Fragmentation in the tropics precludes the movement of animals and plants across the agricultural landscape. If forest species do not reach pastures, natural succession back to forest will never occur despite abandonment or exclusion of pastures. To test dispersal limitation in a fragmented landscape, 120 1-m² seed traps were located in a 3 X 8 grid of fenced 30 X 30 m plots located in active pasture separated 35 m from each other and in near primary and secondary forest in November 2006 in Los Tuxtlas, Veracruz, Mexico. The grid is on a hill along an elevation gradient from 180 to 260 m above see level where upper plots are ca. 150 m from forest.

Results/Conclusions

In the annual seed rain of pastures and forest we collected 73 893 seeds of woody plants from 57 species. The richness of species and density of seeds in the seed rain were significantly higher in the secondary forest (2.35 sp/m² and 445 seeds/m²) and forest (1.72 sp/m² and 20 seeds/m²) than in pastures (0.23 sp/m² and 1.24 seeds/m²) at any distance from the forest. The smooth curves of accumulated richness from December 2006 and 2007 showed that the seed rain in pasture showed up to two late-successional species while the seed rain in the forest and the secondary forest showing up to nine species is far from reaching an asymptote. An non-metric multidimensional scaling analysis including seed traps with more than four woody species (early and late-successionals) showed that there is a high connectivity from the forest to the secondary forest while species falling in pastures comprise a different group of species: most of the species in the seed rain of the forest and secondary forest were late-successional dispersed by animals (21 sp) while pastures received mostly pioneer species dispersed by animals (11 sp) or wind (7 sp). Natural succession in pastures back to forest will proceed at extremely slow pace due to strong and pervasive dispersal limitation of forest species. Further, if forest species attractive to animals are not available in these eroded lands, low-diversity secondary forest will prevail. Seed-dispersal limitation is a general challenge in re-vegetation of cleared agricultural land; therefore, plantations of species that attract animals are needed to reinstate the connectivity in this landscape.