Journal of Tropical Ecology (2010) 26:497–508. Copyright © Cambridge University Press 2010 doi:10.1017/S0266467410000271 Seasonal variation in soil and plant water potentials in a Bolivian tropical moist and dry forest Lars Markesteijn ∗, † ,1 , Jos ´ e Iraipi†, Frans Bongers ∗ and Lourens Poorter ∗, † , ‡ ∗ Forest Ecology and Forest Management Group, Centre for Ecosystem Studies, Wageningen University, P.O. Box 47, 6700 AA, Wageningen, the Netherlands † Instituto Boliviano de Investigaci ´ on Forestal, P.O. Box 6204, Santa Cruz de la Sierra, Bolivia ‡ Resource Ecology Group, Centre for Ecosystem Studies, Wageningen University (WU), P.O. Box 47, 6700 AA Wageningen, the Netherlands (Accepted 7 May 2010) Abstract: We determined seasonal variation in soil matric potentials (ψ soil ) along a topographical gradient and with soil depth in a Bolivian tropical dry (1160 mm y −1 rain) and moist forest (1580 mm y −1 ). In each forest we analysed the effect of drought on predawn leaf water potentials (ψ pd ) and drought response (midday leaf water potential at a standardized ψ pd of −0.98 MPa; ψ md ) of saplings of three tree species, varying in shade-tolerance and leaf phenology. ψ soil changed during the dry season and most extreme in the dry forest. Crests were drier than slopes and valleys. Dry-forest top soil was drier than deep soil in the dry season, the inverse was found in the wet season. In the moist forest the drought-deciduous species, Sweetia fruticosa, occupied dry sites. In the dry forest the short-lived pioneer, Solanum riparium, occupied wet sites and the shade-tolerant species, Acosmium cardenasii drier sites. Moist-forest species had similar drought response. The dry-forest pioneer showed a larger drought response than the other two species. Heterogeneity in soil water availability and interspecific differences in moisture requirements and drought response suggest great potential for niche differentiation. Species may coexist at different topographical locations, by extracting water from different soil layers and/or by doing so at different moments in time. Key Words: Acosmium cardenasii, Ampelocera ruizii, Bolivia, drought, leaf water potential, soil depth, soil water availability, Solanum riparium, Sweetia fruticosa, Trema micrantha INTRODUCTION Tropical lowland forests are found under different rainfall regimes. The majority of tropical forests have a pronounced dry season, and, even in perhumid forests, extended periods of drought can occur (Burslem et al. 1996, Potts 2003, Walsh & Newbery 1999). Tropical tree diversity and species distribution are to a great extent explained by the amount of annual precipitation, length of the dry period and the cumulative water deficit (Bongers et al. 1999, Engelbrecht et al. 2007, Gentry 1988, Killeen et al. 2007, Poorter et al. 2004, Swaine & Becker 1999) and within forests, topographical variation in water availability is an important factor influencing species distribution (Clark 1999, Comita & Engelbrecht 2009, Valencia et al. 2004, Webb & Peart 2000). Topography controls the distribution of water and, through surface run-off or lateral flow, sediments and 1 Corresponding author. E-mail: lars.markesteijn@wur.nl solutes are redistributed over the landscape. This affects soil depth, the ground water depth and soil properties (Lavelle & Spain 2002, Sollins 1998). On crests, ridges and steep upper slopes, high sediment removal rates result in shallow soils with a high sand content. Lower slopes and flat valleys, where weathering rates may exceed sediment removal, have deeper soils with higher clay and silt contents (Itoh et al. 2003, Johnsson & Stallard 1989, Lescure & Boulet 1985, Pachepsky et al. 2001). Resulting differences in soil texture influence plant water availability as fine soils have a higher water retaining capacity than coarse soils (Jenny 1980) and thus soil water availability generally increases down- slope (Becker et al. 1988, Daws et al. 2002). Furthermore, in valleys or on lower slopes the duration of drought can be shorter than at higher elevations, effectively shortening the dry season in these habitats (Daws et al. 2002). This variation in soil water availability with topography can have a strong effect on patterns of seedling emergence and mortality (Daws et al. 2005).