Factors limiting tropical rain forest regeneration in abandoned pasture: Seed rain, seed germination, microclimate, and soil

Vegetation, seed rain, seed germination, microclimate, and soil physical an d chemical parameters were measured in a recently abandoned pasture and adj acent primary rain forest in southern Costa Rica. The goal of this study wa s to assess the importance of these factors in limiting forest regeneration in abandoned pastures. Seed rain of anima dispersed species decreased dram atically in the pasture >5 m from the forest/pasture edge; fewer wind dispe rsed seeds fell in the pasture than in the forest, but the difference was m uch less than for anima dispersed seeds. Percent seed germination of most s pecies studied was similar in the forest and in pasture with grasses; seed germination was lower during the dry season in areas of pasture cleared of grasses. Air temperature, vapor pressure deficit (VPD), and photon flux den sity (PFD) were much higher in the pasture than in the forest at 1 m above the ground. VPD and PFD at ground level and soil temperature were similar i n the pasture and the forest, indicating that pasture grasses strongly modi fy microclimatic conditions near the soil surface. The lowest gravimetric w ater content recorded in the pasture during the dry season was 0.5 and leaf relative water contents of the two species measured in the forest and past ure were identical, suggesting that plants in the pasture were not water st ressed. Levels of most soil nutrients were lower in the pasture as compared to the forest; however, aboveground and root biomass for seedling grown in pasture and forest soils did not differ significantly. Although a number o f factors impede forest recovery in abandoned pastures, these results sugge st that the most important limitation is lack of seed dispersal.