Forest Research develops superior macrocarpa and lusitanica

Gerry Vincent, Luis Gea, Jacqui Aimers-Halliday, Charlie Low and other Future Forests staff - Forest Research, Rotorua

Introduction

Macrocarpa (Cupressus macrocarpa) and lusitanica (Cupressus lusitanica) are the most important cypress species for the New Zealand forest industry. Macrocarpa is particularly renowned for its wood properties, and good stands yield high-quality special-purpose timbers, which are in demand locally and internationally.

Unfortunately, macrocarpa is currently under serious threat from cypress canker, particularly in northern areas. Lusitanica is generally more resistant to canker and grows well on warmer sites. On fertile, sheltered sites, intensively managed stands of both cypress species can attain mean top heights of 30 metres and total stem volumes of 500 to 800 cubic metres per hectare in 30 to 40 years.

Demand for superior macrocarpa and lusitanica planting-stock currently exceeds supply, as evidenced by 100% sales of improved Forest Research Cypress™ seed through Proseed NZ and strong demand for limited numbers of cypress clones.

The macrocarpa and lusitanica breeding programmes

Breeding programmes for both lusitanica and macrocarpa were initiated by Forest Research in the early 1980s. Breeding programme material has been systematically tested in genetic field trials. Selections have been made for both breeding population advancement and seed orchard purposes. Improved seed is currently available from genetic trials via Proseed NZ, and will soon be available from seed orchards.

To ensure product reliability, seedlot or clonal performance must be adequately tested in well-designed field trials. The type of site on which the trees are grown, genetics, and the way the trees are managed, such as spacing, all influence the appearance of any individual tree or stand. Clones with few and fine branches may not necessarily be the ones that give the greatest value at the end of the rotation. They are usually of smaller size and are often physiologically aged.

The potential for family forestry and clonal forestry

Genetically superior material can be deployed via family forestry or clonal forestry Family forestry is the planting of single blocks with seedlings or cuttings derived from select families. (A family is a group of individuals originating from the same parent tree, which has been selected as superior in the breeding programme.) Individual genotypes or clones are not identified. Greater gains will be achieved if control-pollinated seed is the starting point for family forestry. Controlled pollination involves transfer of pollen from a selected male parent to receptive flowers of a selected female parent.

Clonal forestry is the deployment of top clones that have been field tested, and here the identity of the individual genotype is important. Clonal forestry can increase genetic gains over and above that captured by family forestry. Also, increased crop uniformity can improve efficiency in forest management, harvesting and processing of end products.

Vegetative propagation and clonal testing

Work has started on developing a new group of tested clones of lusitanica and macrocarpa. Wood quality and increased resistance to canker are major objectives as well as growth and form traits. Seed was collected from the best individual trees from the better families in the lusitanica genetic trial.

Table 1: Mean number of macrocarpa cuttings produced, from stool beds of four different physiological ages, over a period of 3 years.

This was sown in 2000, and stool-beds of 150 clones were established. Cuttings were set in April 2001. These will be planted in clonal field trials to identify the best clones for future use and the clones retained in hedged archives for future multiplication. A similar programme has been started for macrocarpa with the seed being collected from selected trees in genetic trials in March 2001. This will be run in parallel with a programme for early screening for canker resistance in macrocarpa.

A simple, cost effective stool bed system for propagation of juvenile cuttings was developed by Forest Research (see the article on page 23 of the February 2000 Tree Grower). A stool-bed system is where a plant is topped to produce adventitious shoots for the production of rooted cuttings. This permits the multiplication of scarce and expensive seed orchard seed.

Stool beds have been established from plants of various physiological ages or maturation states. A drop in cutting production with increasing physiological age of the stool beds was observed, and this has important implications for clonal propagation. The effect of physiological age on the field performance of cypress cuttings, and the maintenance of juvenility in stool-beds over time, are important questions currently being researched.

Why is physiological age so important?

Physiological ageing or maturation is the process of change from a juvenile to a mature state. The apparent physiological age of a tree may be different from its chronological age (the time taken to grow from seed) because of environmental influences and cultural practices, such as cutting propagation. The effects of physiological ageing in the parent tree will be, to a large degree, carried over into the vegetative propagules.

Physiological ageing is, therefore, a barrier to successful clonal forestry. When tree breeders have selected mature trees with the very best growth, form, wood characteristics and disease resistance they would like to be able to clonally propagate and reforest with these same genotypes. Normally however, if mature trees are vegetatively propagated, the propagules do not grow well because they 'remember' that they came from mature trees. In other words, cuttings taken from an 8 year old cypress tree will retain many of the characteristics of that original tree, including slower diameter growth and finer branching, compared with seedlings derived from that same tree, which will have a juvenile form and more rapid early growth.

This means that either clonal material must be rejuvenated, subsequent to testing, or that physiological ageing must be arrested, or at least minimised, in a clonal storage system.

Forest Research has thoroughly researched this with radiata pine. While rejuvenation is very much the ideal, it is technically very difficult and not yet operational. However, there has been some success with clonal storage systems, which control physiological ageing and allow true to type clonal propagation, subsequent to clonal testing. One cost effective system that has been developed for radiata pine, and is also showing promise for the cypresses, is the hedging of nursery stool plants.

With radiata pine, both advantages and disadvantages are associated with physiological ageing of vegetative propagules. Significant physiological ageing results in mature plants that are difficult to propagate, with low multiplication rates and poor field performance, compared with juvenile plants. However, there is an optimum physiological age of three to four years, when there are advantages of improved stem form with some ageing, but not the disadvantage of early loss of diameter growth associated with older physiological ages. We believe that similar trends will be observed with the cypresses. The trick is to sufficiently control physiological age so as to gain the advantage of improved form without the disadvantage of slower diameter growth.

Location of possible cutting types on a 1 year-old cypress seedling.

Simple stool-bed techniques for cutting propagation

Stool beds

Macrocarpa and lusitanica nursery stool beds, for cutting production, can be established with seedlings or physiologically juvenile cuttings. The propagation system developed at Rotorua's Forest Research nursery starts with seed sown into seed trays in autumn. Germinants are then pricked out into single-celled containers in June. A free draining potting medium containing a slow-release fertiliser is used. Early sowing of the seed takes advantage of increased spring growth, producing a more robust stool plant for cutting production.

In October, the parent stool plants are lined out in the nursery bed at 1.0 metre by 1.5 metre spacing. Better cutting production is achieved if the stool plants are not topped or root-pruned in their first year. The stock plants can be managed over a period of about four years, with the production of cuttings increasing with time. Stool beds are hedged every winter after cutting collection. The first year's trim is at about 10 cm, and as cuttings are taken each year, the topping height is raised up to 30 cm, depending on the growth during the previous season. Hedging helps maintain juvenility. Stools established from physiologically aged cuttings are less productive than those grown from seedling stock, and cuttings derived from them will be physiologically mature.

Cutting size and setting time

Cuttings are taken from the stool plants in late April or early May, about a year after sowing. The form of cypress seedlings varies considerably from tree to tree and cuttings may differ according to the condition of the seedling wood, and the required multiplication rate.

Three types of cuttings are recognisable. If a lower multiplication rate is acceptable, then robust, semi-lignified, first, and second-order cuttings (12 to 15 cm and 6 to 10 cm, respectively) can be taken. If a higher multiplication rate is desired, then smaller 3 to 4 cm, and more succulent cuttings can be taken. Rooting of the medium-sized 6 to 10 cm, second-order cuttings is more reliable, compared with either the larger or smaller cuttings.

The larger first and second-order cuttings are set into single-celled containers of about 100 ml, with a surface diameter of about four centimetres. A free-draining potting medium such as peat: pumice: perlite in the ratio 3:1:1 is used. No fertiliser is added to the rooting medium.

The small cuttings are set into containers of about 50 ml, with a surface diameter of about three centimetres.

Rooted lusitanica cuttings removed from containers.

Propagation environment

Good rooting percentages of 80% to 90% have been achieved for both macrocarpa and lusitanica cuttings at the Forest Research nursery in Rotorua, in a polythene tent in a greenhouse. Lusitanica cuttings root better if given a cool pre-treatment and are kept outside in a shade frame for 20 days before being moved into the greenhouse environment. Rooting of macrocarpa cuttings is improved with IBA hormone Seradix 2, and to some extent with Clonex, but lusitanica cuttings root better without IBA treatment. Response to rooting treatments varies considerably with different macrocarpa clones and in different propagation environments and the timing of IBA application is critical. There may be other hormone treatments, which we have not tested, that may prove effective.

Cuttings of both species require a high humidity for rooting: close to 100% relative humidity. Initially the temperature is kept low: about 8°C to 10°C at night and cooled to about 15°C to 18°C during the day with shade and fan cooling.

After macrocarpa and lusitanica cuttings are rooted, they are slowly hardened to withstand the lower greenhouse humidity, and eventually an outside environment by spring. Once they are hardened, the cuttings are given complete liquid foliar, or bottom-fertiliser treatments prior to planting.

Macrocarpa tree badly infected with canker, Strathallan genetic field trial, age 11.

Inoculation of macrocarpa with canker.

Developing canker-resistant macrocarpa clones

Cypress canker is the single greatest hurdle to more widespread deployment of macrocarpa, our most valuable cypress species. If we are to overcome the canker problem, many genetically diverse genotypes with multiple resistance mechanisms must be identified. If limited numbers of resistant genotypes are deployed, we run the risk of resistance being quickly overcome by genetic shifts in the canker pathogens.

Forest Research pathologists are developing a system to screen large numbers of genotypes for canker resistance in a greenhouse. We will identify resistant genotypes by screening clones that are derived from families showing resistance in the breeding programme. This would then become the basis of a family forestry programme, and eventually a clonal forestry programme. At the same time, Forest Research will establish clonal field trials to validate the effectiveness of early screening and also allow for further selection for growth, form and wood traits.

Forest Research plans to supply growers with canker-resistant cuttings from superior families, via family forestry. Royalties from sales of Forest Research branded cypress will help finance further breeding and delivery of superior cypress planting-stock to industry.

Another possibility for delivering resistant genotypes, which Forest Research is investigating, is hybridisation. There are about 18 species and 8 varieties of cypresses in New Zealand, some of which are highly resistant to canker. Apparent hybrid vigour has frequently been reported in spontaneous hybrids and it appears that some of the macrocarpa hybrids have resistance to cypress canker. However, any success with hybrid breeding will depend on a continued commitment to the breeding of the pure species and on further research on controlled pollination.

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