BIOTROPICA 38(4): 468–478 2006 10.1111/j.1744-7429.2006.00168.x Edaphic and Floristic Variation within a 1-ha Plot of Lowland Amazonian Rain Forest 1 Axel Dalberg Poulsen 2 Royal Botanic Garden, 20A Inverleith Row, Edinburgh EH35LR, Scotland Hanna Tuomisto Department of Biology, Universityof Turku, FI-20014 Turku, Finland and Henrik Balslev Institute of Biological Science, University of Aarhus, Universitetsparken, DK-8000 Aarhus C, Denmark ABSTRACT Several studies in lowland tropical rain forests have documented effects of local-scale topographic variation on plant species distribution and abundance patterns. Few studies have compared the distribution patterns of more than one plant group, however, and even fewer have related these to measured physical and chemical soil characteristics. Here, we document such soil characteristics within a square 1-ha plot in Amazonian Ecuador, and compare them to the distribution patterns of terrestrial pteridophytes, angiosperm ground herbs, and palms. Substantial variation in soil properties was found within the plot. The three plant groups showed highly correlated floristic patterns within the 1-ha plot even after the effect of geographical distances had been taken into account. Mantel tests yielded significant correlations between edaphic patterns, as measured by distances in various soil and topographic characteristics, and floristic patterns. For all three plant groups, differences in elevation within the plot were highly correlated with floristic distances, and for terrestrial pteridophytes and palms, distances in soil calcium content and sand content were also important. Our results resembled those obtained at wider spatial scales with the same plant groups, which indicates that soil factors may play an important role for distribution and beta diversity of plants, even at the local scale. RESUMEN Varios estudios en bosques tropicales lluviosos han documentado los efectos de la variaci´ on local de la topograf´ ıa sobre los patrones de distribuci´ on y abundancia de especies de plantas. Sin embargo, pocos estudios han comparado los patrones de distribuci´ on en m´ as de un grupo de plantas, y a ´ un m´ as escasas son las investigaciones que relacionan ´ estas a medidas f´ ısicas y qu´ ımicas de las caracter´ ısticas de los suelos. En el presente estudio, documentamos estas caracter´ ısticas de suelos en una parcela cuadrada de 1 ha en la Amazonia ecuatoriana, y las comparamos a los patrones de distribuci´ on de pteridofitas terrestres, hierbas angiospermas terrestres y palmeras. Se encontr´ o una gran variaci´ on en las propiedades de los suelos dentro de la parcela. Los tres grupos de plantas presentaron patrones flor´ ısticos altamente correlacionados dentro de la parcela a´ un cuando se tom´ o en cuenta el efecto de distancias geogr´ aficas. Pruebas de Mantel resultaron en correlaciones altamente significativas entre patrones ed´ aficos, medidos como distancias en varias caracter´ ısticas de suelos y de topograf´ ıa, y patrones flor´ ısticos. Para los tres grupos de plantas, diferencias en elevaci´ on dentro de la parcela fueron altamente correlacionadas con distancias flor´ ısticas, y para pteridofitas terrestres y palmeras, distancias en el contenido de calcio y de arena en el suelo fueron importantes tambi´ en. Nuestros resultados son parecidos a aquellos obtenidos en escalas espaciales mas amplias para los mismos grupos de plantas, indicando que factores ed´ aficos pueden ser importantes para la distribuci´ on y la beta diversidad de plantas aun en una escala local. Key words: Ecuador; ground herbs; local scale variability; Mantel test; multiple regression; palms; soils; species composition; terrestrial pteridophytes, tropical rain forest. STUDIES CONDUCTED IN DIFFERENT PARTS OF THE TROPICS at different spatial scales differ in how much importance they place on environmental factors, especially soil properties, for plant species distributions. At the landscape scale (100–10,000 km 2 ), edaphic gradients have been found to be important for the distribution of trees in several studies (Baillie et al. 1987, Duivenvoorden 1995, Ruokolainen et al. 1997, Ruokolainen & Tuomisto 1998, Pyke et al. 2001, Duque et al. 2002, Phillips et al. 2003), but oth- ers have reported that most tree species occur in more than one forest type (Pitman et al. 1999). Studies on palms, Melastomat- aceae, and pteridophytes have invariably found that landscape-scale edaphic differences are related to floristic patterns (Ruokolainen 1 Received 14 April 2005; revision accepted 8 September 2005. 2 Corresponding author; e-mail: axel@dalbergpoulsen.com et al. 1997; Ruokolainen & Tuomisto 1998; Tuomisto et al. 2002, 2003a, 2003b; Vormisto et al. 2004a, 2004b). At the mesoscale (1–100 km 2 ) in the Neotropics, two studies on trees and palms (ter Steege et al. 1993; Clark et al. 1995, 1998, 1999) and one study on herbs (Costa et al. 2005) found that distributions were related to soil and topography. At the local scale (<1 km 2 ), there seems to be general agreement that topographic factors are important for the distributions of at least some tree species (Lieberman et al. 1985, Hubbell & Foster 1986, Sabatier et al. 1997, Webb & Peart 2000, Harms et al. 2001, Debski et al. 2002, Valencia et al. 2004). Such topographically determined patterns are also common in palms (Svenning 1999, 2001; Vormisto et al. 2004a) and herbs (Poulsen 1996; Poulsen & Balslev 1991; Poulsen & Nielsen 1995; Tuomisto et al. 1995, 1998; Tuomisto & Poulsen 2000). 468 C  2006 The Author(s) Journal compilation C  2006 by The Association for Tropical Biology and Conservation