Effects of enemy exclusion on biodiversity-productivity relationships in a subtropical forest experiment

Interspecific niche complementarity is a key mechanism posited to explain positive species richness-productivity relationships in plant communities. However, the exact nature of the niche dimensions that plant species partition remains poorly known.Species may partition abiotic resources that limit their growth, but species may also be specialized with respect to their set of biotic interactions with other trophic levels, in particular with enemies including pathogens and consumers. The lower host densities present in more species-diverse plant communities may therefore result in smaller populations of specialized enemies, and in a smaller associated negative feedback these enemies exert on plant productivity.To test whether such host density-dependent effects of enemies drive diversity-productivity relationships in young forest stands, we experimentally manipulated leaf fungal pathogens and insect herbivores in a large subtropical forest biodiversity-ecosystem functioning experiment in China (BEF-China).We found that fungicide spraying of tree canopies removed the positive tree-species richness-productivity relationship present in untreated control plots. The tree species that contributed the most to this effect were the ones with the highest fungicide-induced growth increase in monoculture. Insecticide application did not cause comparable effects. Synthesis. Our findings suggest that tree species diversity may not only promote productivity by interspecific resource-niche partitioning but also by trophic niche partitioning. Most likely, partitioning occurred with respect to enemies such as pathogenic fungi. Alternatively, similar effects on tree growth would have occurred if fungicide had eliminated positive effects of a higher diversity of beneficial fungi (e.g. mycorrhizal symbionts) that may have occurred in mixed tree species communities.

Multiple stages of tree seedling recruitment are altered in tropical forests degraded by selective logging

Tropical forest degradation is a global environmental issue. In degraded forests, seedling recruitment of canopy trees is vital for forest regeneration and recovery. We investigated how selective logging, a pervasive driver of tropical forest degradation, impacts canopy tree seedling recruitment, focusing on an endemic dipterocarp Dryobalanops lanceolata in Sabah, Borneo. During a mast-fruiting event in intensively logged and nearby unlogged forest, we examined four stages of the seedling recruitment process: seed production, seed predation, and negative density-dependent germination and seedling survival. Our results suggest that each stage of the seedling recruitment process is altered in logged forest. The seed crop of D. lanceolata trees in logged forest was one-third smaller than that produced by trees in unlogged forest. The functional role of vertebrates in seed predation increased in logged forest while that of non-vertebrates declined. Seeds in logged forest were less likely to germinate than those in unlogged forest. Germination increased with local-scale conspecific seed density in unlogged forest, but seedling survival tended to decline. However, both germination and seedling survival increased with local-scale conspecific seed density in logged forest. Notably, seed crop size, germination, and seedling survival tended to increase for larger trees in both unlogged and logged forests, suggesting that sustainable timber extraction and silvicultural practices designed to minimize damage to the residual stand are important to prevent seedling recruitment failure. Overall, these impacts sustained by several aspects of seedling recruitment in a mast-fruiting year suggest that intensive selective logging may affect long-term population dynamics of D. lanceolata. It is necessary to establish if other dipterocarp species, many of which are threatened by the timber trade, are similarly affected in tropical forests degraded by intensive selective logging.