Numerous mechanisms have been proposed to explain the island species–area relationship (ISAR), including the passive sampling effect, the habitat diversity effect, ecological drift and a group of mechanisms jointly termed environmental filtering (or abiotic selection process). However, determining which of these hypotheses underpins the ISAR in a given system remains an open and controversial question, particularly with regards to the environmental filtering. The effect of environmental filtering on the ISAR for woody plants was tested on 29 islands in a subtropical land‐bridge archipelago. Island area had a significant effect on species richness, while isolation was not found to have an effect. After controlling for the passive sampling effect and habitat diversity using a systematic sampling design, we found that a positive ISAR remained. After controlling for island area and isolation, species richness was significantly affected by environmental conditions (e.g. total phosphorus, soil depth and bulk density). When a null model was used that randomly assigned individuals to islands from the regional pool, the difference between the observed and expected richness was greater for smaller islands relative to larger islands. Synthesis. Our results point to an overlooked role of environmental filtering in driving the island species–area relationship (ISAR). Consequently, our study highlights the importance of considering deterministic process in future analyses of the ISAR.

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Microclimate and biotic factors drive wood decomposition. Yet, relatively few studies have been conducted in warmer tropical and subtropical forests compared with colder temperate and boreal forests, where climate might have stronger limitations on decomposition. Besides, conclusions on decomposition are generally based on single wood species or standardized materials. Thus, a general assessment if wood species identity alters dominant factors of decomposition in subtropical forest could improve the predictability of current C cycle models. We measured fine wood massloss rates of seven widespread tree species in southeast China at two time steps (after 1 and 2 years) in subtropical plantation forests with tree species diversity manipulated. We found that termites exceeded fungi in consuming fine woody debris, and termite foraging intensity decreased with lignin content. Tree species diversity increased fungal hyphal coverage on fine wood samples and had different effects on wood mass-loss rates depending on wood species. Higher minimum temperature and amplitude of relative humidity suppressed termite foraging intensity, with humidity having inconsistent effects on mass-loss rates among wood species. Interestingly, microclimate outweighed biotic factors for Castanopsis eyrei but not for the other species. Three traits, that is, wood density, phenolics, and lignin content, were higher than the average for C. eyrei, making it the least attractive species for decomposers. Our findings suggest that species identity influences the effects and relative importance of biotic factors and microclimate for wood decomposition in subtropical forests. Furthermore, species identity effects are possibly mediated by multiple wood traits.

Island biogeography theory describes the relationship between island size, isolation and biodiversity, but it does not address the effects on ecosystem processes such as wood decomposition. Wood decomposition rates can be expected to increase with decomposer diversity, which increases with island size and decreases with increasing island isolation. However, island size and isolation effects on wood decomposition may also act by impacting substrate composition, forest composition and biotic interactions among decomposers. In this study, branch‐wood decomposition rates of three locally dominant tree species deployed at 154 plots on 15 islands in Thousand Island Lake, China, were measured over one year. The results showed that decomposition rates in coarse‐mesh bags where both termites and fungi were allowed access increased with island size for the two tree species preferred by the termites. By contrast, decomposition rates in fine‐mesh bags which only fungi could access decreased with island size for the two tree species preferred by fungi. The positive effects of island size on termite feeding activity were mediated by a higher deadwood diversity, a more diverse forest composition and a higher soil moisture content, with forest composition shaping soil moisture. For fungal diversity, the negative effects of island size were attributable to a higher soil moisture content and termite feeding activity. Island isolation decreased fungal diversity but increased wood decomposition rates. Our study indicates that island size affects wood decomposition by altering the distribution of decomposers and their relative contributions to wood decomposition. Size–decomposition relationship further depends on the affinity between substrates and decomposer taxa. In general, our results show that ecosystem processes may be affected by island size and isolation, but the patterns are more complex than expected from the island biogeography theory. This discrepancy is largely due to the different responses and trade‐offs among different functional groups.

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