54 Plant Protection Quarterly Vol.16(2) 2000 weeds 20. Mimosa pigra L. Plant Protec- tion Quarterly 4, 119-31. McCullagh, P. and Nelder, J.A. (1983). Environmental weeds of Christmas Island (Indian ‘Generalized linear models’, 1st edi- Ocean) and their management tion. (Chapman and Hall, London).

Miller I.L., and Lonsdale, W.M. (1987). A B Early records of Mimosa pigra in the J.T. Swarbrick and R. Hart A Northern Territory. Plant Protection Weed Science Consultancy, Toowoomba, Queensland Quarterly 2, 140-2. B Parks Australia North, Christmas Island Miller, I.L. and Lonsdale, W.M. (1992). Ecological management of Mimosa pigra: use of fire and competitive pas- Abstract tures. In ‘A guide to the management of The environmental weeds of Christmas which time it was covered by primary rain Mimosa pigra’, ed. K.L.S Harley, pp. Island (Indian Ocean) are mostly exotic forest on the deeper soils of the plateau 104-6. (CSIRO, Canberra). tropical rainforest trees, shrubs and vines and by marginal rainforest on the shal- Miller, I.L., Napompeth, B., Forno, I.W. that persist in areas that have been reha- lower soils of the cliffs and terraces. The and Siriworakul, M. (1992). Strategies bilitated after being mined for phosphate area has a strongly monsoonal tropical cli- for the integrated management of Mi- but which now form part of the Christmas mate, dominated by southeast trade mosa pigra. In ‘A guide to the manage- Island National Park. Other environmen- winds throughout much of the year. ment of Mimosa pigra’, ed. K.L.S Harley, tal weeds occur within the rainforests, Soon after settlement several areas pp. 110-14. (CSIRO, Canberra). along rainforest margins and in other situ- were cleared and planted with potential Miller I.L., Nemestothy S.E., and Pick- ations throughout the island. rubber bearing trees, but the industry was ering S.E., (1981). ‘Mimosa pigra in the The major environmental weeds of not pursued and most of these areas were Northern Territory’. Department of Pri- Christmas Island are Adenanthera pavonia, subsequently destroyed by mining. About mary Production, Division of Agricul- Aleurites moluccana. Castilla elastica, 25% of the original vegetation has since ture and Stock, Technical Bulletin No. Clausena excavata, Cordia curassavica, been cleared for phosphate mining and 51, pp. 1-22. Delonix regia, Ficus elastica, Hevea other purposes. The post-mining land- Miller, I.L. and Siriworakul, M. (1992). brasiliensis, Mikania micrantha, Mucuna scape consists of tall coralline pinnacles Herbicide research and recommenda- albertisii, Pithecellobium dulce, Pterocarpus and deep pits, and is initially almost ster- tions for control of Mimosa pigra. In ‘A indicus, Schefflera actinophylla, Spathodea ile biologically. guide to the management of Mimosa campanulata and Tecoma stans. Minor envi- Some of the exhausted mine fields have pigra’, ed. K.L.S Harley, pp. 86-91. ronmental weeds of the island include been rehabilitated by bulldozing the pin- (CSIRO, Canberra). Antigonon leptopus, Barringtonia asiatica, nacles and resoiling with phosphate rich Sessional Committee on the Environment Ceiba pentandra, Imperata cylindrica, subsoil. The surfaces were then ripped (1997). Report of inquiry into matters Leucaena leucocephala, Muntingia calabura, and planted with a range of exotic trees, relating to the occurrence, spread, im- Nephrolepis biserrata, Nephrolepis multiflora, shrubs and forage legumes. pact and future management of Mi- Pluchea indica, Psidium guajava, Ricinus Mining, rehabilitation, clearing for mosa pigra in the Northern Territory, communis, Senna sulfurea and Syzigium other purposes and natural regeneration pp. 60. (Legislative Assembly of the spp. have left a complex mosaic of vegetation Northern Territory, Darwin). An integrated system of environmental types on Christmas Island. Primary rain- Siriworakul, M. and Schultz, G.C. (1992). weed control is being developed on the is- forest is still dominant on the deeper soils Physical and mechanical control of Mi- land. It includes strict quarantine, the of the plateau, as is marginal rainforest on mosa pigra. In ‘A guide to the manage- eradication of major weeds of limited dis- the shallower soils of the cliffs and ter- ment of Mimosa pigra’, ed. K.L.S Harley, tribution, the development of local legisla- races. Extensive rainforest margins exist pp. 102-3. (CSIRO, Canberra). tion to prohibit the growing of certain along linear disturbances such as roads plants, the encouragement of competition and railway lines, as well as around the by native rainforest plants, limited physi- edges of abandoned and rehabilitated cal and biological control, and progressive mine fields. chemical control of the remaining weeds Much of the island is now national followed by rehabilitation with native park, including much of the remaining rainforest seedlings. rainforest and several partly or wholly re- habilitated mine fields. Largely as a result Introduction of this change in ownership and manage- Christmas Island is a small (135 km2) iso- ment the exotic trees within the rehabili- lated Australian Overseas Territory in the tated areas of the national park are now Indian Ocean, about 400 km south of the perceived to be environmental weeds western tip of Java. The island arose from (Swarbrick 1997). Parks Australia North is the depths of the Indian Ocean, and con- actively removing competitive and inva- sists of a basaltic core overlaid by lime- sive environmental weeds and replacing stone shelves originating from raised coral them with native rainforest trees. reefs. It has an undulating summit plateau This paper lists and discusses the actual 300–360 m high, surrounded by cliffs and and potential environmental weeds of terraces. The island has never been part of rainforests, rainforest margins, rehabili- a larger land mass, and its flora is there- tated areas and other parts of Christmas fore restricted and includes many Island (Table 1), and considers their inte- endemics (Australian Biological Re- grated control and management. sources Study 1993). Christmas Island was uninhabited Environmental weeds of rainforests prior to European settlement in 1888, at Relatively undisturbed primary rainforest Plant Protection Quarterly Vol.16(2) 2000 55 Table 1. Environmental weeds and other plants of conservation significance surrounding disturbed rainforests. Its on Christmas Island. ability to germinate and grow as a epi- phyte is of concern, since it may germinate Adenanthera pavonia Rehabilitation areas, widespread in the inaccessible crowns of the native Aleurites moluccana Rehabilitation areas, widespread palms. S. campanulata is present in many Antigonon leptopus Sea cliffs and rehabilitation areas, widespread rehabilitation areas, from whence the Barringtonia asiatica Rehabilitation areas, widespread wind blows their seeds long distances. Castilla elastica Rainforest, localized Whilst seedlings are unlikely to survive Ceiba pentandra Rehabilitation areas, localized under intact primary rainforest, they rap- Clausena excavata Rainforest and margins, widespread idly become established in light gaps and Cordia curassavica Rehabilitation areas, widespread may invade the more open marginal rain- Delonix regia Rehabilitation areas, widespread forests. Ficus elastica Rainforest, localized Hevea brasiliensis Rainforest, localized Environmental weeds of rainforest Imperata cylindrica Roadsides, localized margins Leucaena leucocephala Rehabilitation areas and roadsides, widespread Clausena excavata is widely established Mikania micrantha Rainforest, widespread along rainforest margins, but also occurs A Mucuna albertisii Rainforest and margins, localized throughout disturbed rainforests in the Muntingia calabura Rainforest, widespread northeast of the island. It is a small rap- Nephrolepis biserrata Rainforest margins and rehabilitation areas, widespread idly growing tree which thrives in me- Nephrolepis multiflora Rainforest margins and rehabilitation areas, widespread dium shade, and there are many kilome- Pithecellobium dulce Rehabilitation areas, widespread tres of mine field edges and roadsides Pluchea indica Sea cliffs, localized throughout the island at risk from this Psidium guajava Roadsides, widespread weed. Mikania micrantha is a rapidly grow- Pterocarpus indicus Rehabilitation areas, widespread ing perennial herbaceous vine, which also Schefflera actinophylla Rainforest, localized grows best in disturbed well lit forest mar- Senna sulfurea Rainforest, localized gins. It is widely established along road Spathodea campanulata Rainforest and margins, localized and track sides in the centre of the island, Syzigium spp. Rehabilitation areas, widespread and since its small light seeds are distrib- Tecoma stans Rainforest margins and rehabilitation areas, widespread uted in the turbulence behind the large A Controlled and perhaps eliminated. trucks used to haul phosphate from stock- piles to the driers it is likely to become more widespread. Mucuna albertisii still covers much of the Christmas Island rapidly spreading through rainforests on F.Muell. (D’Albert’s creeper) is a rampant plateau. It consists of evergreen trees, Christmas Island, particularly the terrace liane from Papua New Guinea which palms and vines 30–40 m tall with forest in the northeast corner of the island. dominated about a kilometre of forest emergents to 50 m. Trees in the marginal Muntingia calabura was introduced for its margin at one place on the island, where it rainforests on the slopes and terraces are edible fruit, and is widely planted as a formed climber towers and smothered ad- shorter (20–30 m) and more open and nurse crop for rainforest rehabilitation. jacent rainforest and regeneration. M. these areas contain many semi-deciduous The minute seeds are distributed by birds albertisii was introduced between 1935 and trees, making them more liable to invasion but only germinate in light gaps, where 1945, possibly in association with the by exotics. The floor of both types of forest the seedlings are unlikely to survive later Japanese occupation of the island in 1943. is closely scavenged by herbivorous crabs, competition for light from rainforest trees. It has recently been controlled and hope- which eat plant litter and most tree seed- The ornamental tree Senna sulfurea fully eliminated from the island through lings. Both primary and marginal rainfor- (Collad.) Irwin & Barneby has been col- the activities of a Green Corps program. est suffer from damage and disturbance lected at one location along the northeast Although the two sword ferns caused by falling trees, especially during terrace, but it is not known how or when it Nephrolepis multiflora (Roxb.) F.M.Jarrett storm events and cyclonic activity in the arrived on the island. ex C.V.Morton and N. biserrata (Sw.) wet season. Potential environmental weeds of rain- Schott are native to the island and are pri- Serious environmental weeds of rain- forests on the island include Ficus elastica mary colonizers of resoiled mine fields, forests on Christmas Island include the Roxb. ex Hornem. (India rubber), Mikania they cause problems. They compete two planted latex bearing trees Castilla micrantha Kunth (mikania), Schefflera strongly with native rainforest seedlings, elastica Sessé (Panama rubber) and Hevea actinophylla (Endl.) Harms (Queensland and in the dry season their dense foliage brasiliensis (Willd. ex Juss.) Muell. Arg. umbrella tree) and Spathodea campanulata dries out and becomes fire-prone. The re- (para rubber), seedlings of which are con- P. Beauv. (African tulip tree). F. elastica sulting fires have a deleterious effect on fined to a small area around parent trees will only become a problem if it flowers rainforest expansion and regeneration. in the remaining plantation. The seeds of and the specific wasp necessary for polli- Rehabilitation plantings intended to re- C. elastica are being spread by birds for up nation is also present on the island. If this store rainforest are managed to ensure to 0.5 km around the original trees, whilst occurs birds would spread the seeds to the rapid canopy establishment and shading the explosive fruits of H. brasiliensis are canopies of rainforest trees, from which it of the soil within a few years, providing less effective dispersal agents. Clausena would be impossible to remove the seed- conditions unfavourable for fern growth. excavata Burm.f. may have been intro- lings. The wind-blown seeds of M. Spathodea campanulata occurs along duced in error instead of the similar micrantha are unlikely to germinate or sur- many rainforest margins on the island, es- Murraya koenigii (L.) Spreng. (curry leaf), vive under the dense vegetation of pri- pecially near rehabilitation areas into and is now very widespread in regenerat- mary rainforest except along the margins, which they were previously planted. ing rainforest in the northeast and in one but may invade the more open marginal Tecoma stans (L.) Juss. ex Kunth (yellow area near the centre of the island. Its seeds rainforests in the future. S. actinophylla is bells) is a small shrub with a prolific pro- are distributed by birds, and the land being grown in the urban area, from duction of wind-dispersed seeds. It is crabs ignore its seedlings. C. excavata is whence birds are spreading its seeds into abundant along the margins of rainforests 56 Plant Protection Quarterly Vol.16(2) 2000 where they are cut through by roads and Mikania micrantha is present in a Leucaena leucocephala is a very serious in- tracks, since its seeds too are dispersed by number of rehabilitation areas, where it is vader of all disturbed areas throughout vehicle turbulence. Muntingia calabura, persisting as a ground cover. The two na- the island including roadsides, urban gar- Leucaena leucocephala and to a lesser extent tive Nephrolepis spp. tend to persist during dens and parks and industrial sites, which Ricinus communis L. (castor oil) also occur rehabilitation of mined areas and can be a require frequent slashing to keep them along disturbed rainforest margins problem. The large thorny tree Pithecell- open. throughout the island. obium dulce (Roxb.) Benth. (Madras thorn) Muntingia calabura is probably ubiqui- was originally planted around the golf tous throughout the island by now as dor- Environmental weeds of course after the war and has been used in mant seeds in the soil, but since the seed- rehabilitated areas several rehabilitation areas. Native birds lings are grazed by crabs and it requires Many locally available ornamental tropi- are now dispersing its seeds. Pterocarpus sunlight for growth it is mainly found on cal trees and shrubs were planted on indicus (Willd.) Nara (angsana) is a tall roadsides and in gardens and industrial resoiled mine fields between 1975 and southeast Asian rainforest tree which has areas. Surviving seedlings in light gaps in 1987 to establish ground cover, break up also been widely planted for rehabilita- the rainforest are quickly smothered by and add organic matter to the compacted tion. Seedlings arise close to parent trees taller and more vigorous native rainforest subsoil, and encourage the introduction of in open areas or where parent trees have pioneers. Pluchea indica (L.) Less. is a the seeds of native rainforest plants been killed. widespread shrubby weed of tropical through bird dispersal. Unfortunately few The most serious environmental weed coastlines and saline areas and is estab- records were kept of what was planted of rehabilitation areas within Christmas lished on cliffs and in industrial areas where, and with what success. The change Island National Park is Spathodea around the wharf, from where it may in- in management of these areas from min- campanulata. It grows rapidly, flowers af- vade the rest of the coastline. Psidium ing lease to national park and the fact that ter only a few years, and produces quanti- guajava L. (guava) is widespread through- these plants are not native to the island ties of fruits high in the canopy, each con- out the island but is not an aggressive has meant that they are now mostly per- taining many wind-blown seeds. The weed, being largely confined to heavily ceived to be environmental weeds. steady trade wind throughout much of the disturbed sites such as along tracks Thirteen species of exotic trees and year with stronger and more turbulent through the rainforest, where its growth shrubs occur in rehabilitated areas on winds in the wet season disperse seed- is severely limited by competition from Christmas Island. Adenanthera pavonia L. lings up to a kilometre from parent trees. encroaching rainforest trees. (red bead tree) has been widely planted Several species of Syzigium (lilly pillies) throughout these areas and is reproduc- have been planted in previous rehabilita- Management and control of ing freely, resulting in dense masses of tion areas, where they are now in fruit. environmental weeds on Christmas rapidly growing seedlings. Fortunately Their seeds and those of Muntingia Island the seeds are relatively immobile and calabura are dispersed by the Christmas Is- It is essential to prevent the introduction mostly germinate close to the parent trees. land Imperial Pigeon (Ducula whartoni) of further environmental weeds to the is- Aleurites moluccana (L.) Willd. (candlenut and possibly the Christmas Island Fruit land, and the Christmas Island Quaran- tree) is even more widespread and be- Bat (Pteropus melanotus natalis), and seed- tine Ordinance should be strengthened haves in much the same way, although lings are being noticed near parent trees. both to prohibit known environmental seeds have been transported into intact Tecoma stans are also common in many of weeds liable to become problems on the rainforest by water movement in at least these areas, but seedlings are intolerant of island and to require the rating of other one location. Germination is suppressed shade and only persist along the edges of plants before introduction. All proposed by heavy growth of ferns and shrubs, but rainforest. plant imports should be assessed for stimulated by the clearing that necessarily A few Ceiba pentandra (L.) Gaertner (ka- potential weediness, with plants of high precedes replanting with native rainforest pok) trees occur in at least one of the par- potential environmental weediness being tree seedlings. Barringtonia asiatica (L.) tially rehabilitated areas, and although prohibited entry. Kurz. (sea putat) is a small shoreline tree fruits have been seen no seedlings have Some existing environmental weeds that has been much less widely planted, yet been found. are still restricted in number or area, and and since its single-seeded fruits are large should be eradicated from the island and heavy (c. 500 gm) seedlings only oc- Other plants of conservation where possible. These include Hevea cur under the widely scattered parent concern brasiliensis, Pluchea indica, Senna sulfurea trees. Cordia curassavica (Jacq.) Roem. & Seven exotic plant species are of environ- and Schefflera actinophylla. Ficus elastica Schult. is a shrubby weed that is very mental concern on the island outside the and Pluchea indica are the most feasible. F. widespread throughout the tropics. It is National Park. Antigonon leptopus Hook. & elastica is represented only by a few large unlikely to have been deliberately planted Arn. (coral vine) is rampant on sea and and easily accessible trees which have not in rehabilitated areas, but its seeds are inland cliffs and in previously mined ar- yet set seed, and P. indica is a highly vis- readily dispersed by birds and it is now eas in the northeast of the island, where it ible shrub which is still confined to cliffs widespread. Delonix regia (Bojer ex Hook.) may be hampering the annual migration and wasteland around the wharf. The area Raf. (poinciana or flame tree) has been ex- of crabs and interfering with natural re- of Senna sulfurea is probably still quite lo- tensively planted during mine field reha- generation. Cordia curassavica is wide- calized, and if so it should be eradicated bilitation and forms dense monospecific spread throughout the island, especially when it is relocated. Schefflera actinophylla thickets. About 20 Ficus elastica trees were along roadsides, in abandoned mine- is restricted to cultivation in the urban planted throughout rehabilitated areas, fields, and in heavily disturbed industrial area and to adjacent severely degraded with mixed results. Some have remained and urban areas in the northeast corner. rainforests, and its eradication should be relatively small with open bases up to 4 m Imperata cylindrica (L.) P.Beauv. (blady attempted. A constant watch needs to be across, but those in better conditions have grass) is established along a mown track kept in the area previously infested with grown extremely well and form impen- carrying a road and power line in the Mucuna albertisii to detect and control etrable masses of aerial roots and spread- northeast of the island and in the urban seedlings before they flower. ing branches up to 15 m across. No fruits area, but requires constant mowing to Local council legislation is needed to have yet been seen on or under these maintain its dominance since it cannot prohibit the growing of certain plants on trees. compete with encroaching rainforest. private land, especially Ficus elastica, Plant Protection Quarterly Vol.16(2) 2000 57 Pithecellobium dulce, Pluchea indica, Acknowledgments Schefflera actinophylla, Spathodea campanu- Both professional and technical staff of lata and a recently introduced species of Parks Australia North assisted the first au- Leucaena. The implementation of such leg- thor during his time on Christmas Island. islation needs to be coordinated with the The work was carried out as a consultancy progressive control of these plants on pub- to the Australian Nature Conservation lic land. Agency, to whom the first author is very Many of the environmental weeds of grateful for the opportunity to visit the is- Christmas Island occur along rainforest land and to investigate its environmental margins and in clearings, and wherever weeds. the rainforest is able to overtop and shade out these weeds it should be encouraged References to do so. This can be achieved either by al- Australian Biological Resources Study lowing rainforest to recolonize clearings (1993). ‘Flora of Australia’, Volume 50, and the edges of unduly wide linear dis- Offshore Islands. (AGPS, Canberra). turbances, or by replanting. Weeds that Swarbrick, J.T. (1997). Environmental may be restricted in this way include Weeds and Exotic Plants on Christmas Mikania micrantha, Psidium guajava, Island, Indian Ocean. Report to Parks Leucaena leucocephala, Tecoma stans and Australia, Canberra. Cordia curassavica. Physical methods have very little place in the management of environmental weeds on Christmas Island, since there are few woody plants that do not regrow after cutting down. Those that are susceptible to felling include Areca catechu L. (betel nut palm) which is regenerating on several re- habilitation areas, and the locally sterile Elaeis guineensis Jacq. (oil palm). Effective biological control is available for Cordia curassavica, against which the combination of a leaf-feeding beetle and a gall-forming wasp has been effective else- where in the tropics, including the Indian Ocean. The majority of the environmental weeds on Christmas Island are rainforest trees which typically have slender erect single trunks, thin bark, and clean boles to at least several metres tall. Plants of this type are particularly susceptible to both cut stump application of water-based her- bicides such as glyphosate and to basal bark application of oil-based herbicides such as triclopyr. Both systems of herbi- cide application are efficient in terms of labour and the amount of herbicide used, and both are safe to use in national parks. Cut stump application of glyphosate has been used on the island for several years, and basal bark application is now proving to be very effective. Further research is needed to elucidate the most cost-efficient methods and herbicide rates for the differ- ent environmental weeds of Christmas Is- land. Effective control of environmental weeds within the national park is unfortu- nately often followed by massive germi- nation of further environmental weed seedlings which require repeated control. These areas should be planted with a nurse crop and native rainforest tree seed- lings as soon as possible, with follow-up control of remaining or regerminating ex- otic weeds.