Ornamental species make up the majority of recently naturalized plants in the developed world in countries as contrasting as Japan and Australia. At the present time in developing countries, ornamental species are becoming an increasing threat to agriculture and natural ecosystems. This particularly applies to aquatic plants, vines, lianas, shrubs and trees. In addition, species introduced for one specific purpose, such as forestry, may invade other habitats and become weeds. More than any other type of plant, exotic trees have the potential to change the face of landscapes around the world. This is one such tree.
African tulip tree, Spathodea campanulata Beauv. is a native of equatorial Africa and a member of the family Bignoniaceae that contains a number of other woody ornamental plants such as Jacaranda and Tecomaria. It is also known as Fountain Tree, Indian Cedar or Santo Domingo Mahogany. It is a tall tree reaching up to 30 m in some habitats. It has attractive, dark green, compound leaves and spectacular broadly trumpet-shaped, orange-red flowers in terminal clusters. It has been widely introduced throughout tropical and subtropical regions of the world as an ornamental and street tree. In subtropical areas it is often deciduous and is benign, but in some tropical areas such as the Pacific islands it has flourished and spread.
African tulip tree has escaped cultivation and invaded agricultural land, forest plantations and natural ecosystems in the Cook Islands, Fiji, Guam, Hawaii, Samoa and Vanuatu. In Fiji it is particularly troublesome. Originally introduced in the 1930s as a street tree and household ornamental, it has been recognized as an increasing problem for more than 12 years. In the last five years the problem has intensified.
Fiji consists of some 300 islands situated between the equator and the Tropic of Capricorn. It has a land area of 18 300 sq km although its territorial limits cover 1.3 million sq km. Over 90 percent of the population of 800 000 inhabits the two largest islands, Viti Levu and Vanua Levu. Agriculture is the largest sector of the economy. Crops include sugarcane, coconut, ginger, rice, vanilla and a range of fruit and vegetables, and cattle grazing is conducted for meat and milk production. However only about 16 percent of the land area of Fiji is suitable for agriculture and this puts significant pressure on these areas and can also result in clearing of land unsuitable for cultivation. Traditional shifting cultivation farming is still practised in south-east Viti Levu and this is particularly susceptible to invasion by weeds, especially African tulip tree.
Secondary regrowth forests previously cleared for agriculture make up more than 20 percent of the forest cover in Fiji. The predominant tree in these forests is now African tulip tree. In addition, the species is also a weed of intensive cultivation. It is difficult to remove from land because of regrowth from broken root pieces and reinfestation from seedlings. In a recent survey in three provinces, African tulip tree was found on 98 percent of farms visited and on many farms, the tree occurred on over 25 percent of the land area.
As well as its impact on agriculture and forestry, African tulip tree represents a threat to the biodiversity of the Fijian islands. The tree is naturalized in both the western (drier) and eastern (wetter) sides of the main islands. Lowland and upland rainforests as well as dry forests are all susceptible to invasion by this tree. Its high sexual reproductive capacity and windborne seeds provide it with the means to colonize disturbed areas at the expense of native species. These disturbed areas may be created without the intervention of humans by tropical storms and cyclones. Once established in an area, the tree is capable of vigorous vegetative growth and it can reach and exceed the height of the native flora and shade it. The dominant tree species of the upland Fijian rainforests, Palaquium hornei grows to about 20-25 m and in lowland forests, the tallest trees, Palaquium stehlinii, Myristica castaniifolia, Calophyllum vitiense and Garcinia myrtifolia reach about 25 m. Replacement of native tree species by African tulip tree would have a profound effect on the biodiversity of forests as many native species carry an extensive tree-dependent flora of epiphytes and vines.
HOW HAS AFRICAN TULIP TREE BECOME A PROBLEM?
In Fiji, African tulip tree flowers and sets seeds over several months. Individual trees produce hundreds of fruit capsules (pods) (approx. 22 cm × 5 cm), each containing many hundreds of seeds. The seeds are elliptical and broadly winged (approx. 2.5 cm × 1.5 cm), very light and easily windborne. Released from a great height these propagules are capable of widespread dispersion especially in windy weather. The pods can also float and facilitate invasion of riparian areas.
The tree readily suckers from roots and regenerates from broken root pieces. The wood of the tree is soft and branches may be easily broken off in storms. In the moist tropical environment of Fiji, it can then sucker from detached branches lying on the soil surface. It is shade-tolerant, so once established in an area it is difficult to control.
The tree has been widely planted in towns and around more remote villages for many years. In addition, woody stems of African tulip tree have been used as living fence posts around farms. These have often become trees again and produced seeds.
Hence there is a huge and widespread source of seeds and vegetative propagules throughout the islands to continue to invade agricultural lands, forest plantations and natural ecosystems. Moreover there do not appear to be any natural enemies of the tree in the Pacific.
MANAGING THE PROBLEM
Farmers who practise shifting cultivation and others who use more intensive production methods all have problems in dealing with African tulip tree. Recent research by the Fijian Governments Ministry of Agriculture Fisheries and Forests (MAFF) has provided chemical control recommendations using 2,4-D+dicamba for ring bark or cut-stump application to trees and spray treatment of smaller plants. However, chemical control alone is not sufficient or appropriate for such a widespread and complex problem. A multifaceted approach is needed.
Initially, a community awareness programme is required, involving both farmers and the wider community, warning people of the threat of this plant and advising them to stop planting it as an ornamental and as living fence posts. This should include advice on removing young trees before they flower, with visual aids depicting seedlings, to allow their early recognition and manual removal.
Unlike the adult plant, young seedlings do not have compound leaves. Cotyledons have a characteristic kidney shape and the following leaves are simple, in opposite pairs, with alternate pairs at right angles to each other. After the first six pairs of leaves, compound leaves begin to form.
Any further work on integrated control of the species would require more knowledge of its ecology, including the following:
period over which seeds germinate;
degree of shade tolerance of seedlings;
age at which plants first flower;
period over which suckers form;
ages and minimum sizes of root and stem pieces required to form new individuals;
age of suckers when first flowering;
period over which plants flower; and longevity of the seed bank.
With this information, management programmes tailored to specific farming and forestry systems could be devised and implemented.
Large trees in inaccessible areas will continue to be a source of reinfestation unless controlled. Classical biological control is a possible option. Seed or flower-attacking organisms could reduce the impact of these trees. Preliminary surveys in Africa discovered an eriophyid mite that attacks leaves of the species causing galls and eventually die-back of the stem.
This raises the possibility of conflict of interests arising in the community. Many mature African tulip trees are part of the flora of parks and gardens in the capital, Suva and in other towns. Many townspeople may not like to see the spectacular flowers and leaves of these trees disfigured. Clearly, a consensus among the community would need to be reached before any irreversible steps were taken to control all African tulip trees.