Interactions of gap size and herbivory on establishment, growth and survival of three species of neotropical pioneer trees

1 The difference in gap size requirements observed for neotropical pioneer species is a classic example of habitat partitioning among tropical tree species. Previous research has shown that the three species Miconia argentea, Cecropia insignis and Trema micrantha differ in the minimum gap size that they occupy as adult plants in semi-deciduous tropical forest in Panama. Here we test whether these differences result from variation in seed germination and seedling establishment requirements, or from differences in susceptibility to herbivores.

2 We planted seeds of Miconia argentea and Trema micrantha into artificially created small (25 m(2)), medium ( 64 m(2)) and large (225 m(2)) gaps in secondary forest, and transplanted seedlings of all three species into small and large gaps inside and outside mesh cages that excluded invertebrate herbivores.

3 Miconia seedlings emerged and established successfully across a broad range of gap sizes, and transplanted seedlings grew slowly but survived well in all gap sizes. By contrast, emergence and survival of Trema seedlings occurred only in the large gaps and transplants died in small gaps. Survival of Cecropia seedlings was intermediate between Trema and Miconia in the small gaps and growth was greater than either species in the large gaps.

4 Herbivore exclosures reduced foliar herbivory on all three species. The median lifespan of Trema seedlings in small gaps was reduced when the seedlings were exposed to herbivores, but survival of the other species was not affected by the exclosure treatment in either gap size. The growth of Miconia seedlings was lower outside exclosures in both gap sizes.

5 Differences in germination and establishment could generate the contrasting gap size distributions observed between Miconia and Trema, but they are partially offset by measured differences in seed bank density. However, patterns of seedling growth and survival reinforce differences among species at the establishment phase and provide a mechanism for the observed differences in gap size distribution at the sapling stage. 'Cross-overs' in species growth rates result in specialization to different environments and reduce their survival in unfavourable conditions. We found no evidence for a definitive influence of herbivory as a mechanism driving differences in gap size distribution among species.