[Show abstract][Hide abstract] ABSTRACT: The Janzen–Connell hypothesis proposes that specialized herbivores maintain high numbers of tree species in tropical forests by restricting adult recruitment so that host populations remain at low densities. We tested this prediction for the large timber tree species, Swietenia macrophylla, whose seeds and seedlings are preyed upon by small mammals and a host-specific moth caterpillar Steniscadia poliophaea, respectively. At a primary forest site, experimental seed additions to gaps – canopy-disturbed areas that enhance seedling growth into saplings – over three years revealed lower survival and seedling recruitment closer to conspecific trees and in higher basal area neighborhoods, as well as reduced subsequent seedling survival and height growth. When we included these Janzen–Connell effects in a spatially explicit individual-based population model, the caterpillar's impact was critical to limiting Swietenia's density, with a >10-fold reduction estimated at 300 years. Our research demonstrates the crucial but oft-ignored linkage between Janzen–Connell effects on offspring and population-level consequences for a long-lived, potentially dominant tree species.This article is protected by copyright. All rights reserved.
[Show abstract][Hide abstract] ABSTRACT: Questions: How do frequent disturbances of varying intensity affect heteroge-neity 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 fre-quent 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 m 2 , 3.3% of the forest) than the 1995 survey (33 m 2 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 commu-nity similarity (>90%) between gaps and non-gaps. Conclusions: In ecosystems with small canopy gaps resulting from frequent dis-turbance, 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.
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