Genetic variation in the Victorian ash-type eucalypt forests – a discussion paper on its relevance to regeneration and reforestation practices.  L. A. Pederick.  March 1990.  25pp. (unpubl.)

SUMMARY

As specified in the Code of Forest Practices, the seeds to be collected for the regeneration of Victorian ash eucalypt forests should be of "species native to the area" and collected from the same general locality in order to "maintain species patterns and genetic pools present prior to harvesting". This prescription has been criticised by some as being insufficiently rigorous to ensure a replacement of the forest with trees genetically adapted to the site. It has been proposed that the seeds should he collected from close to the coupe if not the coupe itself.

This report presents a discussion of scientific evidence relevant to any decision regarding the sources of seed that would be satisfactory for maintaining the native 'ash-type' forests of Victoria in a genetically well adapted condition. It applies only to forests managed for wood production and not to National Parks or scientific reference areas which have strict requirements that the indigenous gene pools on each site be preserved.

The results of provenance trials with E. regnans, E. delegatensis and E. nitens on typical sites within the centre of each species' natural range, provide no evidence for the need for an exact matching of genotype to site to obtain a healthy vigorous forest. On the contrary, the species frequently grew well when seed was moved up to 100 km and sometimes further. Some provenances of each species could be identified as slower growing or less well adapted, but these were usually found to be from particular parts of the species distribution, i.e. outlying parts of the range or small isolated populations. Some evidence of superior provenances (for growth traits) has been obtained.

From isozyme studies of eucalypt species, the number of allelic forms of particular genes has been found to vary, with most of the genes studied having 3-4 forms but some up to 7. Within each enzyme system the same alleles occur throughout the species distribution, though their frequency in populations may vary. However, in E. delegatensis there was more variation between trees within a population than between populations from different areas. This is important as it can explain the good growth of most seedlots of this species from throughout Victoria on two sites near Mansfield. It appears that the genes (alleles) required for well adapted growth at any regeneration site are present in populations distributed over a much broader area than the immediate vicinity of the coupe.

Natural selection has been found to exert a strong effect on survival and growth under some conditions, particularly for cold hardiness at high altitudes. It is also assumed to occur for other site factors, though few examples are documented.

There is mounting evidence that there will be a pronounced change in climate by early next century. The change, far in excess of the rate of previous climatic variation, could result in stands becoming less well adapted to their sites. However, as the process of change advances, any movement of seed might in fact have a beneficial effect by providing a broader spectrum of genetic variation from which a better adapted population for the site may develop.

It is proposed that the genetic objective of the Code, for regeneration of native forests, should be to establish trees genetically well adapted on each site. The optimum gene frequencies for a site may not be the same as occurred in the previous natural stand because of climatic change, chance effects, or other factors. If the gene frequencies in applied seed differ from the optimum for the site, then as a result of natural selection the frequencies in the new stand will change towards the optimum. It is concluded that seeds from well developed stands of the same species from similar sites in the general area of the coupe, preferably within 10-30 km, will provide as well adapted regeneration as it is possible to achieve. However, care should be taken to limit the vertical transfer of seed, perhaps within an upward limit of 200 m, because of cold hardiness considerations.

Seed collected in natural eucalypt forests exhibits an inbreeding depression effect exhibited by slower growth due to the relatedness of neighbouring trees. This growth depression is absent in trees grown from unrelated crosses. Neighbouring trees in plantations derived from a large number of seed parents have minimal genetic relationships, so well-thinned plantations have been advocated as a superior seed source provided the original seed parents are representative of the area to be regenerated.

It is also proposed that selection should occur during seed collection operations for traits associated with improved growth, health and timber quality. Growth and health should be positively related to adaptation, but traits affecting quality such as stem straightness or branch retention are probably neutral to adaptation. Thus, selection of trees for greater average vigour and health, straightness, and finer branches, whenever possible, would have a positive benign effect on the future quality of trees for wood production. The changes, or gains, would be very small, but preferable to the genetic erosion of quality traits that would have occurred in some forests as a result of past selective logging practices.

The use of more intensive techniques to improve the genetic quality of seeds or planting stock is discussed. These include use of more productive provenances, seed orchards, hybridisation, and vegetative propagation. Such improved genetic stock should be grown in plantations rather than in the conditions of the natural forest. Although the Code states that plantations should be "established with species and provenances selected for their vigour, type of wood and productivity", it makes no mention of the type of land to be used for this purpose. Attention to this aspect is needed in view of the recent increase in public interest in plantations and the emphasis in the Code on maintaining the forests on public land in a natural condition.