Plant Functional Traits and the Distribution of West African Rain Forest Trees along the Rainfall Gradient Surya K. Maharjan 1,2 , Lourens Poorter 1,2,5 , Milena Holmgren 2 , Frans Bongers 1 , Jan J. Wieringa 3 , and William D. Hawthorne 4 1 Forest Ecology and Forest Management Group, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands 2 Resource Ecology Group, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands 3 National Herbarium of The Netherlands—Wageningen University Branch, Biosystematics Group, Wageningen University, General Foulkesweg 37, 6703 BL Wageningen, The Netherlands 4 Department of Plant Sciences, University of Oxford, South Parks Road, Oxford OX1 3RB, UK ABSTRACT Plant morphological and physiological traits affect the way plants tolerate environmental stresses and therefore play an important role in shaping species distribution patterns in relation to environmental gradients. Despite our growing knowledge on the role of functional traits in structuring plant communities, few studies have tested their importance at large scales in the wet tropics. Here, we describe the distribution patterns of the most important West African rain forest timber species along the regional rainfall gradient and relate them to their functional traits. We found that the distribution patterns of 25 out of the 31 studied species (80%) were significantly related to mean annual rainfall. Shade tolerance and drought resistance were identified as the main strategy axes of variation. Wood density and leaf deciduousness emerged as the best predictor traits of species position along the rainfall gradient, explaining respectively 32 and 15 percent of the variation. Species traits tended to show stronger relationships with estimated optimum annual rainfall for each species than to the extreme rainfall conditions where they occur. The significant role of rainfall in shaping timber species distribution and the strong relationships between species traits and rainfall indicate that changes in climate, especially declining rainfall, could have strong effects on species composition and abundance in these tropical forests. Key words: deciduousness; drought tolerance; plant strategy; rainfall; shade tolerance; species distribution; upper Guinea; wood density. FUNCTIONAL RESPONSE TRAITS DETERMINE PLANT GROWTH, SURVIVAL AND REPRODUCTIVE SUCCESS, and as a result, they are expected to play an important role in shaping species distribution patterns along en- vironmental gradients. The study of how functional response traits affect the way plants tolerate environmental stresses and cope with disturbances has played a central role in the development of eco- logical theories (Grime 1974, 1977; Keddy 1992; Westoby 1998). Most large-scale generalizations have compared the predominance of morphological traits across vegetation types and evaluated their potential adaptive role along environmental gradients (Ackerly 2004, Thuiller et al. 2004). In contrast, more detailed analysis within vegetation types have concentrated on relationships between functional traits at smaller-scale environmental gradients related to, for example, topography and exposition (Ackerly et al. 2002) and irradiance and soil fertility (Veenendaal et al. 1996, Agyeman et al. 1999). As reliable environmental response curves are lacking for most tropical species, our knowledge of the importance of func- tional traits for species partitioning of environmental gradients is particularly poor for the tropics. This study focused on the rainfall gradient, which is an im- portant environmental factor determining plant species distribu- tion (Swaine 1996, Bongers et al. 1999, Engelbrecht et al. 2007), composition (Hall & Swaine 1976, Bongers et al. 2004, Toledo et al. 2011) and richness (ter Steege et al. 2006) in tropical forests. Rainfall in West Africa has declined more rapidly than in any other rain forest region during the past four decades (Malhi & Wright 2004), which could severely impact plant and animal diversity in the region. Plant species vary considerably in their morphological and physiological traits giving rise to a continuum of plant strategies (Reich et al. 2003). One fundamental axis of evolutionary special- ization across ecosystems and biomes is that of high resource cap- ture (rapid acquisition of resources) at one end of the spectrum and high resource conservation (conservative use of resources) at the other (Diaz et al. 2004). Consequently, along the rainfall gradient rapid water acquisition and conservative water use could be impor- tant strategies for the success of plant species. Conservative water use can be achieved through efficient use of limited water (i.e., drought tolerance) or through drought avoidance (Markesteijn & Poorter 2009). Drought-tolerant species are, in general, character- ized by narrow and small leaves (Fonseca et al. 2000, Thuiller et al. 2004), low specific leaf area (SLA) (Fonseca et al. 2000, Niinemets 2001) and high wood density (Searson et al. 2004, Poorter & Markesteijn 2008). Drought-avoiding species are characterized by deciduous leaves (Borchert 1994, Markesteijn & Poorter 2009) to avoid transpirational water loss in the dry season, and low wood density (Borchert 1994) that allows for water storage in stems. With increasing rainfall, forests also tend to become taller and cast a deeper and more persistent shade. To regenerate successfully in these wetter, more shaded environments species should be more shade tolerant (Smith & Huston 1989), and have larger seeds to Received 29 January 2010; revision accepted 23 September 2010. 5 Corresponding author; e-mail: lourens.poorter@wur.nl BIOTROPICA 43(5): 552–561 2011 10.1111/j.1744-7429.2010.00747.x 552 r 2011 The Author(s) Journal compilation r 2011 by The Association for Tropical Biology and Conservation