Ecological Research (2004) 19: 521–532 Blackwell Science, LtdOxford, UKEREEcological Research1440-17032004 Ecological Society of Japan195521532Original ArticleMicrosite specialization in a clonal palmA. F. Souza and F. R. Martins *Author to whom correspondence should be addressed. Email: fmartins@unicamp.br Received 15 October 2003, Accepted 7 May 2004. Microsite specialization and spatial distribution of Geonoma brevispatha, a clonal palm in south-eastern Brazil Alexandre F. SOUZA 1 and Fernando R. MARTINS 2 * 1 Programa de Pós-Graduação em Ecologia, Instituto de Biologia, Universidade Estadual de Campinas, Caixa Postal 6109, Campinas 13083–970, SP, Brazil, and 2 Departamento de Botânica, Instituto de Biologia, Universidade Estadual de Campinas, Caixa Postal 6109, Campinas 13083-970, SP, Brazil. We tested the hypotheses that: (i) reproductive Geonoma brevispatha, an understorey clonal palm endemic to South American swamps, is most abundant in more brightly lit microsites but the abundance of juveniles is not responsive to light availability, (ii) the species is restricted to the transitional zones between flooded and well-drained microsites, (iii) if hypotheses (i) and/or (ii) are accepted, the microhabitat specialization they represent should be reflected in the spatial distribution of the individuals at distinct scales. The study was carried out in a swamp forest in south-eastern Brazil. Soil moisture was autocorrelated at distances up to approximately 1 m, reflecting a fine-scale microtopographic pattern of flooded pits and channels delimited by drier mounds. The first hypothesis was rejected, but the second hypothesis was accepted. No genets occurred on flooded microsites. A juvenile emergence experiment showed that seeds were unable to develop into juveniles under flooded conditions, but canopy openness did not influence the number of juveniles emerging. The third hypothesis was accepted. Individual genets were randomly distributed at scales corresponding to soil moisture patches, but were aggregated at larger scales. Juvenile genets were positively associated with reproducer genets. Our results indicate that palms may specialize on narrow parts of moisture gradients in swamp forests, and that this specialization can exclude them from even subtly distinct microhabitats. Key words: Atlantic forest; point pattern analysis; Santa Genebra Municipal Reserve; spatial autocorrelation; tropical forests. Introduction Two central paradigms in tropical rain forest plant ecol- ogy are: (i) survival and growth of plants in the under- storey are light-limited, and (ii) resource partitioning between trees exists in their differential regeneration along light gradients created by treefall gaps of different sizes and spatial distribution (Connell 1978; Denslow 1987; Svenning 2001b). Differences in performance associated with a gap-understorey environmental mosaic have been recorded for a number of species at distinct spatial scales (Denslow 1987; Nuñez-Farfán & Dirzo 1988; Bazzaz 1991; Vandermeer et al. 1997; Svenning 2000, 2001b). Indeed, niche differentiation is thought to be an essential mechanism for species coex- istence (Chesson 2000), and the differential utilization of the light environments that forest gaps create is pos- sibly one important factor in the maintenance of the high local species richness in tropical rain forests (Denslow 1987; Svenning 2001b). This view arises as one of the main developments of Grubb’s (1977) regeneration niche hypothesis to explain the mainte- nance of species diversity in forest plant communities. Nevertheless, a large portion of the variance of spe- cies abundance and distribution remains unexplained in the studies relating the occurrence of species in dis- tinct light environments (Lieberman et al. 1995; Harms et al. 2001; Svenning 2001b). There is an ongoing debate on whether this reflects the lack of study of aspects of habitat and microhabitat specialization other than those related to light gaps, or the prevalence of non-equilibrium conditions in most forest communi- ties, by which most species would be generalist and species composition would not remain constant (Chesson 2000; Connell 1978; Hubbell & Foster 1986a; Terborgh et al. 1996). At the spatial scale of habitats, one manifestation of resource-based niche