Journal of Tropical Ecology (2011) 27:35–49. Copyright © Cambridge University Press 2010 doi:10.1017/S0266467410000611 Successional trends in soil seed banks of abandoned pastures of a Neotropical dry region Susana Maza-Villalobos ∗ , Celina Lemus-Herrera† and Miguel Mart´ ınez-Ramos ∗,1 ∗ Centro de Investigaciones en Ecosistemas, Universidad Nacional Aut ´ onoma de M ´ exico, Antigua Carretera a P ´ atzcuaro # 8701, Col. Ex-Hacienda de San Jos ´ e de la Huerta, CP 58190, Morelia, Michoac ´ an, M ´ exico † Facultad de Biolog´ ıa, Universidad Michoacana de San Nicol ´ as de Hidalgo, Ciudad Universitaria Av. Fco. J. M ´ ugica S/N., CP 58030, Morelia, Michoac ´ an, M ´ exico (Accepted 18 September 2010) Abstract: A chronosequence was used to study seed-bank communities during the first 12 y of tropical-dry-forest regeneration in abandoned pastures in Chamela, Mexico. Prediction that seeds of woody species replace those of herbaceous species during succession was tested and mechanisms of species replacement (facilitation, tolerance, inhibition) were assessed. Four successional categories (three sites each) were considered: pasture (0–1 y since abandonment), early (3–5 y), intermediate (10–12 y), and old-growth forest. At the end of the dry season, 20 cylindrical soil samples (10 cm diameter, 15 cm depth) were randomly obtained within a 20 × 50-m plot in each site. Seeds ≥1 mm were counted and identified. Overall, 2941 seeds and 102 morphospecies (52 taxonomically identified) were recorded. Seed bank density reduced, species diversity remained fairly constant and seeds of herbaceous species were replaced by those of woody species over the chronosequence. A clear species-by-species replacement pattern was detected, as expected under a mechanism of succession by facilitation. Twelve years after abandonment, a diverse seed bank of woody species did exist, indicating a fast recovery of the tropical-dry-forest regenerative potential; nonetheless, the structure and composition of the seed bank was still different from that in the old-growth forest. Key Words: Chamela–Cuixmala, chronosequence, Mexico, natural regeneration, regenerative communities, secondary succession, tropical dry forest INTRODUCTION Tropical dry forests (TDF) are greatly threatened by their conversion to agricultural land, making the future of this biome largely dependent on conservation of remaining old-growth forests and regeneration and restoration of secondary forests (Dirzo et al. 2010, S ´ anchez-Azofeifa et al. 2009). This posits a challenge because understanding the ecological factors, processes and mechanisms that allow TDF regeneration is far from being complete (Quesada et al. 2009). For example, currently no more than five published studies on TDF seed-bank communities in old- fields exist (Gonz ´ alez-Rivas et al. 2009, Lemenih & Teketay 2006, Miller 1999, Rico-Gray & Garc´ ıa-Franco 1992, Vieira & Scariot 2006). Colonizing herbaceous plants are highly dependent on light to produce abundant seeds, which usually are able to stay dormant in the soil for long periods (Fenner 1985). 1 Corresponding author. Email: mmartine@oikos.unam.mx Seed bank density of such herbaceous plants is expected to decline as TDF succession advances and light resources reduce in the understorey. In contrast, seeds of most woody species lack prolonged dormancy and suffer high predation rates (Briones-Salas et al. 2006, Garwood 1989, Janzen 1981, Khurana & Singh 2001), which makes their abundance in the soil largely dependent on local seed rain ( ´ Alvarez-Buylla & Mart´ ınez-Ramos 1990, Dalling & Denslow 1998). Thus, the abundance and diversity of seeds of woody species in the soil is expected to increase as more reproductive woody plants become established during succession. The temporal changes undergone by seed-bank communities may allow the exploration of mechanisms underlying species replacement during succession (sensu Connell & Slatyer 1977). Such analysis is possible if the abundance and composition of mature plant communities are mirrored in the characteristics of the seed banks. There is some evidence indicating that this is true for woody species (Ceccon et al. 2006, Dalling & Denslow 1998). Three possible patterns can be expected to occur in