PROVENANCE COLLECTIONS OF GREVILLEA ROBUSTA (SILKY OAK) ¹

by

C. Harwood
CSIRO Division of Forestry and Forest Products
P.O. Box 4008, QVT
Canberra 2600, Australia

BACKGROUND

Grevillea robusta (silky oak, family Proteaceae) is a tree species with a limited natural distribution in subtropical and warm temperate eastern Australia, its natural range extending from about 26° to 30°S and 100–160 km inland from the coast (Figure 1). It was introduced to other countries in the nineteenth century, and is now common in subtropical and tropical highland areas in Asia, Africa and the Americas. Important economic uses have developed, initially as a shade tree for tea and coffee plantations and more recently as a farm forestry tree in the east African tropical highlands where it is used for row and boundary plantings, providing sawn timber, poles, firewood and leaf mulch while interfering relatively little with adjacent cash and food crops.

In a recent annotated bibliography, Harwood (1989) noted that nothing was known of genetic variation within the species. However, Booth and Jovanovic (1988) have demonstrated that there are substantial differences in climate across the natural range, because of the effects of altitude (the species occurs from sea level to 1100 m) and a general decrease in rainfall with increasing distance from the coast. Mean annual temperatures at 25 sites of natural occurrence (selected to span the range of climates across the natural range varied from 14.7 to 20.1°c, and rainfall from 720 to 1710 mm. The species occurs naturally in two distinct habitats: Araucaria-dominated vine forests, and alluvial flats along rivers and streams, on a variety of soil types. It is therefore quite possible that there are provenances with differing morphological and/or physiological characteristics, which may be of value to countries using the species. Much of the original natural habitat of G. robusta has been cleared for agriculture, and some modified for forestry plantations. However, many local populations remain across the geographic range.

Historical evidence indicates that the overses populations were derived from introductions in the nineteenth century, with narrow genetic bases. The first recorded introduction was to Sri Lanka in the early 1860's, from seed collected from an artificial stand planted at the Royal Melbourne Botanic Gardens, Australia (Anon 1885).

In 1989, the International Council for Research in Agroforestry (ICRAF) and collaborating African national research institutions included G. robusta among four agroforestry tree species selected for intensive programmes of genetic improvement for use in agroforestry systems in east and central Africa. Efforts are now under way to compare G. robusta provenances from across the natural range with the local “land races” in countries using the species, and to broaden and improve the genetic base available to users. Workers involved in these programmes attended an international Workshop on the species held at ICRAF Headquarters, Nairobi, Kenya in August 1990.

COLLECTIONS OF NATURAL PROVENANCES

CSIRO's Australian Tree Seed Centre, in collaboration with the Queensland Forest Service and private collectors, has made extensive seed collections of natural provenances over the 1989–1990 summer. The collection strategy was to obtain at least 100 g of seed per tree from each of four trees for each local provenance sampled. Wherever possible, the four trees were at least 100 m apart to reduce the likelihood that they were closely related. 40 provenances were targeted for collection, covering the altitudinal and geographic range of sites at which the species occurs. Where possible, smaller quantities of seed were collected from an additional six trees per local provenance, for isozyme studies.

The 1989–1990 summer was a poor seeding year in some parts of the range, and not all provenances yielded the quantity of seed planned. Nevertheless, an extensive collection of natural provenances has been assembled (Figure 2, Table 1). All seed has been stored as individual tree seedlots. Provenances of particular interest are the high-altitude, western locations (Bunya Mountains, Porters Gap, Maclagan and Guy Fawkes) which may show good frost resistance. The main gaps in the collection are the Mary River Valley in the northern part of the range, and the lower Brisbane River and adjacent catchments near Brisbane. Efforts will be made to cover these regions in 1990–1991.

It is intended that most of the seed collected should be used to establish provenance/progeny trials which will be converted by thinning to seed production/genetic resource stands, in countries that use the species. To ensure that the seed can be distributed widely, only small quantities of seed will be supplied (no more than 5 g per individual tree per customer). Viability tests, still in progress, indicate that for most seedlots 5 g will yield at least 250 germinants. Persons interested in obtaining seed should contact:

Australian Tree Seed Centre
P.O. Box 4008, QVT
Canberra 2600
Australia
Telex: AA62751
Fax: 61-6-2818312

ACKNOWLEDGEMENTS

The Australian Tree Seed Centre acknowledges the assistance of the Queensland Forest Service, Messrs. Geoff Borsham, Jim Johnston and Phil Ross and Ms. Robyn Bell in assembling these collections. The Japan International Cooperation Agency and Swiss Intercooperation supported the collections through advance purchase of some of the seed.

REFERENCES

Anon. 1885 Letter to the Editor, Tropical Agriculturalist. Tropical Agriculturalist, Colombo, Ceylon (Sri Lanka).

Booth, T.H. & Jovanovic, T. 1988 Assaying natural climatic variability in some Australian species with fuelwood and agroforestry potential. Commonwealth Forestry Review 67, 27–34.

Harwood, C.E. 1989 Grevillea robusta: an annotated bibliography. ICRAF, Nairobi.

Figure 1. Locations of some natural occurrences of Grevillea robusta. Many other locations are not shown on the map. The 1600, 1200, 1000 amd 800 mm median precipitation levels (50 percentile values) are shown. These median values would be similar to the corresponding mean precipitation values.	Figure 2. Locations of Grevillea robusta provenances referred to in Table 1.

TABLE 1. NATURAL PROVENANCES OF GREVILLEA ROBUSTA AVAILABLE FROM AUSTRALIAN TREE SEED CENTRE

* Number of individual tree seedlots with 20 g or more of seed

¹ Manuscript received July 1990