| Agroforestry in the Pacific Islands: Systems for Sustainability (1993) |
|5 Agroforestry in Polynesia|
Tongatapu island, Tonga
Tongatapu, the largest and most populous island of the Tongan group, is an upraised limestone island, approximately 257 sq km in area, lying just within the tropics. Its mild and moist climate, with a mean annual temperature of 23°C, a seasonal range of only 4°C, and a mean annual rainfall of 189 cm, provides a favourable agricultural environment. The soils are fertile, deep friable clays of prehistoric volcanic origin that overlay the upraised coral-limestone base of the island. Little soil erosion occurs on the mostly flat surface, the highest elevation being only 83 metres above sealevel. Major constraints to agriculture include the absence of surface streams or surface water, undependability of rainfall, and frequent tropical cyclones and galeforce winds (Thaman 1976).
Owing to population pressure (over 251 per sq km in 1986) and over 3,000 years of human occupance (Groube 1971), very little primary vegetation remains. Vegetation communities currently that exist include: coastal littoral forest, coastal savanna, swamp forest, marshland, mangrove swamps, and isolated stands of tropical lowland forest, plus the highly modified vegetation communities and fallow and ruderal vegetation found in active agricultural areas and villages and towns.
Agricultural land use
A bush-fallow agroforestry system is practiced on some 5,652 bush allotments ('apt 'uta) averaging about 3.2 ha (8.25 acres by law) in area, to which individual farmers or farm families have individual tenure rights. The characteristic vegetation on Tongatapu bush allotments can be best described as a dynamic mosaic of individualized allotments, each containing a diversity of subsistence and cash ground crops and secondary vegetation in various stages of regrowth, spread throughout a matrix of coconut palms and a wide variety of useful tree species. Also found on many allotments, and considered integral and useful components, are small protected areas of old fallow forest, coastal strand forest, or mangrove forest. Most families also have an additional town allotment ('apt kolo), ranging from 0.1 to 0.16 ha (0.250.4 acre), located in town reserves, where they maintain their permanent residences, a home garden, and associated structures. Some families, however, have permanent residences on their larger bush allotments, with many families maintaining temporary thatched shelters or small houses there for use when needed (Thaman 1975, 1976).
The cropping cycle begins with the clearing of secondary vegetation, including many pioneer tree species, while protecting (although sometimes pruning or pollarding) useful species, the most common being coconut, breadfruit, mango, citrus, Tahitian chestnut (Inocarpus fagifer), oceanic Iychee (Pometia pinnatae), candlenut (Aleurites moluccana), and koka (Bischofia javanica). The dried plant remains are then usually burned, and the first crop, often yams (Dioscorea alata), is planted, and usually intercropped with plantains (Musa AAB Group), giant taro (Alocasia macrorrhiza), or other crops. After the yams are harvested, true taro (Colocasia esculenta) and tannia (Xanthosoma sagittifolium) or cassava are generally the next crops in the succession. Other crops, including sweet potatoes, bananas, peanuts, paper mulberry (Broussonetia papyrifera), or Lava (Piper methysticum), are also integrated into the cropping sequence and may be planted at any time during the sequence. If taro has been planted, cassava often follows and is sometimes planted up to four or five times consecutively on a given plot. Then, after the cassava is harvested, the land will be allowed to revert to fallow, although some cassava is commonly left as a component in the tumble-down fallow vegetation (Thaman 1975, 1976).
In a sample survey of 101 bush allotments comprising 326 ha, conducted in 1971-1972, approximately 111 ha, or 34 per cent of the area, was under cultivation, with the balance under various stages of fallow regrowth (Thaman 1976). There were 87 allotments with some cultivation and 14 that were not cropped at all. With respect to cultivated land, it was often difficult to differentiate between subsistence and commercial cropping, because crops such as coconuts, bananas, water melons, pineapples, garden vegetables, and peanuts, which are commonly grown for export or local sale, are also used for home consumption. Conversely, many important subsistence crops such as cassava, yams, sweet potato, tannia, and plantains are also produced commercially. On this basis, 97 of the 111 ha under cultivation, or 86.5 per cent, constituted subsistence cropping; 12 ha, or 10.7 per cent, commercial cropping; and 3 ha, or 2.7 per cent, a combination of the two. Of the 87 allotments under some form of cultivation, all had subsistence crops, whereas only 31 had "commercial" crops (Thaman 1975, 1976).
Of the eight species of staple subsistence ground crops, cassava covered 37 per cent of the area, tannia (Xanthosoma taro) 24 per cent, yams 21 per cent, sweet potatoes and taro 3 per cent each, a combination of tannia and Colocasia taro and plantains 2 per cent each, and sweet yam (Dioscorea esculenta) and giant taro (Alocasia macrorrhiza) 1 per cent each. These figures represent areas for these crops as dominants on a given plot and, in the cases of plantains and giant taro (which are commonly intercropped with yams and infrequently planted alone), their relative importance was greater than these figures indicate. Similarly, the relative importance of sweet potato is probably understated because of the time-period selected for the inventory.
A range of supplementary food crops also plays an important role in the Tongan diet. Because most supplementary food plants are either intercropped with staple crops or protected in the fallow vegetation, their frequency of occurrence was surveyed rather than the area they occupied. Of these plants, the papaya was the most widespread, occurring in either a cultivated or semiwild state on 88 per cent of the sample allotments. In only three cases was it planted in rows resembling a plantation. Supplementary food plants present on over 50 per cent of the sample allotments included Cordyline fruticosa, chill) peppers (Capsicum frutescens), and bananas (Musa AAA Group) (planted as isolated or small groups of plants). Plantains (Musa AAB Group), hibiscus spinach (Hibiscus manihot), pineapple, sugar cane, and pata variety bananas (Musa ABB Group) were found present on at least 25 per cent; wild or cherry tomatoes (Lycopersicon esculentum var. cerasiforme), pumpkin, kava (Piper methysticum), and corn on at least 10 per cent; with the remaining supplementary food plants found on the bush allotments (which bring the total to 18) including green or bunching onions (Allium spp.), coffee, lemon grass (Cymbopogon citratus), Amorphophallus campanulatus, and granadilla (Passiflora quadrangularis).
Throughout the matrix of staple and supplementary food crops and secondary vegetation are a number of important non-food plants. The most common non-food plants, which are both cultivated and protected in a natural state, are a number of species or cultivars of pandanus (Pandanus spp). These provide fibre for baskets, mats, and other handicraft items and were present on 76 per cent of all allotments. On four of these allotments, solid plantings averaging about 0.1 ha were found. Second in frequency to pandanus, but occupying considerably more area, was paper mulberry (Broussonetia papyrifera) (used for making tape cloth), which was planted on 38 per cent of the sample allotments and comprised about 3 per cent of the subsistence area. Other plants useful for construction, wrapping food, decorative purposes, handicrafts, scenting oil, adhesives, and medicinal purposes included bamboo (Schizostachyum glaucifolium and Bambusa vulgaris), Heliconia indica, cycad (Cycas circinalis), Job's tears (Coix lachryma-jobi), members of the ginger family (Zingiber zerumbet and Amomum cevanga), sword grass (Miscanthus foridulus), Polynesian arrowroot (Tacca leontopetaloides), maile (Alyxia stellata), cotton (Gossypium barbadense), tobacco, tumeric (Curcuma longa), and 'uhf (Euodia hortensis).
Of the non-tree crops classified as commercial - although many of them are also of subsistence value - bananas (Musa AAA Group) covered 42 per cent of the area, water melons 17 per cent, plantains (Musa AAB Group) 13 per cent, pineapples 11 per cent, tomatoes and sweet capsicum, or bell pepper (Capsicum annuum var. grossum), both 6 per cent, and peanuts 4 per cent. The balance of the area was made up by rock melons (Cucumis melo var.), corn, and vanilla (Vanilla fragrans), all in contiguous plots. Subsequent to the survey, there has been an expansion of monocropping of butter squash (Cucurbita sp.) for export to Japan. All of these crops, with the possible exception of vanilla, also have subsistence value that, in some cases, may be greater than their commercial value. Bananas in particular, which are cooked in a green state, were, in 1971, one of the major staple foods, together with cassava, American taro, and yams (Thaman 1976).
Trees and agroforestry practices
Although the major staple and commercial crops dominate the cultivated area of bush allotments, the most prominent plants are trees. The coconut palm, which was present on over 98 of the 101 surveyed bush allotments, was by far the most common and most important cash crop on Tongatapu, as well as being the most frequently consumed subsistence crop. Over half of the allotments had over 200 mature palms, whereas some had 500 or more, the median coverage being about 250 palms. Although 56 allotments also had immature replanted coconuts, in only a few instances were they wellmaintained (Thaman 1976, 257-258).
In addition to coconut palms, there were at least 25 other fruit trees among the plant associations. Trees, primarily important because of their edible fruit, present on over half of all sample allotments, included mango (Mangifera indica), citrus (including Citrus sinensis, C. reticulate, C. hystrix, C. aurantium, and C. aurantiifolia), oceanic Iychee (Pometia pinnata), breadfruit (Artocarpus altilis), Tahitian chestnut (Inocarpus fagifer), and guava (Psidium gunjava). Other common species included avocado (Persea americana), beach almond (Terminalia catappa), soursop (Annona muricata), Polynesian vi-apple (Spondias dulcis), and Malay apple (Syzygium malaccense). Other species such as sugar apple, or sweetsop (Annona squamosa), rose-apple (Syzygium jumbos), cocoa (Theobroma cacao), cashew nut (Anacardium occidentals), the common fig (Ficus carica), apple (Malus pumila), and peach (Prunus persica) were also found as isolated individuals on allotments on some occasions.
Of even greater importance in terms of numbers and frequency of occurrence are at least 67 other non-food tree species, all useful in some way to Tongan society. They are generally found in a protected state, although sometimes deliberately planted. Apart from the value of their timber, their fruit, seeds, leaves, bark, wood, and roots are used in the preparation of medicines, dyes, handicrafts, cordage, scent for coconut oil, soap substitutes, handicrafts (including wood carving), canoe construction, and other material goods. The most common trees in this category include Bischofia javanica, Aleurites moluccana, Hibiscus tiliaceus, Morinda citrifolia, Macaranga harveyana, Rhus taitensis, Grewia crenata, Erythrina variegate, Glochidion spp., Pandanus tectorius, Ceiba pentandra, Santalum yasi; Cananga odorata, Pittosporum arborescens, Pisonia grandis, Elatto stachysfalcata, Garciniasessilis, Adenantherapavonina, Mertyamacrophylla, Ficus scabra, Micromelum minutum, and bamboos (Schizostachyam glaucifolium and Bambusa vulgaris), all of which were found present on over 10 per cent of all allotments.
Other species of widespread agroforestry importance throughout the Pacific, found on over S per cent of the sample allotments and which, in some cases, constitute endangered species, include Cerbera manghas, Diospyros laterifolia, Jatropha curcas, Ervatamia orientalis, Grevillea robusta, Syzygium corynocarpum, Xylosma orbiculatum, Plumeria rubra, Syzygium clusiaefolium, Neisosperma oppositifolium, Trema orientalis, and Dysoxylum forsteri. Other species of particular cultural significance or utility, found on less than 5 per cent of allotments, some of which must be considered endangered species, include Garuga floribunda, Parinari glaberrima, Fagraea berteriana, Diospyros ferra, Ficus obliqua, Calophyllum inophyllum, Myristica hypargyraea, Pimenta racemosa, Burkella richii, Casuarina equisetifolia, Tarenna sambucina, Homolanthus nutans, Aglaia saltatorum, Syzygium neurocalyx, and Premna serratifolia.
Most of the tree species found on bush allotments are native to Tonga or are aboriginal introductions, although recent introductions such as Leucaena leucocephala, kapok (Ceiba pentandra), Jatropha curcas, and frangipani (Plumeria rubra) were also common. There are also a number of species introduced by the government as part of reforestation schemes; these include the silky oak (Grevillea robusta), blue gum (Eucalyptus spp.), and cigar box cedar (Cedrela odorata), which have been planted, either as small woodlots, border plantings, or scattered individual trees, on some allotments.
Fallow and natural vegetation
Another dominant vegetational feature on bush allotments was the fallow or secondary vegetation, which has a very important role in the cropping succession. Approximately two-thirds of the combined area of all sample bush allotments were under some stage of bushfallow or natural vegetation, with some allotments (14 per cent of the sample) completely under various stages of fallow vegetation. In no case was an individual allotment under 100 per cent cultivation. There were, however, very few areas under fallow for sufficient time for forest to regenerate. Only 9 per cent of the uncultivated area had been under fallow over 10 years, the majority of which comprised uncultivable stands of coastal strand or swamp forest. Another 21 per cent had been under fallow for approximately 5-10 years, with the remaining 70 per cent having been under fallow for less than 5 years and being characterized by bushes, grasses, and ephemeral pioneer species, most of postEuropean-contact introduction, and few trees. Extensive areas of fallow vegetation, especially in the eastern, or Hakake, district, were under almost monospecific stands of Leucaena leucocephala, which spread so fast and is so difficult to clear that people considered it a weed (Kunzel 1989, 41), while recognizing the tree's value as an enhancer of soil fertility and a major source of saleable firewood.
Similarly, many of the other fallow species (which number more than 60) are useful as sources of food, medicines, firewood, fodder, fish poisons, and other valuable products; but some, such as Cyperus rotundas, Commelina spp., and Psidium gunjava, spread so rapidly as noxious weeds that they have become the focus of control programmes.
As suggested above, in addition to the cultivated and protected plant species and fallow vegetation, a number of allotments also had uncultivable areas under "natural" vegetation. These allotments were usually found along the coast and included areas of littoral or coastal strand forest (on the higher windward southern portions of Tongatapu) and swamp or mangrove forest (on the low-lying leeward northern coast). Although of limited agricultural potential, such areas constitute sources of many useful products, and embody gene pools of species rapidly disappearing from a majority of Tongatapu bush allotments, as well as serving as buffer zones protecting crops and soils from sea spray and salt-water incursion.