Tree community structure and stem mortality along a water availability gradient in a Mexican tropical dry forest Gerardo Segura 1,4 , Patricia Balvanera 2, *, Elvira Durán 1 and Alfredo Pérez 3 1 Departamento de Ecología Funcional y Aplicada, Instituto de Ecología, Universidad Nacional Autónoma de México, Mexico; 2 Departamento de Ecología de los Recursos Naturales, Instituto de Ecología, Universidad Nacional Autónoma de México, Xangari, Apdo. Postal 27-3, Morelia, 58089, Michoacán, Mexico; 3 Instituto de Biología, Universidad Nacional Autónoma de México, Mexico; 4 Current address: Secretaría del Medio Ambiente y Recursos Naturales, Mexico; *Author for correspondence (e-mail: pbalvane@ate.oikos.unam.mx) Received 22 May 2001; accepted in revised form 25 August 2002 Key words: Dominance/diversity, Elevation gradient, Moisture gradient, Species diversity, Stand structure, Woody debris Abstract We document spatial changes in species diversity, composition, community structure, and mortality of trees across a gradient of water availability in a tropical dry forest in western Mexico. This gradient occurs along the main stream of a small watershed of less than 1 km in length. Four 30 × 80 m plots were established systematically to include the driest (ridge top of the watershed) to the wettest sites (watershed bottom) within this watershed. All stems larger than 5 cm were identified, and measured for diameter and height. Dead stems larger than 5 cm were measured and classified as: a) found on live or dead trees, and b) standing (snags) or lying (downlogs) on the ground. The number of recorded species per plot declined from 73 to 44 species as water availability decreased. A decline in estimated total richness, and in Shannon-Wiener and Simpson diversity indices was also observed in the drier plots. Species composition strongly changed along the gradient, with the two ends of the gradient shar- ing only 11% of the species. Stem density and percentage of dead stems and trees increased in abundance and basal area from the wetter to the drier sites. Tree and stem size (basal area, height and stem diameter) showed the opposite trend. Nonetheless, total basal area of live trees was largest at the two end gradient locations and os- cillated between 12.22 m 2 ha -1 and 7.93 m 2 ha -1 . Proportion of snags increased towards the driest site (from 46 to 72%), while that of down logs decreased. Overall, our results suggest that small-scale gradients of water avail- ability play a paramount role in the spatial organization of tree communities in seasonal tropical environments. Introduction The unique vegetation structure found in tropical dry forest (TDF) communities has evolved under a pecu- liar habitat, one with two to six months of severe to absolute drought (Mooney et al. 1995). In such a dras- tic environment, soil water availability appears to be the main limiting resource to plant growth and sur- vival, influencing individual species performance, distribution (Borchert 1994) and phenology (Bullock and Solis-Magallanes 1990; Olivares and Medina 1992; Borchert 1994). Within a biogeographical context, it has been shown that community attributes of TDF trees, such as species diversity, species dominance, species com- position, tree density, biomass, and canopy cover and biomass, vary with climatic, edaphic and topographic variables associated with water availability. Thus, a positive relationship between species diversity and to- tal annual precipitation has been found for TDF in Mexico and other Neotropical regions (Gentry 1995; Trejo 1998). Differences in species composition and dominance/diversity in TDF have been related to cli- matic, topographic and edaphic (water availability re- lated variables) factors in Kenya, Senegal, Brazil, and 259 Plant Ecology 169: 259–271, 2003. © 2003 Kluwer Academic Publishers. Printed in the Netherlands.