NOTES FROM THE IUFRO MEETING ON FROST-RESISTANT EUCALYPTS FRANCE, SEPTEMBER 1983

Of the approximately 500 species of eucalypts, less than 1/10 occur in their natural habitat in areas which experience sub-zero temperatures; the limit of occurrence in Australia of the genus is 45°S latitude and, at higher altitudes (200 m a.s.l.), 35°S latitude.

In their natural environment, the species of the genus Eucalyptus are highly adapted to the sites on which they occur. Frost damage in the genus in Australia is extremely rare and only about half a dozen cases of such damage (of limited and localized extent) has ever been recorded. However, when grown as exotics, damage by low temperatures is a common problem in eucalypt plantations in many countries, including developing countries in southern, eastern and northern Africa; the Mediterranean; and South America.

The meeting, organized by two IUFRO Working Parties on eucalypts in collaboration with AFOCEL, France and CSIRO, Australia, had been convened to discuss problems related to plantations grown in adverse conditions, with special emphasis on eucalypts grown as exotics in climates with occasional sub-zero temperatures. The meeting itself, preceded by two Study Tours, was divided into 7 Sessions. Some 40 papers were presented to the meeting, and they will - in due course - be published by AFOCEL (Association Forêt-Cellulose, Domaine de l'Etançon, F-77370 Nangis, France).

Some points of interest and general conclusions are given below:

• Few countries in the world plant eucalypts in cold areas, except for on an experimental scale.

• Adequate exploration/evaluation for frost resistance has not yet been done. Eucalypts possess resistance greater than that implied by their present, natural ranges in Australia, and only range-wide, locally established trials in potential plantation areas in the introducing country can determine this resistance.

• The causes and mechanisms of frost damage are poorly understood; a species may be highly tolerant on one site, yet suffer considerable damage in non-severe temperatures on others. In addition to adequate hardening in advance of the frosts (in which minimum night temperatures during the days preceeding the frost seem to be of decisive importance), factors related to soil and soil temperatures, soil and air moisture, and the general physiological state of the plant, seem to play an important role in frost resistance and tolerance. It was generally felt that only through understanding the physiological causes leading to frost damage and/or tolerance (which are symptoms rather than absolute processes), can any progress be made in this field.

• Discussions on breeding strategies, which formed the subject of one of the Sessions, were of particular interest. In addition to the “classical path” of exploration, collection, evaluation and improvement of seed and seedling material, recent advances in vegetative propagation techniques have lead to alternative strategies including the use of inter and intraspecific hybrids and clonal forestry. Intensive nursery and management methods have, as a consequence, been developed in a number of countries to cater for the appropriate use of material in which per-unit value is very high as compared to normal seedlings, but in which potential returns could also be manifold. Some specific observations on these points are:

  1. Vegetative and sexual reproduction are always complementary paths;
  2. Conservation of a base population with broad genetic base is of fundamental importance irrespective of strategy, but the needs are highlighted if vegetative propagation and clonal forestry (which minimizes genetic variation in the plantations) are used;
  3. All countries which presently use clonal forestry, plant a mosaic of monoclonal blocks of up to 50 or more hectares of one single clone, rather than a mixture of clones. New selections are made continuously, with the expected “life-time” for the use in plantation forestry of any one clone of no more than 5 years (i.e., in the best of cases, equivalent to 1 rotation);
  4. Micropropagation in vitro is used generally to propagate material of particular value, followed by “classical” vegetative propagation methods to further increase the material; and in some cases for rejuvenation purposes. A notable exception to this is reported from USA (mass-propagation through in vitro methods), however, on a very limited, experimental scale only.
  5. Most older (10–15 years+) introduced genetic material is of unknown origin. Returning to the original populations in Australia and selection of the best provenance(s) followed by selection and breeding, has generally given better results than further work on the unknown and often haphazardly introduced “land race”. This point is accentuated by the fact that early introductions of eucalypts often originate from one mother tree only, rather than being a representative sample of a specific population with a wide range of inherent variation which can be used as a basis for local selections and further breeding. Commercial collections by seed dealers often still today suffer from this grave defect. The indispensable value of knowledge of both origin/provenance and number of mother trees represented in the material to be introduced into a country, cannot be overly emphasized;
  6. The question of (i) selecting a frost resistant species/provenance as a basis for further selection and breeding for growth and yield, versus (ii) the breeding of frost resistance into a highly productive species/provenance, was discussed at length without much consensus. In some cases, intraspecific hybridization has been used to compromise the two, especially in countries like Brazil where rotations, and therefore breeding cycles (and results), are fast. In most cases, however, the expected occurrence of bad frosts versus rotation and expected yields must be considered, and an economically viable choice of genetic material made on calculated risks, on a case-to-case basis.
  7. Although generally overcome to date (however, often at a considerable cost), insect and disease damage to eucalypts must be considered as a highly probable event if large plantations of single species are grown, especially if these are established in marginal conditions. Good examples of such damage are the stem canker (Cryphonectria cubensis syn. Diaporthe cubensis) in E. grandis in Brazil (now largely overcome by change of provenance, selection within the new provenance, and breeding); and the widespread damage by the borer, Phorocantha semipunctata in the Mediterranean, triggered by a series of drier-than-usual years in Tunisia/Morocco (the insect has recently also spread to Italy).
  8. The recently steeply increasing use by florists in other continents of live branch material air-freighted from Australia for flower arrangements and as decoration, is seriously increasing the risk to forest plantations of introduced pests and diseases from Australia into earlier disease-free areas, and should be vigorously opposed in favour of local cultivation of such material to decrease this r
  9. Recent taxonomic studies in Australia, to be published within the next few years in Flora Australiensis, will botanically decimate the genus Eucalyptus, and only maybe 1/5 of the present Eucalyptus species will in the future bear that name. Even with the compromise of continuing to refer to the species as “eucalypts”, strong feelings were expressed in the meeting against this massive reorganization and changes in established nomenclature.