Journal of Tropical Ecology (2015) 31:345–359. © Cambridge University Press 2015 doi:10.1017/S0266467415000231 Does the abundance of dominant trees affect diversity of a widespread tropical woodland ecosystem in Tanzania? Deo D. Shirima ∗, † ,1 , Ørjan Totland ∗ , Pantaleo K. T. Munishi † and Stein R. Moe ∗ ∗ Department of Ecology and Natural Resource Management, Norwegian University of Life Sciences, P.O. Box 5003, 1432 ˚ As, Norway † Department of Forest Biology, Faculty of Forestry and Nature Conservation, Sokoine University of Agriculture, P.O. Box 3010, Chuo Kikuu, Morogoro, Tanzania (Received 24 November 2014; revised 4 May 2015; accepted 4 May 2015) Abstract: Dominant woody species can determine the structure and composition of a plant community by affecting environmental conditions experienced by other species. We explored how dominant tree species affect the tree species richness, diversity, evenness and vertical structural heterogeneity of non-dominant species in wet and dry miombo woodlands of Tanzania. We sampled 146 plots from eight districts with miombo woodlands, covering a wide range of topographic and climatic conditions. We recorded 217 woody plant species belonging to 48 families and 122 genera. Regression analysis showed significant negative linear associations between tree species richness, relative species profile index of the non-dominant and the relative abundance of the dominant tree species (Brachystegia spiciformis and Brachystegia microphylla in wet, and Brachystegia spiciformis and Julbernardia globiflora in dry miombo woodlands). Shannon diversity and evenness had strong non-linear negative relationships with relative abundance of dominant tree species. A large number of small individual stems from dominant and non-dominant tree species suggesting good regeneration conditions, and intensive competition affecting survival. We suggest that dominant miombo tree species are suppressing the non-dominant miombo tree species, especially in areas with high recruitments, perhaps because of their important adaptive features (extensive root systems and ectomycorrhizal associations), which enhance their ability to access limited nutrients. Key Words: coexistence, heterogeneity, richness, structure, tree species INTRODUCTION Dominant plant species may regulate surrounding environment to influence other plant species diversity and composition (Angelini et al. 2011, Peh et al. 2011). According to Grime (1998), ecosystem properties, such as biomass production and diversity, are determined by the traits of the dominant species. Dominant plant species are termed foundation species if they determine the structure and composition of communities at local and regional scales (Caro 2010, Dayton 1972, Ellison et al. 2005). However, increasing abundance of the dominant plant species may have contrasting effects on co-occurring species (Dickson & Gross 2013). For example, a Gilbertiodendron dewevrei-dominated forest at Ituri reserve in the Democratic Republic of Congo had a comparable tree species richness (dbh 10 cm) with adjacent mixed forest (Djuikouo et al. 2014, Makana 1 Corresponding author. Email: dshirima2@gmail.com et al. 2004), while tree species richness (dbh 10 cm) was lower in G. dewevrei-dominated forest in Dja Faunal reserve of Cameroon compared with adjacent mixed forests (Peh et al. 2014). Removal of dominant plant species may have a significant impact on the remaining species (Dayton 1972), because dominant species can create and maintain habitats that support other taxa of a community (Martin & Goebel 2013, Smee 2012). Miombo woodlands, dominated by the genera Brachystegia and Julbernardia, are the most extensive (range: 2.7–3.2 million km 2 ) deciduous woodland type in south-central and East Africa (Campbell et al. 1996). However, plant species structure and composition in miombo woodlands has recently changed rapidly due to anthropogenic activities, such agricultural expansions, and local-climatic variability in the region (Frost 1996, Spinage 2012). These changes may cause decline in species richness or abundance and consequently influence species recruitment patterns and succession (Back´ eus et al. 2006). For example, intensive removal of species of Brachystegia and Julbernardia, which