The effect of typhoon-related defoliation on the ecology of gap dynamics in a subtropical rain forest of Taiwan

Questions: How do frequent disturbances of varying intensity affect heterogeneity of canopy openness and resulting regenerating tree diversity in subtropical rain forests? How does canopy gap pattern and regeneration time of a forest experiencing frequent but unpredictable typhoon disturbance vary over 17 yr? How is the understorey light environment influenced by canopy gaps in a forest experiencing frequent defoliation? Compared to non-gap areas, do gaps provide unique niches reflected in different seedling communities? Location: Natural evergreen hardwood forest, northeastern Taiwan. Methods: We examined gap characteristics, light availability and variability, and seedling diversity in both gaps and non-gaps in a subtropical forest with frequent typhoon disturbance, and compared the results to a survey at the same site conducted in 1995 following six typhoons in 1994. Results: Gaps were smaller and covered a much smaller proportion of the forest in the 2012 survey (mean gap size 10 m², 3.3% of the forest) than the 1995 survey (33 m² and 10%). Canopy regeneration times estimated from the 2012 survey (200-400 yr) were much longer than those based on the 1995 survey (50-175 yr). Neither light availability nor variability differed between gaps and non-gaps, so gaps ostensibly did not provide a unique niche space. There were no seedling species occurring exclusively in gaps, and very high plant community similarity (>90%) between gaps and non-gaps. Conclusions: In ecosystems with small canopy gaps resulting from frequent disturbance, gaps play a neutral role in understorey light environment and thus provide no unique niches and play a neutral role in plant diversity. Our study also suggests that measures of gap area, occurrence and forest turnover should be taken multiple times, during years with varying background disturbance rates.