Choosing the right seed source is one of the most important aspects of forest establishment. It can have huge implications for the early growth and survival of a plantation, and in later years on its productivity and wood quality. If the genetic quality of the planting stock is not correct from the outset, then there is little that can be done to improve a plantation other than costly replacement. Once planted, all subsequent operations and associated costs are applied to this material, it is therefore essential that it is capable of producing a good return if the plantation is to be economically viable.
Foresters, like farmers, pay great attention to the genetic quality of their crops. For foresters it is perhaps even more important as it can be many years before mistakes become apparent. By then it is often too costly to replace a plantation so the crop is retained, resulting in much lower yields, poorer wood quality or lesser returns than originally expected.
Seed for the production of planting stock in Irish forests is obtained from three main sources:
- Seed Stands
- Seed Orchards
- Imports from the species natural range
These are superior forest stands that have been identified as being suitable for seed collection. The trees are generally of known parental seed origin, healthy, show good growth rate, and have straight stems free of defects. Many of the stands have been grown for one or two generations in Ireland from seed that was originally collected in the species natural range. Collecting seed from these superior stands ensures that the genetic quality of the next generation will be as good if not better than the original stand.
A National Catalogue of Irish Seed Stands is maintained by the Forest Service, the designated authority for the selection and registration of seed stands in Ireland. Only seed from registered stands can be used in plantations that receive grant aid from the Forest Service.
EU Council Directive 1999/105/EC sets out the main criteria for seed stand selection and also conditions for the marketing of this material in EU countries.
Registered seed stands for the production of home grown tree seed
The importance of seed stands in broadleaved forestry
EU list of forest reproductive material
Seed orchards are designed and managed to produce seeds of superior genetic quality compared to those obtained from seed stands, or unimproved stands. The seed from the latter is generally wild in character i.e. very similar to the natural stands in the species native habitat. The seed from seed orchards, however, is of improved genetic quality having been derived from selected and bred material from a breeding programme.
Seed orchards can be composed of grafts (vegetative copies) of selected superior trees (plus trees) or sometimes seedlings based on the progeny of selected superior trees. They are generally located on sites isolated from surrounding stands of the same species to avoid contamination from foreign pollen. The trees are widely spaced to ensure good crown development which encourages flower production. Seed production can vary significantly from year to year depending on the weather conditions in the year prior to flowering. Seed orchard managers, however, sometimes use flower induction techniques such as partial girdling or the injection of hormones (gibberellins) into the tree stems to encourage and/or enhance flowering.
Seed orchards have traditionally been field based but in recent years indoor orchards based in polytunnels are proving to be effective. While these orchards require intensive management they are particularly suited to the control of climatic conditions which encourage flower production such as drought stress and higher temperatures. Indoor birch and alder seed orchards have been established by Teagasc and are producing improved seed in commercial quantities.
In the past, Ireland has had to rely on the importation of seed from abroad, particularly for non-native tree species. The cool moist summer climate of Ireland is generally not conducive to seed production and frequent shortages of seed have occurred. Buying in seed from abroad was therefore the only option to ensure a regular and sustained supply of plants for the annual planting programme.
Importing seed can be a risky business as sometimes the best sources are not available when required. Unknown provenances in the past have resulted in costly crop failures as happened with the LuluIsland provenance of lodgepole pine and the brown bud ash. Today, however, there are much better controls on the marketing of seed internationally and also there is more information available on its genetic quality. Obtaining seed from home grown sources is, however, the best option as the parent material has proven its worth in Ireland.
The supply of home grown seed for the planting programme improved greatly with the establishment of a National Tree Seed Centre by Coillte in Ballintemple Nursery in Co. Carlow. The Centre is providing the forest sector with state of the art seed collection, processing and storage facilities which has greatly improved the supply of home collected material to public and private nurseries.
Vegetative propagation (cuttings)
In recent years improved genetic material is becoming available from tree breeding programmes. This material is only available in small quantities and it can take many years using the conventional route of seed stands or seed orchards to produce it in commercial quantities. Tree breeders, however, are using vegetative propagation as a means of mass producing it by taking cuttings from progeny of improved families and clones and rooting them in greenhouses. The subsequent rooted cuttings are then raised in a similar fashion to seedlings. By this method it is possible to produce 200-300 transplants from a single seed of an improved family, or theoretically an unlimited number of transplants from hedges of improved clones.
Rooting cuttings has been a traditional method of vegetative propagation for centuries. While this method works well for species that are easy to root e.g. poplars and willows there are limitations to the extent to which it can be economically used for more difficult species such as our main conifers and broadleaves. Today, however, scientists and breeders are developing advanced lab based propagation techniques such as micropropagation and somatic embryogenesis to produce improved material in commercial quantities. Much research has been carried out on forest trees with some success. While these techniques are possible with some species, the high cost of production is proving to be a limiting factor in their wide scale use. They are however being used as a research tool to produce stock plants for conventional cutting production.
The advantages of vegetative propagation are that plants, with identical characteristics to the improved parents, can be mass produced in large quantities; also they are not subject to the vagaries of weather conditions that greatly affect seed production.
For further information see:
Why plant Sitka spruce cuttings?
Vegetative propagation techniques for oak, ash, sycamore and spruce