Radiata pine makes up almost 90% of New Zealand’s plantation forests. Because it grows quickly, responds well to intensive management treatment aimed at producing high quality logs, and provides suitable timber for a wide range of end uses, radiata pine is likely to continue to be the major plantation tree species.
Reproduction
When pine trees reach sexual maturity (from 6–12 years old), they produce male pollen catkins and female flowers. The female flowers are fertilised by the pollen which is mostly blown in the wind. The female flowers develop into pine cones.
More than two years after pollination, the cone dries and opens and the mature seeds are dropped from the tree. Some seeds drop directly below to the forest floor. Radiata pine seeds have wings attached to help ensure they are distributed over a wider area than around the base of the parent tree. This reduces competition for light, nutrients and water, and increases their chances of survival. This naturally occurring selection does not, however, result in uniformly high quality forests for human utilisation. In nature, the genetic composition of offspring is controlled by barriers to self pollination, and natural selection for the fittest offspring.
Natural regeneration in New Zealand’s native forest differs from that in a pine plantation. Seedlings of many native trees will germinate and grow in diffuse light. In their early stages of growth, these native seedlings require the mature trees to act as a ‘nurse crop’ to offer them protection. In a pine forest, very few pine trees regenerate naturally while the mature pine trees are standing, except on the stand edges. When the pine trees are felled, natural regeneration of the seedlings occurs. They need good light for their early growth. Most of our native tree species have a gap-phase regeneration mode; young trees being released when over mature trees die and leave gaps in the canopy. Radiata pine, however, is a ‘coloniser’ – germinating and growing quickly in clear areas. This is why radiata pine is highly amendable to plantation forestry.
In order to improve the quality of the radiata pine trees, a genetic research programme was developed, starting in the 1950s. This programme has led to a continued improvement in the genetic quality of the trees.
Radiata pine trees can be grown from:
• Seeds
• Cuttings
• Clones produced by micropropagation (e.g. tissue culture)
and
• Somatic embryogenesis (multiple embryos generated from
seed tissue in laboratory conditions)
Seeds
Thirty years ago, growers of radiata pine had little choice in the type of seed. All the seed was unimproved seed from a bulk supply or from good-looking trees selected in forest stands. This meant that the type of tree that grew was unpredictable, and similar to existing stands.
The radiata pine improvement programme classifies the improved material into breeds, based on the selection criteria. The main breed is based on growth rate and stem form (GF). The level of improvement is indicated by an open-ended scale of numbers, with 0 being unimproved, and higher numbers showing the level of improvement. To date tree breeding programmes have resulted in seed with a GF rating of up to 30. More recently, GF plus seed has been developed, which includes breeding for wood quality as well as growth and form.
| Breeds of Radiata Pine, Their Codes and Characteristics |
| Breed |
Code |
Major Characteristic |
|
|
| Growth and Form |
GF |
Improved growth rate and more merchantable volume |
|
|
| Long-internode |
LI |
More clearwood with minimal pruning |
|
|
| Dothistroma-resistant |
DR |
Better growth rate on high Dothistroma-risk sites |
|
|
Modern seed is the result of years of tree breeding to reproduce the characteristics desired by tree growers. Those characteristics include good growth rate, good form including log straightness, and no major defects, disease resistance and more recently, improved wood properties. This is achieved through selection and mating of high quality parent trees.
Seed orchards were first developed in New Zealand to facilitate the breeding of elite trees. In the first seed orchards, pollen blew onto the female flowers from adjacent orchard trees, and also from trees outside the orchard (open-pollination). Because unimproved wild pollen could fertilise the flowers, the level of genetic improvement was reduced. A better system is control-pollination, where only pollen from selected trees can fertilise the flowers. The pollen (the male gamete) from the pine pollen catkin is transferred by hand to the female flower on the selected trees.
The male parents are chosen one year before they are required for pollination, so that their pollen can be collected. The pollen is carefully dried and stored in a refrigerator or freezer. Before the female flowers in the selected trees are ready to be pollinated, the tree shoots bearing the female flowers are covered with a plastic bag. When the female flower is developed and ready to be fertilised with the pollen, the male pollen (which was collected during the current season or the previous year) is injected into the plastic bag and fertilises the female flower. The plastic covering ensures that all other pollen is excluded from that flower and that the parents of the seed produced (i.e. its genetic makeup) are accurately known.
Cuttings
A cutting is a small shoot taken from near the end of a branch or the stem of a plant. It is placed in the ground, and will produce roots and develop into a new plant which will be genetically identical to the original plant.
Radiata pine seed of high genetic quality can be produced by choosing the best parent trees and mating them using controlled pollination. This technique is expensive and the seed can be in short supply. To overcome this problem and to take full advantage of the seed produced, the seeds are planted and grown for a year. When the seedlings are at least one year old, a number of cuttings are taken from them.
In the simplest system, cuttings about 7cm in length are removed from the parent seedling with secateurs. These cuttings, planted directly into the beds in the forest nursery, will have rooted and be ready to plant out into the forest about a year later. The original seedlings are kept trimmed as low hedges, so that cuttings can be collected from them each year. These hedged seedlings are called stool plants and, if managed appropriately, juvenile cuttings can be collected from them for 4 or 5 years.
Using this simple method of propagation, between 4 and 20 cuttings can be taken from a single seedling in a year. Each of these cuttings and the subsequent tree that will grow from it has exactly the same genetic qualities as its parent. This means that from one high quality seed we now have up to 20 young trees after the first year and more in subsequent years. Not all plants are propagated easily via cuttings, but it is fortunate that the techniques for doing so with radiata pine are now well known.
Cloning
A clone is a group of offspring which are genetically identical. Each individual in the clone has the same genes as the original individual from which the clone was made. Cuttings are clones of the original mother plant.
Cloning of radiata pine can also be carried out in a micropropagation laboratory by organogenesis, or tissue culture. The propagation process starts with a seed. The seed is bleached to sterilise it, soaked overnight and the seed embryo is removed. This embryo is placed in tissue culture medium, which sustains growth and promotes development of shoots. Multiple shoots are cultured from each seedling. These shoots are separated and each is cultured and multiplied for a further period of time. Shoots are divided every 6 to 8 weeks. After sufficient multiplication of shoots is achieved, the shoots are treated to promote root growth. The resulting plantlets are planted out into a glasshouse under high humidity conditions. The glasshouse conditions are gradually changed until they are similar to the nursery environment.
All tissue culture propagation must be carried out under sterile conditions. In tissue culture each original seed can yield up to 100,000 plantlets, depending on the time the material can be maintained in culture.
Another propagation technology which is being developed is somatic embryogenesis – the formation of multiple embryos in culture. Embryogenic cell lines are established from immature seed, and millions of immature embryos of individual genotypes can be multiplied from each seed. These embryos are developed and matured under controlled laboratory conditions and then can be germinated like natural seeds. The efficiency of this process is still low, but the technology has the potential to produce unlimited quantities of embryos of desirable genotypes at costs cheaper than current control-pollinated seed prices.
The advantage of growing forests using clones developed from high quality radiata pine genotypes trees is that it will result in mature trees which are more uniform in height and diameter. This will enable more mechanical systems to be used in the tending of the trees and in their harvesting and processing. The wood properties will also be more consistent and may be selected to suit particular end uses.
Ultimately, foresters will request that trees be cloned which have the particular characteristics required to flourish and produce high financial returns in a specific forest area, or for a specific end use. Clonal forestry, the planting of known, tested clones, is becoming a reality with radiata pine in New Zealand. There are risks associated with clonal forestry, which are being addressed. This includes risks associated with genetic uniformity, reduced genetic diversity, plus technical and logistical difficulties associated with large-scale clonal propagation and clonal storage. These risks can be minimised with risk spread, forward planning, and full researched and carefully managed clonal forestry programmes.
Nursery to Forest
The re-establishment of plantation forests relies on good quality tree-stock being grown and prepared in the nursery. The lifting of seedlings from the nursery, and handling and transportation through to the planting stage requires extreme care. Good land preparation, careful handling practices, and care in the selecting of seedlings to be planted, together mean that 90% of all planted seedlings having the ability to survive through to the forest harvest.
In the nursery the following practices are carried out:
Spacing – the seed is sown precisely in rows, so that each seedling has enough space to grow properly.
Root wrenching – once seedlings are established and growing well in the nursery bed, their roots are regularly wrenched. This involves passing a machine with a horizontal blade at a set distance under the seedlings and cutting off their deeper roots. The removal of the tap root results in the seedling growing more roots near to the soil surface. These extra roots give the seedling a better chance of survival when planted out in forest areas.
Careful lifting – seedlings are carefully lifted from their seedling beds. Care is taken to ensure minimal damage. Some soil and mycorrihizal fungi are left on the roots to help seedlings re-establish after planting.
Root trimming – the longer straggly roots of seedlings are cut back to about 100 mm. This is done because seedlings with longer roots are difficult to plant and their roots may often be bent and remain deformed. Root deformation may stunt a seedling’s growth and make it prone to toppling by wind.
Careful packing – the lifting, trimming and packing of the seedlings is often done by the same person or group of people. The seedlings are packed horizontally to avoid root damage and minimize breakage during transportation. The seedlings are usually lifted directly into and planted out of the same planting box so minimising handling and exposure to the elements. In other cases, seedlings may be packed in plastic bags which provide less support but are equally good at retaining moisture.
Root moistening – care is taken to ensure seedling roots do not dry out during transportation. Wet jute hessian is sometimes placed over roots in the planting boxes to keep them moist.
Direct transport – seedling planting boxes are packed into crates which reduce damage during transportation, double as a cool storage in the field and as movable field dumps. These are loaded at the nursery and taken directly to the planting site overnight or even on the same day.
Good management of all nursery operations will lead to seedlings arriving on the planting site in good condition. In the forest the following operations ensure that the seedling has the best conditions for its growth:
Check on stock quality – seedlings are checked in the crates and during planting to ensure that they are of a good condition. Roots should be moist and seedlings of an even size.
Choose a good planting spot – obstructions such as stumps and tree roots from previous tree crops, and rocks should be avoided. Otherwise these obstructions may cause root distortion, retard the seedling’s growth and make it prone to toppling.
Cultivation – manual cultivation is done with a spade. The soil is cultivated to a full spade blade depth. This makes easy planting and allows new seedling roots to grow straight. Hard soils can be mechanically cultivated, usually to a depth of 600 mm. Deep cultivation improves conditions for root development.
Manual Planting
• Cultivate the soil and open up a good sized slot in the soil
with the blade of the spade. This allows the seedling roots
to spread more naturally. A narrower V-shaped slit will
sweep up roots as the seedling goes in, leading to lop-
sided root development and risk of toppling of the
seedling by wind.
• Place the seedling in the opened slot. Replace the
soil around the roots.
• The seedling is pulled upwards, about 50–100 mm, to
straighten any roots that may be twisted or swept up.
Using the sole of the boot, not the heel, the soil is firmed around the seedling. Planters use the same technique for each seedling to ensure uniformity throughout a stand.
Experienced contractors will plant between 800 to 1,500 trees per day.
Mechanical Planting
Several types of mechanical tree planters are in use:
• Continuous furrow planters
• Intermittent tree planters.
Speed and accuracy of spacing between plants are the main reasons for using machines. However their use is limited in New Zealand. The factors in favouring manual planting over mechanical, include:
Site conditions – steep grades, irregular terrain and debris are factors that mechanical planters have difficulty dealing with.
Variable seedling size – too much variation in seedling size may create problems with planting machines designed for a particular size.
Planting Density
Establishment of a forest requires forward planning. The forest site and the expected use of the mature tree, will determine the number of seedlings planted per hectare, and sometimes even the species that should be planted.
Planting more than the planned final tree crop stocking allows for thinning to select the best trees for the final crop.