Multiple canopy opening effects on recruited saplings in a typhoon-disturbed tropical rainforest,Taiwan

To investigate the influence of multiple canopy openings on the composition and diversity of recruited saplings in a forest frequently disturbed by typhoons. We conducted tree-by-tree censuses (diameter at breast height ≥ 1 cm) and mapped gaps (canopy height < 5 m) in 1993, 2000, 2008, and 2013 in a tropical mountain zonal foothill evergreen broad-leaved forest in Nanjenshan Nature Reserve, Taiwan. We analyzed the composition and diversity of recruited saplings within a 2.1 ha plot (840 sampling quadrats (5 m × 5 m)) with variable numbers of canopy openings recorded during the study period. Composition of recruited saplings was dissimilar between quadrats that stayed opened and those that stayed closed throughout the study period (pairwise similarity estimates C₀₂ = 0.52, 95% CI = 0.38–0.66). The quadrats under closed canopy had high diversity when weighting rare species (species richness), whereas quadrats with one or two gap opening records during the past 20 years had high diversity when weighting the abundance of species. Although canopy openings provided establishment conditions for saplings of some shade-intolerant species, due to the nature of small gap size, such habitats do not favor the dominance of shade-intolerant species. Even in a frequently disturbed forest, species composition and richness of recruited saplings were mainly contributed by shade-tolerant species. Although multiple canopy openings facilitated the establishment of shade-intolerant species, species diversity in the study forests is possibly mainly mediated by coexistence mechanisms of those shade-tolerant species rather than light-gap-related species strategies.