Report: Trees for steep slopes
Sustainable Forest Solutions
Reviewed by Mike Marden, July 2018.
PDF download of this report ».
Please note that the web report is regularly updated whereas the pdf download above is dated July 2018.
|Species rating *|
|Early growth rate||7|
|Root decay rate||7|
In a nutshell
Suitable for planting in gullies and other sheltered locations with adequate soil moisture and reasonable shelter. Requires form pruning at regular intervals.
The extensive root system offered by blackwood deserves more attention in stabilising erodible hillsides (Brown, 2006a). Blackwood coppices and suckers freely and fixes nitrogen into the soil, so can be used as a colonising species for permanent erosion control in fairly harsh sites (Van Kraayenoord et al. 1986).
Although blackwood will grow almost anywhere in New Zealand, on dry exposed locations it is slow growing and poorly formed (Brown, 2005a). Blackwood is best suited to lower valley slopes and moist gullies (Brown, 2006a) where fertility is moderate to good and drainage is not severely impeded (Jackson, 2006). If the species is to be managed for timber, blackwood is highly site selective (Brown, 2005a). For fast growth and production of straight logs, blackwood requires reasonable quality soils, sufficient soil moisture and shelter (Brown, 2005a). Eroding gullies, where sheltered and with plenty of soil moisture are suitable sites for productive blackwoods (Brown, 2005a). Gullies with difficult access may suit planting in blackwood because the value of the resulting timber would justify extraction (Brown, 2006a).
Blackwood "has little apical dominance and will form a large branched crown at the first opportunity on as short a stem as possible" (Jackson, 2006). Therefore, form pruning and thinning are essential for sawlog production (Brown, 2005a, 2006a; Jackson, 2006) and "unlike radiata or cypress there is no fallback option of leaving blackwood unpruned." (Jackson, 2006).
The specific siting and silvicultural requirements for blackwood, along with the limited information available on costs and returns currently deters commercial growers from planting the species (Brown, 2006a).
Form pruning must be undertaken annually until at least year eight (Brown, 2006a). The best growth response is produced when blackwood is pruned in spring rather than winter (Brown, 2008).
Thinning must be undertaken in a timely manner, preferably down to 200 stems per hectare to achieve valuable large diameter logs (Brown, 2006a). Ringbarking is effective but is best done in spring when the bark strips easily (Brown, 2008).
Blackwood, being a pioneer species, is intolerant of shade (Brown, 2006b). Jackson (2006) found that good side shade improved form, by row-planting with good site preparation in the rows, but retaining scrub between the rows. Brown (2006b) found that although mixed planting offers benefits of rapid height growth and good form, the response needs to be carefully managed by removing the nurse crop on time (Brown, 2006b). Brown (2006b) suggested form pruning is preferable to the complicated management of nurse crops. Millen (2017) favoured continuous cover management with a gradual removal of trees when they reach millable size, which offers side shade for regeneration, which is prolific and can be managed rather than replanting with new stock.
Coppice grows best in Autumn and can be managed for timber production because it tends to grow straight and with good branch suppression (Brown, 2008). Brown (2008) also suggested that thinning of coppice growth can be delayed until the coppice shoots are over eight metres tall, producing acceptable trees with little effort from regrowth.
Average rotation length is 35 years (Brown, 2006a). Bigger, older trees tend to produce the best wood (Esson, 2006).
Insect pests of blackwood in New Zealand are regarded as significant and can affect both growth rates and form (Brown, 2006a).
Blackwoods do not tolerate heavy frosts (Brown, 2005) and incur limb breakages from snow (J. Fairweather, pers. comm; A. Gordon pers. comm).
The weed potential of blackwood is relatively low, although soil disturbance allows regeneration from seed or suckers where there is sufficient light (Brown, 2006a). Thus sucker growth can be prolific on a harvested site. Once established, blackwood is difficult to remove from a site, potentially an advantage for erosion control and continuous cover management. Selective herbicides are available that kill it to the roots.
The special requirements for milling and drying blackwood require experience and understanding (Brown, 2006a) but sawmilling is fairly straight forward provided blades are sharp (Li Legler, pers. comm).
Blackwood timber is air dried before finishing in a low temperature kiln (Esson, 2006).
The demand for blackwood timber tends to exceed the supply and prices can be very high (Brown, 2006a; Esson, 2006). However, only niche quantities are being produced currently in New Zealand and a sudden increase in supply could potentially destabilise market value. Export markets tend to depend on consistent quantities, which may be possible as supplies increase.
The market prefers lengths that are as long and as clear as possible and heartwood only (Esson, 2006).
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.