- Relationships between soil hydrology and forest structure and composition - 183 Journal of Vegetation Science 18: 183-194, 2007 © IAVS; Opulus Press Uppsala. Abstract Question: Is soil hydrology an important niche-based driver of biodiversity in tropical forests? More specifically, we asked whether seasonal dynamics in soil water regime contributed to vegetation partitioning into distinct forest types. Location: Tropical rain forest in northwestern Mato Grosso, Brazil. Methods: We investigated the distribution of trees and lianas ≥ 1 cm DBH in ten transects that crossed distinct hydrological transitions. Soil water content and depth to water table were measured regularly over a 13-month period. Results: A detrended correspondence analysis (DCA) of 20 dominant species and structural attributes in 10 × 10 m subplots segregated three major forest types: (1) high-statured upland forest with intermediate stem density, (2) medium-statured forest dominated by palms, and (3) low-statured campinarana forest with high stem density. During the rainy season and transition into the dry season, distinct characteristics of the soil water regime (i.e. hydro-indicators) were closely associated with each vegetation community. Stand structural attributes and hydro-indicators were statistically different among forest types. Conclusions: Some upland species appeared intolerant of anaerobic conditions as they were not present in palm and campinarana sites, which experienced prolonged periods of saturation at the soil surface. A shallow impermeable layer restricted rooting depth in the campinarana community, which could heighten drought stress during the dry season. The only vegetation able to persist in campinarana sites were short-stat- ured trees that appear to be well-adapted to the dual extremes of inundation and drought. Keywords: Biodiversity; Detrended correspondence analysis (DCA); Mato Grosso; Palm; Primary forest; Soil water; Tropi- cal forest; White sand soil. Abbreviations: DCA = Detrended correspondence analysis; LAI = Leaf area index; θ = soil volumetric water content. Nomenclature: Ribeiro et al. (1999). Relationships between soil hydrology and forest structure and composition in the southern Brazilian Amazon Jirka, Stefan 1* ; McDonald, Andrew J. 1,6 ; Johnson, Mark S. 2,3,6 ; Feldpausch, Ted R. 1,5,6 ; Couto, Eduardo G. 4,6 & Riha, Susan J. 1,6 1 Department of Earth and Atmospheric Sciences, College of Agriculture and Life Sciences, Cornell University, Bradfield Hall, Ithaca, NY 14853, USA; 2 Department of Crop and Soil Sciences, College of Agriculture and Life Sciences, Cornell University, Bradfield Hall, Ithaca, NY 14853, USA; 3 Department of Geography, Simon Fraser University, Burnaby, BC V5A 1S6, Canada; 4 Faculdade de Agronomia e Medicina Veterinária, Universidade Federal de Mato Grosso, Cuiabá, MT 78060-900, Brazil; 5 Earth and Biosphere Institute, School of Geography, University of Leeds, Leeds, LS2 9JT, UK; 6 E-mail addresses ajm9@cornell.edu; msj8@cornell.edu; trf2@cornell.edu; couto@ufmt.br; sjr4@cornell.edu; * Corresponding author; Fax +1 607 2552644; E-mail sj42@cornell.edu Introduction Correlative studies that link the distribution of tropical forest types to environmental factors are common. Soil class (Harms et al. 2001; Duque et al. 2002; Svenning et al. 2004), soil texture (Laurance et al. 1999), flooding regime (Salo et al. 1986), slope angle (Duivenvoorden 1996; Clark et al. 1999), precipitation gradients (Pyke et al. 2001), and topography (Valencia et al. 2004) have all been found to covary with patterns in vegetation distribu- tion. Evidence for explicit controls on tree distribution, however, is scarce and often confined to a particular species or set of species (e.g. Baker et al. 2003; Souza & Martins 2004). Establishing causal relationships is dif- ficult since vegetation can affect soil properties as well as show preferences for specific edaphic site characteristics (Sollins 1998). Soil hydrology is a component of the environment that could play a strong role in shaping tropical forest structure and composition. At one extreme, inundation of soil with water creates anaerobic conditions which can lead to anoxia and cell death in roots of maladapted species (Vartapetian & Jackson 1997). Impeded drain- age, due to either topographic position (Vormisto et al. 2004) or soil type (Pélissier et al. 2001), has been shown to affect plant species distribution. For example, certain palm species grow preferentially in low-lying areas affected by poor drainage (Svenning 2001; Souza & Martins 2005) or seasonal inundation (Frangi & Lugo 1998). At the other extreme, limited water avail- ability under drought conditions can lead to reduced plant growth (Baker et al. 2003), net biomass loss via increased tree mortality (Rolim et al. 2005), increased herbivory (Itioka & Yamauti 2004), and seedling death (Engelbrecht & Kursar 2003; Engelbrecht et al. 2005). Furthermore, prolonged drought may favor the growth of species adapted to dry conditions thereby altering