Grevillea robusta (silky oak)

G. robusta is a tree which is an effective colonizer and has demonstrated invasive behaviour in Australia (New South Wales), New Zealand, French Polynesia, Jamaica, Zimbabwe, and notably in South Africa and the United States (i.e., Hawaii). Contributing to its behaviour is likely to be its early and prolific seed producing habit, and the production of potentially allelopathic compounds. In South Africa, G. robusta has been declared a category 3 invader (Henderson, 2001). It is also invasive in Hawaii (Holm et al., 1979; Cronk and Fuller, 1995; Space and Flynn, 2001), a common weed in Micronesia (Space and Falanruw, 1999) and cultivated in Niue (Space and Flynn, 2000). It seeds heavily and regenerates strongly after disturbance in rainforests and along riverbanks. Several years ago, Binggeli (1999) classified G. robusta as a moderately invasive species: currently, it is listed as invasive and potentially invasive in many countries in South America, Africa, Asia, and on many islands in the West Indies and the Indian and Pacific Oceans (Harwood, 1997; Kairo et al., 2003; Oviedo Prieto et al., 2012; PIER, 2015; PROTA, 2015).  

The family Proteaceae includes 80 genera and 1600 species of shrubs and trees distributed principally in the southern hemisphere, especially across Australia and South Africa. There are about 515 species in the genus Grevillea (family Proteaceae), most of which are endemic to Australia (Stevens, 2012). Grevillea robusta was described by Alan Cunningham, and the name was published in 1830 in the Supplementum Primum to Robert Brown's Prodromus Florae Novae Hollandiae. The specific epithet is derived from the Latin robustus - hard, strong, robust, in reference to the large size of this species in a genus where many species are shrubs. It has no recognized subspecies or varieties, and no hybrids with other species have been recorded (McGillivray and Makinson, 1993).

G. robosta is an erect, single-stemmed tree typically reaching 20-30 m tall and 80 cm in stem diameter. The crown is conical and symmetrical with major branches spaced at intervals of about 1 m and projecting upwards at an angle of 45°. Bark on the trunk is dark grey and furrowed into a lace-like pattern. G. robusta is described and illustrated by McGillivray and Makinson (1993), Boland et al. (1984), and Harwood (1997). Proteoid roots (sections of the secondary roots which develop as dense cylindrical clusters of rootlets) develop in conditions of low phosphorus availability, and are thought to increase the plant's ability to take up nutrients (Skene et al., 1996). Young branchlets are angular and ridged, subsericeous to tomentose but glabrous on older growth. The fern-like foliage of this species is very distinctive. Leaves are 10-34 cm long, 9-15 cm wide, variably pinnate to bipinnate, with a glabrous green upper surface and subsericeous silvery under-surface. Petioles are 1.5-6.5 cm long. The species is semi-deciduous in its natural range, being almost leafless shortly before flowering. The flowers are grouped into compound racemose inflorescences (conflorescences, after Johnson and Briggs (1975)), which themselves are grouped into panicles of one to six conflorescences, borne on older wood. The bright orange flowers, about 2 cm long, are borne in numerous pairs along the flower spikes, on pedicels 1.5 cm long. The perianth consists of 4 narrow tepals, 0.6-1 cm long, with the concave summit of each tepal holding a small anther 0.1 cm long. The ovary surmounts a gynophore 0.2-0.3 cm long. Fruits are two-seeded follicles 2 cm in length, with a slender persistent style. Seeds are winged, 13-19 mm long, 8-10 mm wide, 0.8-0.9 mm thick, with a papery wing around the brown, ovate central seed body.

G. robusta is native to northern New South Wales and southern Queensland, Australia, where it occurs from the east coast to as far west as the Bunya Mountains, Queensland, some 160 km inland. The north-south range of the species is some 470 km, from the Guy Fawkes and Orara Rivers (tributaries of the Clarence River in New South Wales, 30°10'S) to just north of Gympie, Queensland (25°50'S). It is found across a wide range of altitudes from sea level to mountaintop occurrences at 1120 m in the Bunya Mountains (Harwood, 1992a). Outside its native distribution range, this species can be found naturalized in tropical Asia, Africa, America and the West Indies (see distribution table for details; Harwood, 1997; Acevedo-Rodriguez and Strong, 2012; PIER, 2015; PROTA, 2015). 

G. robusta has been introduced to other parts of Australia outside its native distribution range, and is now regarded as invasive around Sydney and is a known weed in the Blue Mountains of New South Wales (BMCS, 2002). It has also been introduced to warm temperate, subtropical and tropical highland regions around the world commencing in the mid to late 1800s, and it is now widely planted in India, Sri Lanka, Central and South America and many countries in Africa (Harwood, 1989). It performs poorly in lowland tropical environments. In Hawaii, it was introduced in 1880 from Australia (Motooka et al., 2003). In the West Indies, G. robusta appears in herbarium collection made in 1899 in Guadeloupe, 1916 in Puerto Rico, 1928 in Dominican Republic and 1932 in Jamaica (US National Herbarium).  

 The risk of introduction of G. robusta is moderate to high. This species has been widely planted in warm temperate, subtropical and tropical regions of the world and it is a prolific seed producer. It behaves as a weed in drier mesic pastures, disturbed areas and forests and it can be found naturalized principally in areas near cultivation (Harwood et al., 1997; ISSG, 2015; PROTA, 2015). 

The seeds of G. robusta are dispersed by wind (Smith, 1998). G. robusta has gained widespread popularity in warm temperate, subtropical and tropical highland regions of many countries, originally as a shade tree for tea and coffee and now as an agroforestry tree for small farms (Harwood, 1989) and intentional introduction is the principal means of long-distance dispersal.

In its native range, G. robusta is commonly found in small, discontinuous stands along the banks of rivers and streams, usually within 30 m of the water's edge, in areas with generally moderate to good soil fertility and water availability. G. robusta is vulnerable to fire and hence is excluded from the fire-prone Eucalyptus forests and grasslands that occupy much of its natural range. Habitats invaded by this species in South Africa include forest edges, coastal forests, disturbed sites, savannah and riparian areas (Henderson, 2001). It is very popular in agroforestry systems and often planted to provide shade for tea and coffee plantations (Harwood et al., 1997).

Genetics

The chromosome number reported for G. robusta is 2n = 20.

No records of hybrids between G. robusta and other species of Grevillea have been published. Isozyme studies of natural populations and land races (Harwood et al., 1997) established that the species has a moderate level of allozyme heterozygosity in natural populations. The genetic diversity of individual populations varied little, and did not appear to be related to their ecological characteristics (araucarian vine forest or riverine habitat types) or their geographic locations. Between-population differences accounted for 17.9% of the total genetic variation. It appears that genetic exchange between populations has been maintained, despite the pattern of natural distribution in small, separated populations. African land races of G. robusta are descended from initial introductions in the late 1800s and early 1900s and have developed in different countries in partial or total isolation from one another, and mean heterozygosity and other genetic diversity measures were substantially lower for the African land races than for the natural populations. The complete absence of a common allele in the two Madagascar populations suggested that these populations were derived from a very narrow genetic base. Provenance-progeny trials testing seed collections from across the natural range have recently been established in several countries including Australia, Kenya and Rwanda.

Physiology and Phenology

Flowering commences in October in lowland occurrences in the native range, and seed is mature by the end of December. At high elevations, flowering and seed production occur 4-6 weeks later (Harwood, 1992a). In the equatorial highlands where rainfall distributions are bimodal, flowering is much less synchronized. In western Kenya, for example, there are two main flowering peaks in March-May and August-October, with some trees flowering throughout the year (Kalinganire et al., 1996). In its native range, the species is partly deciduous, losing much of its canopy in the dry spring months (September-October) and recovering in early summer. In the equatorial highlands there is no clear seasonal pattern of foliage loss.

Reproductive Biology

Under favourable conditions, flowering and seed production of G. robusta commence at age 2-3 years (Kalinganire et al., 1996). In subtropical and warm temperate latitudes, first flowering may occur at 4-6 years (Swain 1928). Flowering in the native range occurs for a few weeks in October-November but in equatorial latitudes the species may flower at any time of year and in Jakarta, Indonesia, it does not flower (World Agroforestry Centre, 2002). There are about 40,000 viable seeds/kg. The species seeds heavily and regenerates strongly after site disturbance in rainforests and along riverbanks.

The breeding system in natural populations was found to be highly outcrossing in isozyme studies of progeny arrays (Harwood et al., 1992). Multi-locus estimates of outcrossing rates of 0.97 and 0.86 were obtained for two natural populations. Observations and experimental studies in a planted stand in western Kenya (Kalinganire et al., 1996) confirmed that the species is fully outcrossing and self-incompatible, and that nectivorous birds are the principal pollinating agents in Kenya, as was found in Australia (Brough, 1933). According to the World Agroforestry Centre (2002) pollinating agents also include honeybees and tree-living marsupials (Phanlangeridae) but Kalinganire et al. (2001) report that ants and bees are nectar robbers that rarely come into contact with the stigmas. The period from the start of inflorescence development to anthesis is about 40 days. Stigmas become receptive 1 day after anthesis, and the period from fertilization to fruit maturity is about 60 days. Hot, dry weather stimulates seed release once the fruits are mature. G. robusta may also vegetatively reproduce by root suckering (World Agroforestry Centre, 2002).

Environmental Requirements

The distribution is in the warm humid to warm sub-humid climatic zones. Climatic analysis of the natural occurrences and locations where the species is successfully grown as an exotic has been carried out by Harwood and Booth (1992) and a modified description of climatic requirements was prepared by CSIRO (Booth and Jovanovic, 2000). Within its natural distribution range, climate varies widely because of the substantial altitudinal range (0-2500 m) and the rainfall gradients created by prevailing weather systems interacting with rugged topography. Mean annual rainfall is in the range 700-2400 mm, but droughts of 6 months or longer will cause death or damage to established trees, although towards the hotter extremes of the tolerated temperature range, the dry season should be no longer than 4 months for good growth. It has some resistance to frost, as during the winter months in temperate latitudes, G. robusta can survive temperatures down to -8°C with little or no damage, but milder frosts of only -2°C or so will cause damage during the growing season.

While it occurs naturally along riverbanks and will survive short periods of flooding by moving water, G. robusta will not grow on poorly-drained, swampy sites where waterlogging occurs for more than a few days at a time. The species is more common on rather fertile soils such as those derived from river alluvia or basalts but will grow on shallower, less fertile soils derived from sedimentary material. Best growth is obtained on sandy loam, loam and clay loam soils and it performs poorly on heavy clays. The pH range for good growth is around 4.5 to 7.5. In highly acid soils, symptoms of boron deficiency (Smith, 1960) and manganese toxicity (Child and Smith, 1960) have been observed. While the species is frequently planted as a windbreak around plantations of food and cash crops, it cannot withstand severe gales or persistent strong winds without damage to the branches. Even low-intensity ground fires will kill seedlings and young trees.

Associations

G. robusta is present in riverine gallery rainforest of the Castanospermum australe (black bean) alliance, the natural vegetation along larger Australian rivers. The frequency of mature trees has, however, been reduced across much of the natural range as a result of past logging and land clearance. G. robusta is also associated with river she-oak (Casuarina cunninghamiana) along smaller streams where rainforest has not developed. The second major habitat type for the species is the Araucarian vine forests and vine thickets dominated by Araucaria cunninghamii. These forests contain rainforest elements but are subject to occasional fires. The species is not known to form symbiotic associations with soil bacteria or mycorrhizal fungi, although it develops proteoid roots that are believed to enhance nutrient uptake. 

In the humid tropical lowlands and other regions with high humidity, G. robusta is vulnerable to attack by fungal diseases such as Botryosphaeria dothidea in Guatemala (Schieber and Zentmeyer, 1978) and Corticium salmonicolor in Karnataka State, India (Nayar, 1987). Pathogenic fungi such as a Amphichaeta grevilleae (Loos, 1950; Venkataramani, 1954), Cercospora sp. (Chiddawar, 1956) and Phyllostica sp. (Rao, 1961) have been observed to cause considerable damage to leaves and stems of young plants in Sri Lanka, particularly if they are over-watered in the nursery.

In lowland environments in the Caribbean, G. robusta is severely attacked by the scale insect Asterolecanium pustulans (Martorell, 1940) effectively precluding its use there. Attack by termites can be a problem when the species is planted on dry sites in Africa.

While the species is frequently planted as a windbreak around plantations of food and cash crops, it cannot withstand severe gales or persistent strong winds without damage to the branches. It has some resistance to frost. During the winter months in temperate latitudes, G. robusta can survive temperatures down to -8°C with little or no damage, but milder frosts of only -2°C or so will cause damage during the growing season. Droughts of 6 months or longer will cause death or damage to established trees. Even low-intensity ground fires will kill seedlings and young trees.

In the humid tropical lowlands and other regions with high humidity, G. robusta is vulnerable to attack by fungal diseases such as Botryosphaeria dothidea in Guatemala (Schieber and Zentmeyer, 1978) and Corticium salmonicolor [Erythricium salmonicolor] in Karnataka, India (Nayar, 1987). Pathogenic fungi such as a Amphichaeta grevilleae [Seimatosporium grevilleae] (Loos, 1950; Venkataramani, 1954), Cercospora sp. (Chiddawar, 1956) and Phyllosticta sp. (Rao, 1961) have been observed to cause considerable damage to leaves and stems of young plants in Sri Lanka. In lowland environments in the Caribbean, G. robusta is severely attacked by the scale insect Asterolecanium pustulans (Martorell, 1940), effectively precluding its use there. Attack by termites can be a problem when the species is planted on dry sites in Africa.

Its use as tea shade has been largely discontinued in Kenya and Rwanda because of the risk of Armillaria and other root pathogens spreading from dead G. robusta roots to those of the tea plants (Tea Research Institute of East Africa, 1969).

G. robusta drops much leaf and fruit litter (Gilman and Watson, 1993). Its leaves produce an allelopathic substance that inhibits the establishment and development of native species. G. robusta also causes changes in patterns of nutrient cycling (ISSG, 2015).

The sap of G. robusta is an irritant to skin and eyelids (Henderson, 2001) and its pollen may trigger hay fever (Motooka et al., 2003). 

G. robusta is a common element in agroforestry and forestry applications, and is one of the most important trees for agroforestry in the tropical highlands of East and central Africa (Harwood, 1992b). It is commonly planted as a boundary tree around the perimeter of small farms, in a single row at 2-2.5 m spacing. It is also planted in rows between small fields, and as scattered individuals over crops such as coffee and maize (Spiers and Stewart, 1992). G. robusta has been planted as a component of contour rows in a number of farming systems that aim to conserve soil on sloping lands in the tropical highlands (e.g. Neumann, 1983). The use of the leaves as a mulch is frequently advocated and practised, and studies have shown that this can reduce soil losses on sloping land (Omoro and Nair, 1993).

The dense, brilliant golden-yellow or orange flower heads, attractive silver, fern-like leaves and symmetrical crown encourage widespread and increasing use of the species as an ornamental. It is used in many tropical and subtropical countries for park and roadside plantings. In the UK and Europe it is commonly available as an indoor plant. In Australia, other Grevillea species and interspecific hybrids are commonly grafted onto rootstocks of G. robusta to produce ornamental planting stock. This practice takes advantage of the resistance of the root system of G. robusta to Phytophthora cinnamomi and its non-susceptibility to phosphorus toxicity, which is a problem for most other Grevillea species (Burke, 1983).

From the late 1800s onwards, G. robusta has been planted extensively as high shade for tea and coffee plantations, and this use continues in many countries. The trees are often pollarded to produce a spreading crown, and have a typical working life of 40-50 years before they become senescent and must be replaced (Rao 1961). Shade trees of G. robusta provided effective protection against frost that caused extensive damage to open-grown coffee plants in southern Brazil at planting densities of 71 and 119 trees per hectare, but not at 26, 34 and 48 trees per hectare (Baggio et al., 1997), and the economic productivity of coffee and G. robusta wood at 34, 48 and 71 trees per hectare was greater than that of coffee in unshaded plantations. Only at 119 trees per hectare was there a significant (15%) reduction of total coffee production to age 10 years, relative to unshaded stands.

The sawn timber is of medium strength and is used for furniture, packing cases, flooring, panelling, plywood and the manufacture of small wooden items such as pencils (Bolza and Keating, 1972; Skolmen, 1974). The wood produces short-fibre pulp of acceptable quality (Ghosh, 1972) but has not been used for pulp production on a commercial scale. Poles are used for house construction, scaffolding and rafters in rural areas (Spiers and Stewart, 1992). The branches and twigs are used for firewood and charcoal (Mbuya et al., 1994).

In addition to their use as a soil mulch, the leaves of G. robusta are used by some farmers in the Embu district of Kenya as a fodder supplement for cattle in the dry season when other fodder sources are scarce (Spiers and Stewart, 1992). They are also used as bedding in livestock stalls. The leaves contain a number of useful chemical compounds, in particular rutin which has pharmacological applications (Cannon et al., 1973). However, the rutin concentration of 0.6% of leaf dry weight is too low to be of commercial value. The gum produced from the wood of trees that are stressed or wounded may have commercial value (Anderson and de Pinto, 1982).

G. robusta is vulnerable to fire, which may be used as a means of control. Also, grazing with goats can contribute to the control of G. robusta (PIER, 2002). Herbicides such as triclopyr ester, picloram and glyphosate may be used against G. robusta, with modes of application including basal bark applications, cut surface, frill cut and continuous frill techniques, though it appears to be resistant to 2,4-D and dicamba (PIER, 2002). Smith (1998) reports that biological control has not been investigated on Hawaii because of conflicts with forestry and farming industries.

G. robusta grows well in line plantings and as scattered trees over food and cash crops in warm temperate and subtropical climates. The species is regarded by farmers as being non-competitive with crops, relative to other tree species such as Eucalyptus spp. Recent studies on root architecture and water uptake indicate that G. robusta is relatively deep-rooted and thus may compete less with crop roots than do other trees (Howard et al., 1996). It tolerates repeated heavy pruning and pollarding, enabling farmers to regulate the degree of competition with adjacent crops. It can also be grown in monoculture in block plantings in woodlots or plantations, but this use is much less frequent.

G. robusta is normally planted in rows with spacing of 2-2.5 m between trees and at 2.5 m P 2.5 m in plantations and woodlots (Kalinganire, 1996; Spiers and Stewart, 1992). Planting in rows and at wider spacings favours faster diameter growth and hence individual tree volume. For firewood and pole production it should be planted at close spacing of 1.5 m P 1.5 m and thinned early (Kalinganire, 1996).

Propagation is usually from seed. There are about 40,000 viable seeds/kg. Seed will retain viability for at least five years if dried to below 8% moisture content and stored in a dry, cool (20°C or lower) environment (Jones, 1967; B.V. Gunn, CSIRO Forestry and Forest Products, P.O.Box E4008, Kingston 2604, Australia, personal communication, 1998). No pre-treatment is required for germination. Seeds are usually germinated on loamy soil under a shallow covering of sand.

Seedlings are pricked out when their second leaf-pair starts to develop, into tubes or plastic bags filled with a fertile, loamy potting mix. Seedlings are grown on for around 4-6 months in the nursery until planting out at a height of 20-40 cm during the rainy season. In many countries, demand for seed currently exceeds supply, so farmers also obtain planting stock by digging up wildings. Cuttings can be easily struck using shoots of seedlings or saplings (Swain, 1928), which can also be air-layered. Rajasekaran (1994) reports a successful method of micropropagation through axillary bud activation.

As the species is usually planted in rows or small woodlots, or as scattered individual trees, mechanical site preparation is seldom used. Planting positions are cultivated using agricultural tools such as hoes. Some control of competing vegetation is required for the first 1-2 years after planting. This is normally achieved by several rounds of manual weeding.

The species is not known to form symbiotic associations with soil bacteria or mycorrhizal fungi, although it develops proteoid roots which are believed to enhance nutrient uptake. Fertilizer is seldom applied: 50 g per tree of an NPK fertilizer (12:12:12) applied shortly after planting would be appropriate for infertile soils. If there are symptoms of boron deficiency, an application at planting of 10 g per tree of elemental boron as borax or, preferably, the less soluble Ulexite (a sodium-calcium borate mineral) is recommended.

While it occurs naturally along riverbanks and will survive short periods of flooding by moving water, the species will not grow on poorly-drained, swampy sites where waterlogging occurs for more than a few days at a time. G. robusta performs poorly on heavy clays.

When planted in woodlots and line plantings, thinning of inferior trees is often carried out at age of around 4-5 years to yield poles and firewood for local use or sale. Farmers in the African highlands commonly harvest branches by high pruning and pollarding every 3-4 years from age 4-6 years onwards (Poulsen, 1983; Spiers and Stewart, 1992). Some African farmers prune the surface roots of G. robusta by digging with a hoe at a distance of around 30 cm around the trunks of trees growing in and around fields, to reduce competition with adjacent crops.

When grown at close spacings in plantations and woodlots the growth rate of the species is relatively modest. For example, the estimated mean annual wood yield in tropical highlands is only some 10-12 m3/ha over 10-15-year rotations at recommended stocking of 800-1200 trees/ha (Pandey, 1987). G. robusta is therefore not a high priority plantation timber species.

When it is planted singly or in lines, annual growth rates of 2 m (height) and 2 cm (diameter) over the first 5 years are commonly achieved in a number of countries where climate and soils are suitable. A survey of many plantings in Rwanda found that the mean annual height increment in young plantations was 1.1-3.1 m and mean annual dbh increment was 0.9-3.4 cm (Kalinganire, 1996). The mean annual increment for height of trees grown on farms was 1.3-2.8 m, and mean annual dbh increment was 1.1-3.7 cm, declining with increasing age of the trees. Growth was slower at high (>2300 m) altitudes. Clay loams and volcanic soils gave better diameter and volume production. Spacing influenced diameter growth and tree volume production, but height growth was not affected by spacing.

Proceedings of an international workshop (Harwood, 1992b) describe the use of G. robusta in agroforestry and forestry applications. The branches are pruned to regulate shading and competition with adjacent crops and are used for firewood, sticks for climbing beans and poles, and the tree grows back well after heavy pollarding and pruning. The main trunk of the tree may be harvested as a sawlog from age 15-25 up to 40 years. G. robusta coppices well after being cut back to ground level at ages of up to two years, but coppicing ability declines sharply thereafter, so management on a coppicing rotation is not feasible.

Provenances

In two field trials in Rwanda (Kalinganire and Hall, 1993), the natural provenances Benarkin, Imbil and Glenbar from low elevations in Queensland grew faster than seedlots collected from plantations in Australia and Rwanda. In a provenance-progeny trial at Malava, Western Kenya, high-elevation natural provenances from Porters Gap and the Bunya Mountains in Queensland displayed poor growth relative to other natural provenances from lower elevations in Queensland and New South Wales. In this trial, significant differences between provenances in branch size and branch angle were also evident (F. Owino and F. Esugu, ICRAF, United Nations Avenue, Gigiri, P.O. Box 30677, Nairobi, personal communication, 1996). Significant differences between provenances were also found in growth and morphological traits in two provenance trials on the Atherton Tablelands in north Queensland. Duck Creek and Tyalgum provenances from lowland New South Wales displayed the best growth in these trials (Sun et al., 1995).

Advanced genetic improvement programs have not yet been implemented. Techniques for controlled pollination (Kalinganire et al., 1996) and vegetative propagation are relatively straightforward and could be used in improvement strategies (Harwood and Owino, 1992). No records of hybrids between G. robusta and other species of Grevillea have been published.

Genetic resources

Isozyme studies of natural populations and land races (Harwood et al., 1997) established that the species has a moderate level of allozyme heterozygosity in natural populations. The genetic diversity of individual populations varied little, and did not appear to be related to their ecological characteristics (araucarian vine forest or riverine habitat types) or their geographic locations. Between-population differences accounted for 17.9% of the total genetic variation. It appears that genetic exchange between populations has been maintained, despite the species' pattern of natural distribution in small, separated populations. African land races of G. robusta are descended from initial introductions in the late nineteenth and early twentieth centuries and have developed in different countries in partial or total isolation from one another. Mean heterozygosity and other genetic diversity measures were substantially lower for the African land races than for the natural populations. The complete absence of a common allele in the two Madagascar populations suggested that these populations were derived from a very narrow genetic base.

Provenance-progeny trials testing seed collections from across the natural range have recently been established in several countries including Australia, Kenya and Rwanda. These trials provide a good genetic base for breeding programs.

Reproductive biology

Under conditions favourable for good growth of G. robusta in the tropical highlands, flowering and seed production commence at age 2-3 years (Kalinganire et al., 1996). In subtropical and warm temperate latitudes, first flowering may occur at 4-6 years (Swain 1928). Mature trees are heavy seed producers. The breeding system in natural populations was found to be highly outcrossing in isozyme studies of progeny arrays (Harwood et al., 1992). Multi-locus estimates of outcrossing rates of 0.97 and 0.86 were obtained for two natural populations. Observations and experimental studies in a planted stand in Western Kenya (Kalinganire et al., 1996) confirmed that the species is fully outcrossing and self-incompatible, and that nectivorous birds are the principal pollinating agents in Kenya, as was found in Australia (Brough, 1933). The period from the start of inflorescence development to anthesis is about 40 days. Stigmas become receptive 1 day after anthesis, and the period from fertilization to fruit maturity is about 60 days. Hot dry weather stimulates seed release once the fruits are mature.

Webb et al. (1967) considered that an auto-allelopathic compound associated with the living roots of G. robusta was responsible for the poor performance of plantations in Australia. The good growth of many successful plantations and woodlots in Africa suggests that in most plantations such an effect is minor, if present at all. G. robusta is an effective colonizing species and, in some cases, threatens to be a noxious weed (e.g. in Hawaii, Nelson and Schubert, 1976).

Its use as tea shade has been largely discontinued in Kenya and Rwanda because of the risk of Armillaria and other root pathogens spreading from dead G. robusta roots to those of the tea plants (Tea Research Institute of East Africa, 1969). The trees have brittle branches and can be damaged by high winds. The wood from relatively young, fast-growing woodlots has some notable drawbacks: the sapwood is very susceptible to attack by borers and fungi, and even the heartwood is only moderately durable, and the wood is light. The sawdust from G. robusta wood causes a skin allergy in some people of Caucasian descent, particularly those with fair hair and light skin colouring (Skolmen, 1974).