Tree Improvement
Tree improvement, or as it is often referred to as genetic improvement, is the process of improving the genetic quality of a tree species. Our forest trees are still genetically close to their wild state in their natural range. However, considerable variation exits in economic traits such as growth rate, stem form and wood quality between different populations within a species, and also between individual trees within populations. Opportunities, therefore, exit to improve the silvicultural value of a species by identifying the best wild seed sources; and also to select individuals within these to develop varieties that are considerably better than the wild material.
Two different methods of improvement are generally used:
- Provenance studies – to identify the best wild populations
- Tree breeding programmes – to select and breed from the best individuals within the best populations.
PROVENANCE STUDIES
Many of the tree species common to our forests have wide natural ranges which cover many different habitats and climatic zones. Trees have adapted to these conditions and as a result they can have very different characteristics depending on the origin of the seed source. For example, Douglas fir and lodgepole pine have natural ranges that cover large areas of western North America, from moist coastal regions to high altitude dry interior mountain ranges, and from northern to mid latitudes. They have developed characteristics that allow them to survive and grow in these conditions, and depending on which region the seed is collected from, the performance of these seed sources can be very different when grown in Ireland. To determine the best sources, seed origin or provenance studies, have been carried out by the Forest Service since the early 1960s. These have subsequently been continued by Coillte and Teagasc with funding from Coford.
Oak provenances showing differences in early growth (photo D. Thompson)
To date, provenance studies have been carried out on all the main species planted in Irish forests and also many of the minor species. Field trials of each species have been established across a range of sites types and observations on survival, growth rate, stem form, resistance to damaging agents such as insect pests and diseases, frost, etc have been made over many years. This has allowed foresters to identify the best adapted and most productive tree seed sources for Ireland.
Generally, with some exceptions, the best seed sources are those from areas of similar climate and latitude to this country. It is therefore no accident that most of our main non-native species come from the coastal regions of Washington and Oregon in North West America which, like Ireland, are also on the western edge of a continent and experience cool temperate climatic conditions.
For further information on provenances see links below:
Where should Washington and Oregon sources of Sitka spruce be planted in Ireland
Provenances of beech best suited for Ireland
Seed source significantly influences growth, form and the silvicultural management of oak
Irish oak – genetic diversity and the Iberian connection
TREE BREEDING
The wide variation in traits such as growth rate, stem form and wood quality that exists in many of our tree species provide tree breeders with the opportunity to develop improved varieties. These have the potential to produce larger volumes of better quality wood than can be achieved from wild material.
Tree breeding programmes in Ireland began in the 1970s by the Forest Service and today are being continued by Coillte and Teagasc. Breeding activity has expanded over the years to include both conifer and broadleaved species. Techniques being used by tree breeders are:
- Plus tree selection
- Controlled crossings
- Progeny testing
- Clonal testing
Plus tree selection
Ash clone bank at Kilmacurragh ( photo D. Thompson)
Superior trees (Plus trees) are selected in the best forest stands as breeding stock for the improved varieties. These trees display superior characteristics to the surrounding trees in the stand e.g. straighter stem, larger volume etc. Shoots from these trees are taken and grafted onto seedling rootstock in the nursery. These grafts are clones and are therefore genetically identical to the original plus tree. They are planted out in a clonal archive or clone bank to preserve the genes of the original tree and to be used as breeding material. Plus trees are generally selected on the basis of their appearance (phenotype) in the forest but their breeding value cannot reliably be assessed by the trees outward appearance alone. Their breeding value must therefore be determined by testing their offspring or progeny.
Controlled crossings
Controlled crossings are carried out on flowers produced by the grafts of the original plus trees in the clone banks. Their purpose is to produce seed from selected parents to test their breeding value and/or to combine the best traits of the selected trees for the next generation.
Controlled crossings are carried out in spring when the flowers first emerge. In conifers the male and female flowers are separate and the female flowers are isolated using plastic tubes or pollen proof bags. Pollen is then gathered from the donor tree and injected onto the isolated female flowers when they become receptive. This process requires great attention to detail both in record keeping and observing when the females reach their optimum receptivity and also in collecting and maintaining hundreds of separate seed lots.
The seed from controlled crossings is collected, kept separate by mother tree and sown to produce offspring which are then planted out in progeny tests on typical forest sites.
Progeny testing
In developing improved varieties it is important to test the offspring or progeny of plus trees to determine their breeding value. Only those plus trees that produce superior progeny are used as breeding stock for the next generation. The progeny from a particular plus tree or controlled crossing is known as a family.
The performance of families are determined in progeny tests established on forest sites typical for the species. The families are planted in replicated plots and their growth and development assessed at periodic intervals. It can take many years before reliable information on the performance of the progeny is known – approx 10 for Sitka spruce and many more years for slow growing species such as oak.
The results of progeny tests are used to select the plus trees with the best breeding value. These can be either the original plus trees or superior individuals selected within the progeny test itself. Material (grafts or seed) from these trees is then used create a seed orchards for the production of improved planting stock.
Clonal testing
Within populations (unimproved stands, seed stands or families) individuals are often found with outstanding characteristics which are far superior to the rest. Tree breeders select and vegetatively propagate these individuals to produce superior clonal varieties with the same traits as the parent trees. In developing these varieties it is necessary to test the performance of the clones to ensure that they are indeed superior and will perform consistently over a range of site types. Clonal testing is similar to progeny testing in having the objective of screening the best material for the production of improved planting stock.