Size of sampling unit strongly influences detection of seedling limitation in a wet tropical forest.

Seedling limitation could structure communities, but often is evaluated with sampling units that are orders of magnitude smaller than mature plants. We censused seedlings for 5.5 years in five 1 x 200-m transects in a wet Neotropical forest. For 106 common species (> or = 10 seedlings in a transect), we calculated prevalence (occurrence of > or = 1 newly emerged seedlings per sampling unit) at 1 m(2) and at 1 m x mature crown diameter units by aggregating adjacent quadrats. For most species, prevalence was 2-25% at 1 m(2), but 20-92% at mature crown scales. Increased prevalence arose from broadly distributed seedlings within transects, with unoccupied segments generally shorter than crown diameters. At the landscape scale, 69% of 301 species were locally rare (< 10 seedlings) and only 16% were represented in all transects (maximally separated by 2.4 km). Nonetheless, for more common species, much lower estimates of seedling limitation at mature crown scales suggest weaker influence of seedling limitation on community dynamics than previously assumed.