Jun. 2013, Vol. 7, No. 6, pp. 635-642 Journal of Life Sciences, ISSN 1934-7391, USA D DAVID PUBLISHING Impact of Sphagneticola trilobata on Plant Diversity in Soils in South-East Viti Levu, Fiji Apaitia R. Macanawai Tropical Weed Research Unit, Plant Protection Section, Koronivia Research Station, Ministry of Agriculture, Nausori, Fiji Received: January 03, 2013 / Accepted: March 02, 2013 / Published: June 30, 2013. Abstract: Wedelia (Sphagneticola trilobata (L.) Pruski) has become one of the most dominant invasive plant species in Fiji. However, the soil seed bank of its monospecific stand and its ability to reproduce by seed is relatively unknown. A soil seed bank study was undertaken in a monospecific stand of S. trilobata in Sawani, Natavea and Wainivesi in south-east Viti Levu, Fiji in March 2012. The soil samples were collected from within 1.0 m2 quadrat taken at 10 spots in each of the study areas and spread thinly over a base of Yates Thrive Premium potting mix in seedling trays and placed in a glasshouse at Koronivia Research Station, Fiji. A total of 23, 26 and 33 plant species were found in the soil seed bank in Wainivesi, Sawani and Natavea respectively which may have succumbed to S. trilobata invasiability. There were ca. 3,800 (17%), 2,100 (11%) and 2,600 (6%) germinable S. trilobata seeds·m-2 in the soil seed bank in Wainivesi, Sawani and Natavea areas respectively. This study has demonstrated that S. trilobata seeds may have a role in the spread of the invasive species in Fiji and movement of soil to S. trilobata free areas should be restricted. Key words: Sphagneticola trilobata, wedelia, Fiji, soil seed bank, quadrat, invasive. 1. Introduction nodes [4]. The invasive species spreads vegetatively and encroach the neighbouring area easily and at a ((Spagneticola trilobata (L.) Pruski); Asteraceae) rapid rate. According to Batianoff & Franks [5], the is native to Mexico, Central America, and the highest rate of spread of S. trilobata in Australia was Caribbean and has been introduced widely to tropical recorded at 167 km·year-1. and sub-tropical regions of the world including the From an observation on cultivated S. trilobata in Pacific Islands [1, 2]. In the Pacific Island countries Hawaii, it was reported that S. trilobata matured and territories, S. trilobata is established in American achenes were 4-5 mm long [4]. Similarly, the risk Samoa, Northern Mariana Islands, Cook Islands, assessment for S. trilobata prepared for Australia was Federated States of Micronesia, Fiji, French silent on the seed reproduction capability of the Polynesia, Guam, Hawaii, Kiribati, Kingdom of species [2]. Based on a published literature on biology Tonga, Marshall Islands, Nauru, New Caledonia, of non-native plants in natural areas in Florida, S. Niue, Palau, Samoa and Tokelau [2]. S. trilobata is trilobata fruit is a tiny, brown, pimpled achene, ca. 5 commonly known as wedelia, creeping ox-eye, mm in length [6]. All these authors neither reported Singapore daisy and trailing daisy [2]. It is usually the production capacity nor the fertility of S. trilobata introduced as an ornamental plant, attributed to its achenes. This suggests that there is a gap in fleshy green leaves and bright yellow flowers [3]. It is knowledge on the seed biology of S. trilobata. a perennial creeper that forms dense thickets of However, the reproductive potential of S. trilobata inter-winning stems that shoots and roots at the may have played a substantial role in the success of its naturalization and invasion as occurred in a related Corresponding author: Apaitia R. Macanawai, Ph.D., species, Mikania micrantha H.B.K. [7]. Furthermore, research field: tropical weed research. E-mail: [email protected]. factors influencing the number of propagules 636 Impact of Sphagneticola trilobata on Plant Diversity in Soils in South-East Viti Levu, Fiji produced, mode of spread and rate of recruitments and annual plant found to decrease the growth and nutrient deaths contribute in the spread of invasive species [8]. content of S. trilobata in a field study in China had S. trilobata was thought to have been introduced been recommended as a potential biological control into Fiji in mid-1970s [9]. However, it did not take for the invasive species [14]. Cuscuta species have long to colonise road sides, river banks, drains, been observed on S. trilobata in some sites in Viti pasture fields and vacant lands in major islands in Fiji Levu Fiji (personal observation). According to [9] (personal observation). S. trilobata has extended Englberger [21], S. trilobata is controlled by repeated its boundary to cropping areas and was found as a new physical removal of the plants but this method is labor weed of taro (Colocasia esculenta L. Schott) in the intensive. Herbicide found to be effective on S. taro production areas in south-east Viti Levu [10]. In trilobata was metsulfuron methyl herbicide (600 g/kg) Taveuni Island Fiji, S. trilobata was found infesting a and has been registered for controlling the weed in taro farm in the southern side of the island in October Australia [22]. In Fiji, metsulfuron methyl herbicide 2012 (personal observation). It has very wide has been recommended for the control of S. ecological tolerance range and found to be equally trilobata [23]. Other herbicides found to have had suited to dry and moist sites and seemed to grow very some effect were dicamba and 2,4-D [24], triclopyr well on almost all soil types [2]. Based on its (Garlon 4) and glyphosate (Roundup) [21]. ecological significance, S. trilobata was considered There is insufficient data on impacts of invasive one of the “100 World’s Worst Invasive Alien species in the PICs and strong recommendation was Species” [11]. made for research to be undertaken on the assessment According to Meyer 2000 [3], wedelia is one of the of the impacts of invasive species on the biodiversity most significant dominant invasive plant species in of island ecosystems [25]. S. trilobata is a threat to the most of the Pacific islands. The presence of invasive ecosystem because of its ability to easily colonise an plant species such as S. trilobata in the islands area however, the soil seed bank of a monospecific terrestrial ecosystem poses a major threat to the stands of S. trilobata and its effect on plant diversity is biodiversity of a given community and ecosystem relatively unknown. functions [12, 13]. S. trilobata was found to prevent The presence of a seed bank is essential for the life regeneration of co-occurring species and damage cycle of many plant species [26, 27]. The soil seed riverbanks and wastelands [14]. This could have been bank is “plant biodiversity in storage” and provides attributed to the allelopathic effect of S. trilobata the future re-establishment of variation back into the leachates and residues that caused substantial ecosystem [28]. However, the dominance of a reduction in germination and growth rate of tested particular invasive plant would deplete seed crop and native plants in China [15, 16]. Furthermore, production of other neighboring species and may the high concentration of CO2 found to cause greater reduce the genetic diversity of the community [29, growth stimulation in S. trilobata as compared to its 30]. Therefore, the aim of this study was to determine native congener, (Wedelia chinensis (Osbeck.) Merr.) if S. trilobata is present and if it does, its density in southern China [17]. relative to co-occurring plant species in the soil seed One of the reasons S. trilobata is more aggressive bank community, underneath monospecific stands of S. and dominant in the invaded region is because of the trilobata, with a view to better understand its lack of natural enemies which are capable of invasiveness and to identify better management controlling its population growth [18-20]. However, practices in cropping areas of Fiji. A positive outcome Cuscuta australis R. Brown, a holoparasitic rootless is evidence that S. trilobata can produce fertile seeds. Impact of Sphagneticola trilobata on Plant Diversity in Soils in South-East Viti Levu, Fiji 637 This study may also reveal plant species that S. these samples was estimated by counting the viable trilobata may have displaced, i.e., the history of the seeds germinating from the soil seed bank tray. The site and the potential future plant community if S. soil samples were subjected to conditions favorable trilobata is controlled. for seed germination. Five additional trays, filled only with “Yates Thrive Premium” potting mix, were used 2. Materials and Methods as controls to detect any seed in the potting compost 2.1 Study Site or in the glasshouse environment. The thin layering of the soil sample spread over Three S. trilobata infested fields in Sawani (18°02'S; the potting mix in the tray was undertaken to enhance 178°28'E), Natavea (18°04'S; 178°34'E) and the possibility of germination of the viable seeds Wainivesi (17°47'S; 178°31'E) in south-east Viti Levu present in the sample. Daily observations made on were identified. Each study site has a measurement of newly emerged seedlings and all trays were watered approximately 50 m × 50 m of > 95% monospecific to field capacity when required. The emerged stand and > 30 cm dense thickets of S. trilobata. A seedlings were counted, identified and then removed zig-zag method of sampling and the use of quadrats from the seedling tray. Unidentified seedlings were (1.0 m2) (method modified from that described by removed from the trays and transplanted into Colbach et al. [31] and Mehdi et al. [32] was separate pots and grown to maturity so that they employed to collect soil samples for soil seed bank could be identified at a later date using taxonomical assessment.