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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|Early growth rate||6|
|Root decay rate||9|
In a nutshellRedwood is not well suited to steepland slopes unless sheltered and with reasonable soil depth and fertility. It is very well suited to the lower slopes where it is a very productive species and ideal for land stabilisation. Interplanting with alder offers a simple low cost management option for hill country.
Where there is a high risk of post-harvesting landslides, redwood has the potential to become an important erosion-control forestry species (Phillips et al. 2012). This is because of good growth rates in early years, the ability to produce many fine lateral roots and the ability to coppice once the tree is removed (Phillips et al. 2012).
Redwood is unique among conifers in that it produces coppice growth from roots and stumps, so regenerates prolifically in cutover stands (Berrill & O'Hara, 2007). This gives redwoods exceptionally high soil stabilisation properties (Libby, 2006). Redwood offers good erosion control in steep country because the roots remain alive after harvest and continue to reinforce the soil (Burdon, 1975 as cited in Meason et al. 2012; Phillips et al. 2012; Silcock, 2008; Webster, 2007). Roots also graft with neighbouring trees, forming a continuous network, providing additional reinforcement (Wallwork & Rapley, 2009; Clinton et al., 2009). However little information is available on how many years it takes for young trees to form a live network of roots (Clinton et al., 2009).
Because stumps coppice, rapid restoration of the forest canopy takes place (Burton, 1975 as cited in Meason et al. 2012) and re-establishment costs are minimised (Webster, 2007). However, because regeneration tends to be confined to stumps, this can leave large gaps between trees along with a "clumpy" distribution of trees (Berrill and O'Hara, 2007). The coppice also requires silvicultural management to reduce sprouts to no more than 3 per stump (Berrill & O'Hara, 2007).
Because tree canopy functions, such as the ability to intercept rainfall and transpiration, are lost when trees are felled, the ability for redwoods to coppice does not entirely negate the negative effects of harvesting on soil (Madej, M. 2010; Reid & Keppler, 2011 as cited in Meason et al., 2012). Clearfelling significantly reduces live root mass volume even where stumps stay alive, therefore sufficient root cohesion to prevent erosion may take 10-15 years to re-occur after harvesting (Reid & Keppler, 2011 as cited in Meason et al., 2012).
Redwood also has "the ability to produce adventitious roots near the new ground surface when smothered by silt" which "is an additional advantage of redwood over other species if, for example, any surface erosion or landslides were to occur and deposit sediment within a stand of trees" (Marden 1993 as cited in Phillips et al. 2012). Because redwood is good at withstanding flood events and sedimentation below sites where gully erosion is likely, redwood is a good option for planting in flood zones beside tributaries, to filter debris flows before these reach more significant water courses (P. Silcock, pers. comm.).
Phillips et al. (2012) studied the below ground characteristics of young redwood trees in the context of erosion control and found that at 3 years of age redwood compared favourably with radiata pine. Root biomass, total below-ground biomass and total tree biomass were similar for the two species, although "in some situations radiata is more likely to reach a given RCD in a shorter time from planting than redwood, the differences are not expected to be great for trees less than about 6 years from establishment" (Phillips et al. 2012). This is because along with the use of clonal material (R. Coker pers. comm. as cited in Phillips et al., 2012) "recent experience with pre- and post-planting care, in particular good site preparation, attention to nutrient status, and control of competing plants, now results in much better survival and early growth than previous experience" (Libby, 2007).
In warm North Island sites with sufficient rainfall and good soils, redwood growth rates are comparable to those for radiata pine (S. Rapley, pers. comm). However, siting can have a far more significant impact on redwood growth rates than those for radiata pine. In particular, redwood requires medium to high productivity sites with sufficient soil depth. While radiata pine grows relatively consistently across a site with variable soil qualities, redwood will be extremely variable in terms of growth rates (P. Silcock, pers. comm). In particular, eroded gullies with no topsoil or hard clays underlying shallow soils produce poor growth rates, which may be more soil moisture deficit related than fertility (P. Silcock, pers. comm). On lower productivity sites redwood only grows at half the rate of radiata pine during the early years, potentially extending the rotation length to 60-80 years (S. Rapley, pers. comm.). Redwood performs particularly well below steeper eroding land on the lower slopes where there is sufficient topsoil (S. Rapley, pers. comm).
Redwood can be very productive in terms of annual growth rates, but only if well sited (Libby, n.d.). Redwood is capable of matching the growth performance of radiata pine on warm, moist, sheltered, fertile sites (Webster, 2008). However, redwood is more site-limited than radiata pine and can be "badly affected by chronic winds, particularly if those winds are salt-laden" (Libby, 2007). Brown (2007) described redwood as requiring "a temperate climate, decent soils and regular rainfall". Limitations include heavy frosts, salt spray and strong prevailing winds, although redwood can "withstand occasional gales" (Brown, 2007) and is "surprisingly resistant to toppling and breakage from periodic storms" (Webster, 2008).
Redwood is considered to be a good choice for moist soils in sheltered sites (Thomsen, 2011). Webster (2008) found that best growth rates are found in warm, moist sites in coastal Bay of Plenty, Northland, Waikato, inland Taranaki, the King Country and the East Coast of the North Island. South Island locations suitable for redwood include the Nelson region and sheltered parts of the West Coast (Webster, 2008). Careful site selection would be necessary in the sedimentary soils of the Wanganui hill country exposed to strong coastal winds and seasonal drought (Webster, 2008). Exposed sites in Wairarapa produce wind-affected trees (S. Rapley, pers. comm). Young trees are especially intolerant of exposure and require shelter from strong winds (Knowles & Miller, 1993).
Redwood tolerates periodic storms but is less resilient to persistent prevailing winds, especially where cool or salt laden (S. Rapley, pers. comm). Exposed ridgetops produce "windshorn" trees of little value (P. Silcock, pers. comm). Slopes exposed to persistent wind produce second rate trees, whereas as little as ten metres down the slope on the leeward side of a ridge produces good quality trees (P. Silcock, pers. comm). To avoid production of different timber types, ridgetops and exposed slopes in redwood plantations could be planted in the extremely resilient giant sequoia.
Redwood has a preference for mild climates and is vulnerable to out of season frosts (Webster, 2008) such as those that occur on the central North Island volcanic plateau (Dean, 2007). Redwood is resistant to snow damage because its thin horizontal branches shed the snow (Cairns, 2012).
Because initial stocking tends to be low and plants expensive, establishment of redwood requires attention to quality of planting and clonal tree stock, along with timely releasing, to ensure an evenly stocked forest is established (Mineham, 2009). Newly planted trees have a high soil moisture requirement and weed competition, especially grasses, can seriously impede growth (Knowles & Miller, 1993).
Interim recommendations in 2007 were for a final spacing of four to five metres between trees (Brown, 2007). Knowles & Miller (1993) suggested a final crop stocking of between 200 and 300 stems per hectare with pruning up to 10 m.
Redwood can accumulate large volumes of stemwood per hectare and can grow into very large trees of considerable age with few pests or diseases (Wallwork & Rapley, 2009; Libby, 2006). These characteristics led Dean (2007) to suggest that redwoods could be managed under more innovative regimes than clearfell harvesting. Because redwoods are also shade tolerant they are well suited to being managed under continuous canopy systems (Webster, 2007; Wallwork & Rapley, 2009). Being shade tolerant, coast redwood "can recover from almost indefinite suppression" (Knowles & Miller, 1993). Berril and O'Hara (2007) found that stands thinned to half their original basal area "quickly returned to a similar rate of productivity as unthinned stands, but there were many fewer trees, resulting in greater diameter growth". Dean (2007) suggested that a more structured effort is required "to identify and answer the key ‘how to’ questions of uneven-aged stand management".
Selective logging of regrowth redwood forest with haulers is common practice in California. However, selective logging is more expensive than clearfelling and taking into account the time value of money it may be difficult to justify on economic grounds alone (S. Rapley, pers. comm) unless log values were to double from current prices (P. Silcock, pers. comm). Pruned plantations on flatter ground could be harvested by extracting every second row, leaving behind half the volume to grow into large buttlogs for a potentially profitable outcome (S. Rapley, pers. comm). One option for steeper country is for tethered harvesting machines to harvest strips of trees (P. Silcock, pers. comm). Importantly, redwoods can be released at any age, i.e. they are never too old for a growth response after removal of adjacent competing trees (S. Rapley, pers. comm).
On sheltered fertile sites volume production can equal that from radiata pine (Knowles & Miller, 1993). Brown (2007) considered New Zealand's conditions to be "better suited to growing redwood than anywhere else on earth". Brown (2007) and Dean (2007) both suggested clearfell rotations of 35 years, with Rydelius (2007) recommending planting clonal stock at 500 stems per hectare and harvesting at 30 years old.
Redwood is pruned to maximise production of high-value clear heartwood. Pruning of redwood encourages production of epicormic shoots, requiring subsequent removal (Brown, 2007). Epicormic shoots are less frequent if pruning is undertaken in autumn rather than spring (Brown, 2008). Pruning redwoods to a small DOS is desirable in terms of clearwood production, but production of epicormics and bark damage around pruned collars can be a problem in younger trees (Dean, 2007). Delaying of pruning until the bark thickens and leaving more green crown in the trees after each pruning lift negates formation of epicormic shoots and has less impact on growth (Dean, 2007). Silcock (2008) suggested that ultra-high pruning would be worthwhile, although anecdotal evidence suggests that pruning may allow entry of a native longhorn borer (Dean, 2007). Thirty-eight year old trees that were pruned four times to a height of six metres and thinned twice down to a final stocking of 398 stems per hectare only produced 32% of timber from the pruned logs that was graded clearwood (Silcock, 2009). The defect core in these trees was 305 mm and averaged 60% of the diameter at breast height (Silcock, 2009), suggesting that silviculture should achieve a small defect core in order to justify pruning.
Clones may also become available that offer fine branching characteristics and therefore can be planted at low stockings without branch size becoming excessive and compromising wood quality (Mineham, 2009). However, tree stocking affects growth (Dean, 2007) and low initial stockings may not adequately control width of growth rings, so where planted at low initial stockings of 550 stems per hectare, care is required to ensure every tree grows well (Mineham, 2009). The current recommendation is to plant 600 clonal stems per hectare to give a greater "margin for error" (S. Rapley, pers. comm). Form issues and loss of leader still occurs with clonal material, caused by multiple factors such as wind gusts, cicadas and leafroller (S. Rapley, pers. comm). Only at very high stockings is the tree likely to grow back a single leader on its own and form pruning, although effective, is expensive (S. Rapley, pers. comm).
Clones have good consistency in their branching pattern and growth rates (Gray & Gray, 2012). Clonal stock is currently being selected for rapid growth, good stem form with good coloured heartwood, with low shrinkage and a basic density of greater than 320 kg/m3 (Palmer & Rapley, 2012). Although more variable, seedlings can produce more rapid early growth rates (Gray & Gray, 2012). However, seedlings have higher levels of epicormic shoots and leaders are more likely to snap in the wind (Gray & Gray, 2012).
Although management of redwood is in its infancy and optimum regimes have yet to be developed, the target is to grow large pruned buttlogs and upper unpruned logs with live branches having diameters of no greater than 50 mm (Dean, 2007). Although there is a time delay between branches dying and the formation of bark-encased knots, thinning of redwoods should aim to minimise dead branches (Dean, 2007). Ideally, this can be achieved by maintaining a deep green crown through timely thinning and ensuring that if planted at a high initial stocking, this is not followed by delayed or late thinning (Silcock, 2008). However, in practice at a commercially acceptable final crop stocking, even when well managed, the first log above the pruned buttlog will tend to be the lowest quality log in the tree with abundant bark-encased knots in the sapwood (S. Rapley, pers. comm). Above this, the logs tend to have mostly green knots.
Because early pruning for a small defect core encourages production of epicormic shoots, management to maximise production of clear heartwood can be problematic. To overcome this and other management issues a mixed-species regime was developed for redwood plantations by Rob Webster (S. Rapley, pers. comm). Clonal redwood is planted at 400-500 stems per hectare with 400-500 Italian alders (Alnus cordata) as a companion crop. Italian alders are suitable because they are robust, don't mind weed competition, they grow on infertile ground, fix nitrogen and are a hardy tree that does not out-compete redwood (S. Rapley, pers. comm). The alders limit diameter growth of the redwoods and push them upwards, but without overtopping them. Alders were selected for companion planting because as coloniser species they tend to have vigorous early growth but then "run out of steam". If necessary the dominant alders are thinned out at 12 years of age to then encourage diameter growth in the redwoods (P. Silcock, pers. comm).
Trials have shown that redwoods are not out-competed by alders and after the first decade emerge as dominant (P. Silcock, pers. comm). By shading the pruned section of the redwood trunk, epicormic shoots are minimised (S. Rapley, pers. comm). Competition is not severe enough for the shade-tolerant redwood branches to senesce, but does limit their size. Redwood form is improved by the sheltering effect of the alder intercrop (P. Silcock, pers. comm). Pruning costs are lower because of less taper and smaller branches of lower frequency, therefore achieve greater height per pruning lift (S. Rapley, pers. comm), with 6.5 metres achieved in two lifts (P. Silcock, pers. comm). Furthermore, by constraining diameter growth during the establishment period, the core of lower density, less durable wood is minimised (P. Silcock, pers. comm). To date trials have shown that alders may not even need to be thinned out because natural senescence occurs as the redwoods dominate the site (P. Silcock, pers. comm). Alders also appear to be well enough adapted to hill country and their strong rooting habits would significantly decrease risk of erosion beyond what would be achieved with the low stocking rate for redwood clonal material, while sheltering the redwood crop during the critical establishment phase (P. Silcock, pers. comm).
Insect damage can devalue the timber, in particular native drywood termites (Kalotermes brouni) and a borer (Silcock, 2008). The species of borers and how management affects levels of damage does not appear to have been well studied in New Zealand. Entry of wood-damaging borers appears to be through dead wood that has begun to decay. Pruning out large double leaders can allow entry of borer, as can large pruning wounds where healing is slow, along with decaying dead branches (S. Rapley, pers. comm).
Redwood can be prone to double leaders (Purey-Cust, 2011), caused primarily by wind damage in early years (Gray & Gray, 2012). Where multiple leaders are formed in crop trees, form pruning is required to reduce these to a single leader (Gray & Gray, 2012). Clonal selections may produce less forking than seedlings (Gray & Gray, 2012), but clones are still susceptible to multiple leaders (S. Rapley, pers. comm).
Redwood has a very low potential for wilding spread because of low seed viability and the inability for the seed to travel more than a few metres from the parent tree (Wallwork & Rapley, 2009).
Redwood timber is low density, soft, dimensionally stable and easily worked (Webster, 2007). Redwood is also odour free, non-resinous and straight grained, with little shrinkage (Knowles & Miller, 1993). Heartwood colour varies from pinkish-red to brown and heartwood is naturally durable (Webster, 2007). Redwood has a good reputation and used in the U.S. for exterior and interior joinery and weatherboards (Silcock, 2008) and because the heartwood is resistant to decay it is suitable for outdoor applications like exterior decking, fencing and garden structures (Webb, 2007). In New Zealand redwood complies with the building code as an acceptable solution for exterior cladding but not decking (NZS 3602:2003). Most redwood applications take advantage of its outdoor durability or attractive appearance (Webb, 2007). Redwood timber is considered to be too soft for framing and general uses (Cornell, 2007) although characteristic strength values have been determined for New Zealand material (NZFFA, 2015), potentially creating an opportunity for appearance structural applications using specific design, provided properties for fixings were determined (D. Gaunt, pers. comm).
New Zealand redwood has comparable properties with second-growth redwood from the U.S., which comprises over 95% of that market (Webster, 2007). However, New Zealand material can be slightly lower density, depending on genetics and growth rate (Brown, 2007) but has greater dimensional stability (Palmer & Rapley, 2012). Market value for redwood in California is significantly higher than for Douglas fir and radiata pine; and demand, being driven by sentiment, is high (Brown, 2007). There is also considerable demand for redwood in Asia (Webster, 2007) and opportunities also exist in the local New Zealand market (Brown, 2007). However, Tombleson (2007) suggested that sustained supply and demand is required for improved prices that justify growing the species, after finding that exceptional quality large pruned redwood logs fetched only equivalent to the average pruned radiata log price in the New Zealand market.
Small diameter logs (down to 20 cm diameter) may be sawn for utility uses (Dean, 2007). Sapwood material is commonly used for fencing in California and has proven to be adequate in New Zealand conditions (S. Rapley, pers. comm). Although the sapwood is susceptible to decay, this can be boron treated to resist decay, but not pressure-treated with water-borne preservatives (Knowles & Miller, 1993). Sapwood can also be treated with LOSP preservatives (S. Rapley, pers. comm). Thermal modification might offer opportunities for using sapwood material for applications like fencing where strength is not required.
The Californian market values clear heart grades and these attract a high market premium (Dean, 2007). Tight knot grades also attract a significant premium over timber with bark-encased knots, which potentially influences silvicultural management of redwood in New Zealand (Dean, 2007). Silcock (2009) found, however, that "the value decrease due to dead branches was not as marked as expected".
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.