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Most studies of the effects of fragmentation and landscape patterns on plant communities focus on particular patches and on local species richness (α-diversity), while few studies examine different patch-types at the whole landscape level and address effects on abundance and composition of species or functional groups. The present study aims to identify and characterize relationships between patch-type metrics and species density and abundance of trees using four tropical subdeciduous forest landscapes in the Yucatan Peninsula considering the entire landscape as the unit of study. Species density and abundance of different groups of tree species resulting from hierarchical clustering were related to landscape patterns of patch-types (area, edge, shape, similarity and contrast) using regression analysis and Akaike Information Criterion (AIC). The abundance of tree species in the oldest stages of succession was associated with percentage of land of a patch-type. Total area may favor the establishment of shade-tolerant tree species in the study area because as the area of forest patches increases, the area of forest interior conditions also increases. Conversely, the abundance of species at early and intermediate stages was related to total edge contrast and edge density, respectively. Fragmentation increases the proportion of edge zones of a patch-type, creating contrasting microclimate conditions that could promote the establishment of pioneer and light-demanding species. Thus, the combined effect of total area and edge length of a patch-type may enhance total tree species richness in the study area by favoring species with different life-history strategies. The appearance of area, shape, edge and contrast in most of the regression models suggests that some generalization can be made about the effects of spatial geometry of patch-types on species composition and abundance of tropical trees. Understanding associations between landscape metrics and species density and abundance of objectively derived groups or guilds of species can provide important insights on the effects of fragmentation and landscape pattern on these guilds and on overall α-diversity, as well as guidelines for their conservation and management.

We experimentally investigated interacting effects of canopy gaps, understory vegetation and leaf litter on recruitment and mortality of tree seedlings at the community level in a 20-year-old lowland forest in Costa Rica, and tested several predictions based on results of previous studies. We predicted that experimental canopy gaps would greatly enhance tree seedling recruitment, and that leaf litter removal would further enhance recruitment of small-seeded, shade-intolerant seedlings in gaps. We created a large (320–540 m2) gap in the center of 5 out of 10 40 m × 40 m experimental plots, and applied the following treatments bimonthly over a 14-month-period in a factorial, split–split plot design: clipping of understory vegetation (cut, uncut), and leaf litter manipulations (removal, addition, control). As expected, experimental gaps dramatically increased tree seedling recruitment, but gap effects varied among litter treatments. Litter addition reduced recruitment in gaps, but enhanced recruitment under intact canopy. Species composition of recruits also differed markedly between gap treatments: several small-seeded pioneer and long-lived pioneer species recruited almost exclusively in gaps. In contrast, a few medium-to-large-seeded shade-tolerant species recruited predominantly under intact canopy. Leaf litter represents a major barrier for seedling emergence and establishment of small-seeded, shade-intolerant species, but enhances emergence and establishment of large-seeded, shade-tolerant species, possibly through increased humidity and reduced detection by predators. Periodic clipping of the understory vegetation marginally reduced tree seedling mortality, but only in experimental gaps, where understory vegetation cover was greatly enhanced compared to intact canopy conditions. Successful regeneration of commercially valuable long-lived pioneer trees that dominate the forest canopy may require clear-cutting, as well as weeding and site preparation (litter removal) treatments in felling clearings. Management systems that mimic natural canopy gaps (reduced-impact selective logging) could favor the regeneration of shade-tolerant tree species, potentially accelerating convergence to old-growth forest composition. In contrast, systems that produce large canopy openings (clear-cutting) may re-initiate succession, potentially leading to less diverse but perhaps more easily managed “natural plantations” of long-lived pioneer tree species.