Journal of Forestry Research (2009) 20(1):7-14 DOI 10.1007/s11676-009-0002-6 Species diversity and regeneration of old-growth seasonally dry Shorea robusta forests following gap formation Indra Prasad Sapkota, Mulualem Tigabu, Per Christer Odén Department of Forest Genetics and Plant Physiology, Swedish University of Agricultural Sciences, SE-901 83, Umeå, Sweden. Abstract: Diversity and regeneration of woody species were investigated in two ecological niches viz. gap and intact vegetation in old-growth seasonally dry Shorea robusta (Gaertn. f.) forests in Nepal. We also related varieties of diversity measures and regeneration at- tributes to gap characteristics. Stem density of tree and shrub components is higher in the gap than in the intact vegetation. Seedling densi- ties of S. robusta and Terminalia alata (B. Heyne ex Roth.) are higher in the gap than in the intact vegetation, while contrary result is ob- served for T. bellirica (Gaertn. ex Roxb.) and Syzigium cumini (L. Skeels) in term of seedling density. The complement of Simpson index, Evenness index, and species-individual ratio in the seedling layer are lower in the gap than the intact vegetation. Gap size can explain spe- cies richness and species establishment rate. Gaps created by multiple tree falls in different years have higher seedling density of S. robusta than gaps created by single and/or multiple tree falls in the same year. In conclusion, gaps maintain species diversity by increasing seedling density, and favor regeneration of Sal forests. In addition to gap size, other gap attributes also affect species diversity and regeneration. Keywords: Canopy gap; intermediate disturbance hypothesis; Nepal; Sal (Shorea robusta Gaertn. f.) forest Introduction Understanding of canopy gap dynamics provides an insight into the many debated question whether or not species equilibrium is maintained in a forest community during regeneration processes in gaps (Barik et al. 1992; Zang and Wang 2002). Regeneration processes in gaps depend on several physical and biological fac- tors involving canopy closure, intensive growth of advanced regeneration of pre-disturbance origin, and species colonization (Arriaga 2000). The prevailing regeneration strategy following a tree fall, including seed rain, seed bank, suppressed seedlings and saplings, vegetative regeneration, or lateral growth of peripheral trees, depends heavily upon the characteristics of gaps and dis- turbed areas (Runkle 1985; Lawton and Putz 1988; Brokaw and Scheiner 1989; Arriaga 2000). The size of a gap, its temporal distribution, and severity of disturbance are the determining factors that account for changes in floristic composition, structure, species diversity, and regen- eration. Tree fall gaps offer specialized regeneration niches as a Foundation Project: The study was supported by Swedish International Development Cooperation Agency (SIDA). Received: 2008-08-27; Accepted: 2008-11-25 © Northeast Forestry University and Springer-Verlag 2009 The online version is available at http://www.springerlink.com Biography: Indra Prasad Sapkota (1970-), male, PhD. Forest ecologist, Swedish University of Agricultural Sciences, Department of Forest Ge- netics and Plant Physiology, Tropical Silviculture and Seed Science Group, SE-901 83, Umeå, Sweden. E-mail: sapkotai@yahoo.com Responsible editor: Hu Yanbo result of spatial and micro-environmental heterogeneity (Barik et al. 1992; Arriaga 2000; Li et al. 2005). Even within a gap, dif- ferences in light, moisture and temperature regimes and spatial heterogeneity caused by root, bole and crown zones, create a number of potential regeneration niches. Such heterogeneities are of fundamental importance in maintenance and promotion of high tree diversity in tropical forest communities (Connell 1978; Barik et al. 1992). Gap size is a critical variable for recruitment and establishment of different tree species (Brokaw 1985; Li et al. 2005). Regeneration strategy prevailing in small gaps is intensive growth of advanced regeneration (saplings), whereas the regen- eration path in larger gaps is dominated by seed rain or seed bank of pioneer species (Runkle 1985; Garwood 1989). Small gaps are more likely colonized by clonal expansion of surrounding plants, whereas successful establishment through germination prevails in larger gaps (Bullock 2000), and establishment success from seed increases with distance to the gap edges (Li et al. 2005). Establishment of individuals in gaps depends not only on gap size, but also largely on how these gaps are created (Pakeman et al. 1998). Subtle aspects of tree fall gaps, involving shape (Li et al. 2005), age (Barik et al. 1992; Schnitzer and Carson 2001), number and causes of tree fall (Uhl et al. 1988; Arriaga 2000), gap canopy height and micro-environmental factors (Barik et al. 1992) are of equal importance for post-gap regeneration and diversity of tree species. Despite a few studies in Indian sub-tropical region (Barik et al. 1992), southern tropics of India (Chandrashekara and Rama- krishnan 1993) and China (Zang and Wang 2002; Li et al. 2005; Zang et al. 2005) and temperate forests of Nepal (Vetaas 1997), the role of gaps in maintaining species diversity and regeneration in old-growth seasonally dry Shorea robusta forests (also known as Sal forests) in South East Asia is not fully understood. It is RESEARCH PAPER