing (Miller & Knowles, 1994). Douglas-fir also requires care in nailing and tends to split at board ends (Miller & Knowles, 1994). Douglas fir can be peeled satisfactorily, however the species has not been used in New Zealand for manufacture of plywood and LVL, possibly because local logs peeled poorly in research trials (Miller & Knowles, 1994).
Douglas-fir can be chipped for manufacture of reconstituted boards and pulp (Miller & Knowles, 1994). It is an important pulpwood species in the Pacific Northwest but its use in New Zealand is limited, possibly because of the high bleach requirement for kraft pulps and poor brightness as mechanical pulp (Miller & Knowles, 1994).
Kiln schedules are recommended to be milder than those used for radiata pine, to avoid surface checking (Miller & Knowles, 1994).
Douglas-fir "will not admit water-borne preservatives " and is regarded as "refractory for preservation because of the difficulty of getting chemicals into the heartwood and dry sapwood" (Miller & Knowles, 1994). For internal structural applications Douglas-fir is listed as suitable for H1.2 boron treatment (NZS 3640:2003).
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.