Plantation forestry best practice for Northland
The Northland region has the opportunity to embrace positive land use change, in particular planting of erodible pastoral slopes in timber plantations. This is a goal that both regional and central government share and recognise as having the potential to improve both revenue streams for landowners and environmental outcomes for the region.
To be able to achieve this goal, landowners will require adequate knowledge on tree species, regimes and siting in order to plant the right trees, in the right place, for the right purpose. The purpose of this resource is to provide information on species and regime selection for different sites in Northland along with current best practice in plantation forest establishment.
Northland's land forms provide highly variable micro-sites. For successful outcomes the practitioner must have knowledge on adaptability and siting of tree species, in particular with respect to soil types and exposure to wind. Variables that limit choice of species include soil drainage, structure and fertility while exposure to wind also limits species and influences productivity. The practitioner must understand that each tree species has differing limitations to extremes and that limitations are never black and white. The mantra for growing trees is "eliminate as much risk as possible to improve the odds of success".
Radiata pine has traditionally been a resilient and adaptable species to a large range of sites. This will not necessarily remain the case for all sites throughout the region and indeed limitations are emerging. Red needle cast caused by Phytophthora pluvialis is severely and repeatedly affecting some sites in Northland, in particular high-elevation sites. This highlights the need to site trees within their "comfort zone" – siting is critically important and even radiata has its limitations.
Wind exposure and trees
The severity of wind and how salt laden that wind is, both influence species selection for a site. Radiata pine is fairly resilient to both and is a good benchmark to compare other species with.
Wind can cause toppling of trees and stem breakages. Productivity of the stand tends to reduce as the site gets windier, such as nearer ridge-tops, especially if wind-resistance of the species is lower. Wind in coastal sites can burn foliage of many species, depending on level of salt exposure. It is important to carefully match species with site.
Soil and trees
Soil drainage and fertility are important in the context of species choice.
Evaluating soils for level of drainage is easier in winter when poorly-drained soils become soggy. Another indicator of poor drainage is presence of certain water-loving vegetation types, such as rushes and buttercup. Some tree species such as poplar enjoy wet soils while others such as radiata pine do not thrive at all in poorly-drained soils. Again, radiata is a good benchmark to compare other species with.
Soil fertility can be measured by either testing the soil or by foliar tests of the existing vegetation (e.g. radiata pine). A fertiliser history suggests moderate to good fertility, whereas pastoral farming without addition of fertiliser for decades suggests low soil fertility. Vegetation types also indicate soil nutrient status, for example if the natural bush in the area is dominated by totara and kahiikatea then fertility might be low, whereas if broadleaf trees such as puriri and taraire are common then this might suggest a reasonable soil fertility. If there is no clover in the pasture this suggests low soil fertility. Fertility usually has a direct relationship with tree growth rates. There are negative consequences to both excessively high and excessively low fertility.
Species assessment and selection for site
Northland has a solid history of plantation forestry and species trials, with a good knowledge base that growers can tap into. Successes and failures over time have resulted in a shortlist of well proven "core species" that growers can be confident in planting.
|Species*||Resilience to wind exposure||Resilience to salt wind exposure||Soil fertility requirements||Soil drainage requirements|
|Cedar, Japanese||Very high||Moderate||Medium||High|
|Cupressus lusitanica||Low||Low||Medium||Very high|
|Cupressus macrocarpa||High||Very high||Medium||Very high|
|Cupressus 'Ovensii'||High||Moderate||Medium||Very high|
|Eucalyptus botryoides||High||Very high||Low-medium||Low|
|Poplar - Kawa||Low||Low||Medium||Low|
|Redwood - Coast||Low||Low||Medium||Medium|
|Rata/Pohutukawa hybrid||Very high||Very high||Low-medium||Medium|
*This species list is deliberately not comprehensive, because for each species a minimum scale is required for sufficient market development that maximises economic value for the species. Growers are not discouraged from experimenting with other species.
Detailed information on recommended species is available below.
Planning firstly requires decisions on the appropriate land use, in particular what should be planted where. This means evaluating the environmental sustainability of competing land uses, along with potential returns for each competing land use. Costs involved with establishing plantation forestry on pastoral sites include fencing to exclude grazing animals and suitable roading for both planting and eventually harvesting, along with the costs of tree establishment.
Planting of trees requires forward planning, in particular securing orders for tree stock. This usually needs to be done well in advance, such as a year ahead of planting.
Economics and species choice
Exotic tree species tend to grow considerably faster than native species, usually by a magnitude of at least two, sometimes up to four. Radiata pine is the primary plantation forest species grown in Northland, so again is a useful benchmark to compare other species with for growth rates, both exotic and indigenous.
Productivity and timber value are useful indicators for potential returns from different tree species. Species that grow slower (lower productivity) but that have high-value timber might prove to be more profitable than lower-value species that are more productive.
Timber value and species
Although future timber values are inherently speculative, the following table offers "best bet" estimates to compare timber utility based on species quality scores:
|Species||Durability||Strength||Hardness||Sapwood depth||Quality score|
|Cedar, Japanese||7||2||2||4||15 (=1)|
|Eucalypt, medium durability (Group 1)^||7||7||7||5||26 (=1.73)|
|Eucalypt, medium durability (Group 2)^||7||7||7||8||29 (=1.9)|
|Eucalypt, high durability (Group 3)^||8||8||8||8||32 (=2.13)|
|Radiata pine*||2||5||4||4||15 (=1)|
|Rata/Pohutukawa hybrid||8||8||8||5||29 (=1.9)|
*Radiata pine is a special case whereby its inherently low durability as a result of wood porosity works in its favour currently because this property also makes the species "treatable" with chemical preservatives. Depending on societies perspective towards chemical treatment, this could also be radiata's achilles heel.
^See the Eucalypt chapter for individual species.
Timber and log markets
Export log markets are available for most exotic timber species including eucalypt, cypress, poplar and redwood. However, because these markets are undeveloped, prices can trend towards the lower end of the price spectrum, if wood properties are taken into account. Only with a sufficient scale of resource will a species achieve the necessary critical mass for product differentiation that would attract premiums over radiata. However, it must be understood by the prospective grower that export markets for "alternative" species to radiata pine are not yet realising their market potential because scale is insufficient for this to occur.
Domestic log prices for "alternative" species tend to be low compared with the resulting timber product's premium value, because domestic markets for plantation specialty timber are under-developed and disparate. The quantity of logs required for consistent supply and availability to capture and grow market share for a timber species is not currently available. Furthermore, although niche marketing opportunities exist, constraints such as the building code with its emphasis on treated timber, remain as hurdles. The resource is therefore mined rather than generating optimum value for the grower.
Growers planting trees now can only speculate on level of market development and market value into the future for any timber species. However, the key to viability for a species is a scale threshold whereby active demand is generated in both the export and domestic markets, by generating a predictable and steady supply of logs and a consistent quality product. The grower planting trees now, to be a price setter rather than a price taker, must either become:
- a niche marketer for their product that capitalises on the unique timber properties they offer; or
- part of a marketing network or collective that achieves sufficient scale to capture market share.
Markets depend on sufficient scale and the Northland region is well positioned to generate high-value opportunities for high quality timber. However, to ensure success it will be necessary for industry leadership that encourages and facilitates growers to collectively achieve the scale threshold necessary for their species to generate value premiums. The individual grower must also focus on producing high quality timber.
Combining the individual species productivity with timber quality and introducing an estimate of biological risk reveals a total score for each species that represents potential economic utility:
|Species||Timber quality||Productivity||Biological risk||Score*|
|Eucalypt, medium durability (Group 1)^||1.73||0.8||0.5||1.01|
|Eucalypt, medium durability (Group 2)^||1.9||0.8||1||1.23|
|Eucalypt, high durability (Group 3)^||2.13||0.7||1||1.28|
^See the Eucalypt chapter for individual species.
* A higher score represents a better value proposition.
- That the scale threshold is achieved for each species and market value for timber products is based on timber qualities;
- Timber value is based on physical properties rather than convention or other subjective market values or product part-worths;
- Costs for establishing the plantation are equal between species;
- The primary purpose of plantations is timber production.
Three important factors must be considered when planning a species regime. These are initial stocking, final stocking and rotation length.
Radiata pine is usually planted at a density of between 600 and 1200 stems per hectare. Planting density may be influenced by a number of factors, including species and genetic quality of the planting stock. Planting density is one of the most important decisions to make for a new forestry plantation. Planting more trees requires more work later thinning out unwanted trees, but offers a higher selection ratio, meaning better quality (more valuable) trees are retained as crop trees. The tradeoff is therefore between increased establishment costs and the resulting improved productivity and log quality (i.e. value).
Weed competition and browsing animals are the two biggest threats to establishing trees.
Browsing stock and wild animals
Seedlings must be protected from browsing stock and wild animals. Fences must be stock proof and well maintained for at least a number of years, or best permanently. Rabbits, hares and possums must be well controlled before planting. Last minute animal control is not likely to be as effective as a dedicated control effort over a period of time before planting. Repellant should not be relied on but can be made by mixing 5 fresh eggs, 150 ml of acrylic white paint and 600 ml of water. This is sprayed on the seedlings in the nursery and allowing to dry before dispatch.
Good weed control produces better growth and survival in young trees, especially in the first year after planting. Weeds compete for moisture, nutrients and light and the presence of grasses in particular delays establishment of trees.
Although fire has been used in the past as a tool for land clearance, standard practice land preparation for establishing trees in pasture and forest cutover is to desiccate existing vegetation with herbicides before planting. This can be either "spot spraying" of 1m diameter circles at the required spacing, or blanket desiccation of the whole planting site by aerial spraying. Spot application uses considerably less herbicide but can be more expensive than blanket spraying. Woody vegetation is best sprayed a year before planting to allow it to begin decaying and reduce down before planting. A glyphosate/metsulfuron mix is often used for controlling gorse, tobacco weed, blackberry and brushweeds. If metsulfuron is used the site should not be planted for at least 3 months after spraying.
Cultivation and deep ripping is not recommended. Cultivation is likely to cause erosion and sedimentation and deep ripping creates wet zones and water pooling in the rip zones, which can cause toppling.
Fertiliser is recommended to be applied to the root zone of newly planted trees if the site doesn't have a recent fertiliser history. Fertiliser tablets planted with the tree provide a slow release of nutrients to the tree rather than the competing weeds. Side dressing of small quantities of DAP fertiliser (Di-ammonium phosphate) above the tree is suitable for post-planting application where good weed control is undertaken.
Tree seedlings are produced as either bare-rooted or containerised stock. Container-grown trees can be more expensive and heavier to lug around the hills, but produce faster initial growth and are more resistant to water stress because the whole root system is intact within the "plug". Bare-rooted stock has much of the root system cut back for planting, which is fine for planting in the middle of winter when soil moisture levels are high and desiccation unlikely to occur.
It is generally recommended that bare-rooted stock be planted through winter until the end of August, whereas containerised stock can be planted through winter and spring. Containerised stock may be less susceptible to poor ("slit and stuff") planting practice but if seedlings are pricked out into containers (rather than direct seeded) there may be root distortion present in the core of the plug. Root distortion from careless planting of bare-rooted stock, or from container walls or pricking out in containerised stock, is implicated in increased toppling of young trees, so the grower should be actively minimise this.
Bare-rooted stock should be freshly dug, but can be refrigerated for short periods of time. Bags or boxes of trees will begin composting if not kept cool.
Containerised stock must be soaked before planting. This requires full immersion of the plugs in a water bath, trough or bucket until bubbling stops, for up to an hour. Plants should be handled carefully to minimise damage to the root plug.
Containerised stock can be planted shallower than bare-rooted stock but the plug must not be exposed to the air. If the top of the plug is exposed, this will act as a wick and the plant may dry out.
Planting costs are usually around $0.50 per tree and bare-rooted radiata seedling stock costs around $0.50. Containerised stock is usually more expensive, starting from $0.60 but sometimes more than $1.00 per tree for alternative species.
For more information see NZFFA information leaflet Successful establishment of tree seedlings
Herbicides are used to facilitate establishment of trees after planting. This is called "releasing", because the young trees are released from the weed competition. Manual suppression of weeds can also be practiced, whereby the weed growth is checked using physical means so that the young trees have "room to grow" and do not become smothered by the competing weeds. Manual releasing is more time consuming than chemical releasing and needs to be repeated at a higher frequency, usually 2-3 times in the first summer. However, provided this is executed in a timely manner and slower initial growth is accepted, this method can be successful for landowners who do not want to use herbicides. Releasing is required until the tree is robust enough to not be overtopped by weeds. This will depend on tree growth rate and type of weed competition.
A range of herbicides are available and used for releasing different tree species. Some are selective, which means they might kill some weeds but not others, or some tree species but not others. Selective herbicides are a powerful tool for controlling weeds in tree plantations, provided the practitioner has knowledge about what species each herbicide does and doesn't kill.
Costs for spot spraying and spray releasing can be as low as $0.50 per tree per application. Manual releasing may cost $1 or more per tree, depending on timing and size of weeds.
Residual herbicides control weeds for an extended period but the residual action in the soil can also kill young trees. When applying residual herbicides with a knapsack care must be taken to avoid over-application and spraying the same ground more than once. Spray dye is recommended as an additive for all hand spray-releasing operations.
Terbuthylazine is the most widely used residual herbicide used in tree plantations. It has some knockdown ability and is usually applied once shortly after planting and prior to the spring flush of growth. Terbuthylazine should not be applied if rain is expected within 3 hours, but approximately 10mm of rain is required within a week to be washed into the soil.
Terbuthylazine should not be used on sandy soils as it will leach into the roots and kill the young tree. It is effective on most weed species provided they are no taller than 10 cm. Terbuthylazine can be mixed with Haloxyfop or Clopyralid for a dual knockdown and residual effect on weeds over 10 cm tall.
Rates used for pine, redwood, cypress and other conifer species are 2 ml active per square metre. For Eucalyptus and other sensitive species no more than 1.5 ml per square metre should be used and contact with foliage avoided where possible. Terbuthylazine should only be used around poplars after the first year and at half the rate used for conifers. Terbuthylazine should not be used near acacia species. For knapsack spray releasing the average user might release 20 trees per litre of spray (1 square metre each). At this rate 40 ml active per litre of water will deliver 2 ml terbuthylazine per square metre. Calibrating the sprayer for the user is essential so that the dose per square metre is accurate. It is recommended to spray rectangular sites sweeping one side of the tree then the other, making sure to not double-spray the centre where the tree is. Terbuthylazine should not be used over actively growing trees.
Glyphosate and glufosinate-ammonium (buster) are non-selective knockdown herbicides.
Glyphosate can be used around any tree species for weed control, provided it does not contact the leaves. This is because it is translocated through the plant and small doses can be lethal. Only skilled practitioners should use glyphosate for releasing trees and then very carefully with a spray guard, so that spray does not drift on to the foliage of the crop tree. Glyphosate has no residual effects.
Glufosinate-ammonium (buster) is a non-selective herbicide with no residual effect that does not translocate through the plant. However, although safer than glyphosate being a non-selective contact herbicide, the practitioner should still be careful to not get it on the tree foliage.
Haloxyfop can be used over tree species and only controls grasses. Average knapsack application rate is 3 ml per litre of water to treat 20 trees. Penetrant should not be added. Compatible with clopyralid and terbuthylazine. Can cause some tree scorching if applied over trees that are flushing.
Clopyralid is a selective herbicide used for controlling annual broadleaved weeds and legumes. Clopyralid is classed as a hazardous substance and only available to certified approved handlers. Average knapsack application rate is 3 ml per litre of water to treat 20 trees. Compatible with haloxyfop and terbuthylazine and is suitable for spray releasing most species when not in active growth, but not alders or acacias.
Triclopyr (grazon) can be used as a selective herbicide for spot spraying blackberry, gorse, broom or himalayan honeysuckle in plantations but direct contact with trees must be avoided because triclopyr translocates and is toxic to trees.
Sequence of events for good weed control
Removing weed competition by release spraying improves the growth rate of establishing trees and this can even be continued into the second and third years. However, there can be a downside to the lush tree growth resulting from a "bare earth policy", because by removing competition for nutrients and moisture, the roots get lazy and the tree produces fast height growth with little root growth, increasing the risk for toppling.
There is a tradeoff between the (sometimes dramatically) increased growth rates resulting from complete weed suppression and the (sometimes vastly) increased risk of toppling resulting from inducing such high growth rates. This imbalance results in toppling should a storm event occur involving wind and rain. The practitioner can manage this tradeoff by, for example, only supressing weeds enough to ensure they do not out-compete the trees, and also accepting moderate initial growth rates. Experience helps with judgment calls and practices that achieve the "right" level of weed control for the site. It should also be noted that in erodible hill country minimising bare earth reduces risk for erosion and resulting sedimentation to occur. The more vegetation on the slope, the less risk for loss of soil during a storm event. Erosion results in lower productivity of the land for any land use and mitigating this risk is good land management practice.
For more information see Successful establishment of tree seedlings, NZFFA Information leaflet No. 3 (2005).
Pruning is practiced for production of clearwood. For naturally durable species where the aim is production of clear heartwood, large stem diameters are required because smaller trees will produce low volumes of clear heartwood. Where production of clearwood that includes sapwood is the goal, recoveries can be high even for small diameter trees. The rule of thumb for pruning is that as frequency of pruning is increased (this can be as often as annually), the length of the pruning lift decreases (i.e. more lifts required) but the practitioner has more control over DOS (diameter over stubs). Targeting clear heartwood means keeping the DOS as small as possible.
Cost of pruning on a per hectare basis depends on how many trees are being pruned and the hight the trees are pruned to. Some species also tend to be more expensive to prune than others, for example macrocarpa, because of the high number of branches. Cost also depends on frequency of pruning, therefore how many lifts are required. Radiata pine is often pruned to 6 m in three lifts, with each lift costing approximately $2.40. Therefore, pruning 400 stems to 6 m might cost approximately $3000 per hectare.
For more information see Pruning, NZFFA Information leaflet No. 7 (2005).
The practitioner must understand the tradeoff between thinning too many trees and not thinning enough. Thinning too many trees at once increases the risk of windthrow in the residual crop trees, because these trees are more exposed to the elements. Not enough thinning results in too much height growth in proportion to tree diameter (this is called the crown:stem ratio). The art of thinning is knowing how far to go without going too far in each thinning operation. The more often you thin and the less trees you thin each time, the better the results, but this comes at a cost. Each thinning intervention costs money and so the more interventions there are the more the total cost of thinning will be.
Thinning costs per hectare depend mostly on the number of trees being thinned. Also, the more thinning operations that are required, the higher the cost tends to be per tree.
Chainsaw thinning of radiata pine from 1000 stems down to 500 stems should cost between $500 and $850 per hectare, a cost of between $1.00 and $1.70 per tree. Thinning is usually undertaken as two or three operations, with a greater number of thinning operations being required where the initial tree stocking is high. For example, an initial stocking of 2000 stems per hectare could be reduced to 1200 in the first thin, then 700, then 400 in the final operation, each operation spaced two years apart.
Chemical thinning methods are being developed that offer cost reductions per tree so are suitable for plantations that start with high tree stockings. A price of $0.50 per tree should be achievable but costs and methods are still under development.
For more information see Getting back to basals, NZFFA Information leaflet No. 11 (2005).