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
Significant attention to establishment would be required, especially in steepland settings that are subject to wind exposure, low soil moisture levels and with poor soil depth. A nurse crop such as manuka would be required to successfully establish kauri on steep slopes and rotation length would be significantly longer than for faster-growing species. Kauri requires intensive management input.
Kauri plantations are likely to be subject to significant biological risk because of kauri dieback disease.
Kauri (Agathis australis) has long been regarded as the Lord of the Forest, the dominant tree species of the natural rainforest in northern New Zealand, with diameters of 6 m recorded (Bergin & Steward, 2004). Most kauri forests have been cleared and the small remaining areas in mature forest are now reserves (Bergin & Steward, 2004).
Kauri forms extensive root systems with both deep "peg" roots or "sinkers" that descend from lateral roots and provide anchorage, along with an extensive network of lateral roots and a mat of fine feeding roots (Bergin & Steward, 2004). Fusion of lateral roots has been observed between adjacent trees in mature kauri and stumps may consequently stay alive for considerable periods of time (Bergin & Steward, 2004).
Kauri requires warmth to grow well and does not tolerate heavy frost (Bergin & Steward, 2004). In particular, the shoots of young plants can be damaged, causing multiple leaders to develop (Bergin & Steward, 2004).
Kauri prefers moist fertile soils and is not likely to survive on drought-prone infertile ridgetops (Bergin & Steward, 2004). Application of fertiliser can improve growth where soils are infertile (Bergin & Steward, 2004). Mycorrhizal associations are found in natural kauri forests. These have been investigated and may improve phosphate absorption by roots (Morrison & English as cited in Bergin & Steward, 2004).
Kauri "should be planted on open sites only if these are sheltered and warm" (Bergin & Steward, 2004). Because nursery raised seedlings tend to have poorly developed root systems, planted seedlings tend to be highly susceptible to drought for the first two years after planting (Bergin & Steward, 2004). Site preparation needs to be good and competing plants removed for up to 5 years (Bergin & Steward, 2004).
Small seedlings require shade and shelter, but saplings prefer full sunlight. Side shading is recommended for establishing kauri, such as provided by a manuka nurse crop established before line planting of kauri (Bergin & Steward, 2004). This protects young plants from wind and frost. However, releasing may be necessary to ensure the kauri is not overtopped until at least 1-2 m high (Bergin & Steward, 2004).
Management and silviculture
Seedlings tend to develop a strong taproot and nursery systems need to encourage development of a fibrous root system (Bergin & Steward, 2004). Kauri is usually grown in containers and although kauri can be raised as bare-rooted seedlings, taproots are damaged by mechanical undercutting (Bergin & Steward, 2004). Kauri can be grown from cuttings (Bergin & Steward, 2004). The cost of 50 cm high two year stock in containers was $3-4 per seedling in 2004 (Bergin & Steward, 2004), which is high for plantation forestry requiring high initial stockings.
On a good site, similar volumes might be expected from 60 year old kauri as 40 year old cypress and individuals might be expected to achieve 40 cm diameter and 20 m height in 50 years (Bergin & Steward, 2004). Although some plantations established on fertile sheltered sites with good soil moisture can achieve diameter increments of greater than 10 mm per year if given sufficient space, dense stands with stocking rates of 1000 stems/ha might be required to achieve good branch suppression, with an average diameter of 20-30 cm at 60 years old and a mean annual increment of 20 m3/ha for the first 40 years (Bergin & Steward, 2004). If thinned down to 700 stems/ha at 60 years the residual trees might be a harvestable size once over 100 years old. Trees with diameters of 90 cm can be expected to contain 2 m3 of heartwood in the lowest 6 m log (Bergin & Steward, 2004).
Poor performance of trial kauri plantations is often attributed to lack of adequate releasing from competing vegetation (Bergin & Steward, 2004).
Form pruning may be required to remove double leaders (Bergin & Steward, 2004).
Damage to kauri seedlings and saplings by goats, deer and possums has been observed (Bergin & Steward, 2004).
Pests and diseases
Kauri has few problems with insects, but a serious disease, believed to have been introduced into New Zealand from overseas has become established – kauri dieback disease Phytophthora agathidicida, formerly known as Phytophthora taxon Agathis (PTA). This is believed to have arrived on military equipment in the 1950's that was repatriated from the pacific after World War II (Brown, 2015). The disease is now widespread through Kauri's natural range and continues to spread, with no treatment available (Brown, 2015).
Kauri dieback can kill trees and seedlings of all ages and is spread primarily by movement of infected soil such as on footwear or machinery or by animals such as feral pigs (Bellegard, 2012; Brown, 2015).
The heartwood of mature kauri is "one of the finest softwood timbers in the world" (Bergin & Steward, 2004). Although sapwood has some of the appearance qualities of heartwood, it is lighter in colour than the heartwood, is not as dimensionally stable, lacks durability and is susceptible to Anobium borer (Bergin & Steward, 2004). Sixty-six year old trees from a plantation in New Plymouth were found to contain mainly sapwood (Bergin & Steward, 2004). Stiffness values were higher than for old-growth kauri and radiata pine and wood quality did not vary much between the pith and outer wood (Bergin & Steward, 2004).
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.