Taiwania 67(1): 73‒82, 2022 DOI: 10.6165/tai.2022.67.73 73 Seed biology and early seedling developmental traits of Clidemia hirta, an invasive species of Sri Lankan rainforests compared to two native species sharing the same habitat B.R. Chandima P. SAMARASINGHE 1,2,* , K.M.G. Gehan JAYASURIYA 1,2 , A.M. Thilanka A. GUNARATNE 1,2 , Kingsley W. DIXON 3 , Mahesh SENANAYAKA 4 1. Department of Botany, University of Peradeniya, Peradeniya, Sri Lanka.; 2. Postgraduate Institute of Science, University of Peradeniya, Peradeniya, Sri Lanka. 3. ARC Centre for Mine Site Restoration, School of Molecular and Life Sciences, Curtin University, Western Australia. 4. Forest Department Sri Lanka, Ringgold standard institution, Battaramulla, Western Sri Lanka. * Corresponding author’s emails: brprabodani@gmail.com; Tel: +94770412912. (Manuscript received 19 May 2021; Accepted 16 December 2021; Online published 11 January 2022) ABSTRACT: Comparative studies on invasive species with native species sharing the same habitats are valuable in controlling invasive plants. This study compared seed biology and seedling development traits of two native species, Melastoma malabathricum L and Plectranthus kanneliyensis Willemse, and the invasive species Clidemia hirta (L.) D. Don, which share a tropical rainforest habitat in Sri Lanka. In particular, the adaptability of these species to their habitat was assessed. Seed moisture content, dormancy status and desiccation sensitivity were also tested. Seedling growth and survival was observed through greenhouse experiments. Seeds of all the study species germinated >50% in light/dark conditions, but none germinated in complete darkness or under green light. Time taken to reach 50% germination (T50) and final germination percentage suggested that C. hirta and P. kanneliyensis seeds are non-dormant while M. malabathricum seeds are dormant. All study species produced desiccation tolerant seeds. Under all light conditions, seeds of C. hirta recorded the highest seed germination and seedling survival percentage, while P. kanneliyensis had the highest growth rate. Interspecies difference in photosynthesis efficiency was not observed. These experiments demonstrated that the study species have different strategies to adapt to their shared habitat. Furthermore, higher germination percentage and higher survival may be the key factors that determines the invading ability of C. hirta, enabling them to outcompete M. malabathricum and P. kanneliyensis. Hence, immediate actions must be taken to prevent further invasion of C. hirta into disturbed sites in tropical rainforest regions. KEY WORDS: Invasive species, native species, seed dormancy, seed germination, seedling survival. INTRODUCTION Invasion of exotic species has become a major threat to the biodiversity of tropical habitats, including rainforests (Bradshaw et al., 2009; Meyer and Cowie, 2010; Krupnick, 2013). These species affect native species through competition (Levine et al., 2003), change of the edaphic factors (Jordan et al., 2008) and alteration of fire regimes (Antonio, 2000). Invasive species through these effects lower the native biodiversity with a disproportionately negative impact on rare endemic species (Hejda et al., 2009). Clidemia hirta has been reported as one of the world’s 100 worst invasive species (Global Invasive Species Database, 2020). In its native range (lowlands of Central and South America and the Caribbean islands), C. hirta is known to be found in open areas but can also invade closed canopy forests in its introduced range (DeWalt et al., 2004a). This species can replace native species that are specialized in inhabiting open sites in rainforests by changing habitat conditions (Wester and Wood, 1977). Moreover C. hirta invasion is recorded to alter forest regeneration through alteration of the habitat in many rainforests worldwide (Hawaiian Rainforests [Wester and Wood, 1977], Malaysia [Peters, 2001; Le et al., 2018], Taiwan [Yang, 2001], Fiji [Simmonds, 2009]). Tropical lowland rainforests of Sri Lanka, which are biodiversity rich ecosystems with many endemic plant species (60‒75%) have not escaped from this threat (Kariyawasam et al., 2020; Bambaradeniya, 2002). C. hirta was first introduced to Sri Lanka in 1894 through Peradeniya Botanical Gardens (Wijesundara, 2010). In Sri Lanka it is found in the sub-montane areas of wet zone, and occupies open areas in lowland rainforest edges, roadsides and canopy gaps in rainforests rather than in forest understory and is now spreading to higher elevations in the Kandy and Ambagamuwa areas (Wijesundara, 2010). Although improved management and control practices are crucial to control this invasive plant, while promoting the establishment of native species (Simmonds, 2009), there are no such attempts made so far in Sri Lanka regarding C. hirta. It is imperative to gather information on the life history of the invasive species and native species that share the same habitat to plan for the successful management of invasive species. Seed germination is considered as a transition from the most tolerant developmental stage i.e. the seed to the most vulnerable stage in plant development, the seedling (Castro et al., 2005). Growth of a new plant is associated