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Several ecological and biogeochemical processes in mangroves are influenced by crab activities through bioturbation, leaf litter consumption, or fecal deposition. Bioturbation might be influenced by crab burrow dynamics or surrounding abiotic and biotic factors or both. Therefore, it is necessary to examine burrow dynamics under different abiotic and biotic conditions. This study was conducted in the estuarine mangroves of Fukido River in the northern part of Ishigaki Island, Japan. Two transects consisting of five and seven 14-m diameter circular plots were established perpendicular to the mangrove/ocean interface. Spatial and seasonal burrow dynamics data were collected from five randomly chosen quadrates within each plot. Burrow density, renewed burrows per day, and collapsed burrows per day significantly varied spatially and seasonally. Burrow density strongly depended on sediment bulk density. Numbers of renewed and collapsed burrows per day were the same (R2 = 0.86) though they significantly differed among the four seasons with being lowest in the winter season. This suggests that temperature, precipitation and crabs occurrence might control seasonal burrow dynamics. Proportion of collapsed burrows were strongly and significantly correlated to sediment bulk density, salinity and sediment temperature, which suggests that sediment properties provide structural support for burrows maintenance. On the other hand, vegetation cover and sediment characteristics influence the proportion of renewed burrows. Burrow depth was significantly controlled by sediment particle size, which suggests that crabs do not create deeper burrow if sediments contain higher amount of coarse sand. Entrance diameter was correlated to percentage organic matter, salinity, fine sand and distance from the river that means seaward sites are occupied by smaller and shallow burrows. Our results suggest that the burrow dynamics depend on abiotic and biotic factors and are useful for understanding spatial and seasonal variations for estimating bioturbation effects on mangroves biogeochemical cycle.

Mangroves aboveground biomass (ABG) estimation at a large scale is a crucial to understand their role in mitigating climate change. The large-scale estimation of AGB is generally conducted with remote sensing techniques using stand characteristics such as canopy height and species composition. In the present study, we employed Lorey's mean canopy height (Hm) and basal area weighted mean wood density (ρm) to develop models for estimating AGB of mangroves in Asian regions such as Indonesia, Philippines and Japan, considering its application toward large-scale AGB estimation covering different climatic zones of subtropical zone in the Eastern Asia and tropical zone in the Southeastern Asia. The best power function model for AGB estimation with Hm as a single explanatory variable was selected based on AIC ranks. The Hm – AGB relationship model varied significantly among study sites. We successfully developed common allometric model to estimate AGB for closed-canopy mangroves in the Asian regions. The common allometric model of the Hm – AGB relationship tended to show underestimation for old growth mangroves having AGB > ca. 400 Mg ha−1. The common allometric model showed a similar trend with a previously developed model for mangroves in Bangladesh. On the other hand, the common allometric model for mangroves showed distinct difference from the model for terrestrial tropical forests, i.e. the range of Hm was narrower in mangroves than in terrestrial tropical forests, and AGB was quite higher in mangroves than in terrestrial tropical forests at a same range of Hm. The difference of AGB specific to Hm was explained by higher ρm and cumulative basal area (BA) in mangroves than that in terrestrial tropical forests. In particular, mangrove showed quite high BA in a comparison with terrestrial tropical forests. This study confirmed that developing mangrove specific biomass model is important since mangroves showed its unique characteristics through comparisons with terrestrial tropical forests, and limitations of the use of Hm as a single variable. It should be noted that old growth mangrove AGB is underestimated with the developed common allometric models. For old growth mangroves, carbon stock should be fairly evaluated with more accurate models otherwise their contribution to blue carbon would be overlooked, which could lead insufficient efforts for conservation of old growth mangroves. Further accurate models for estimating mangrove AGB at large scale are urgently required.

Organic matter fluxes into marginal wetland areas require to be quantified given their support for the detrital components. Following their study for 5 years in a mangrove ( Kandelia obovata ) stand near the northernmost limit of the species’ distribution in East Asia, leaf and stipule litterfalls exhibited a clear monthly pattern, with maximum values in July and minimum values in February. In this region, mangroves experience a distinct seasonal climate characterised by low temperatures and frequent typhoons. Propagule litterfall was largest in April and May, whereas no propagule litterfall occurred in summer. Kendall’s coefficient of concordance revealed that the monthly trends of litterfall did not significantly differ among years (branch, p p −2 yr −1 , of which leaf litterfall constituted 49.3–67.9%. Higher annual total litterfall was caused by high production of branch litterfall during typhoons. Stipule litterfall (an indicator of new-leaf flush) decreased with increasing levels of flower/fruit and propagule litterfalls and mean annual leaf litterfall per tree decreased with increasing tree density among plots. Annual leaf litterfall per plot was almost constant regardless of tree density and mean leaf longevity was estimated to be 12.04 ± 0.04 month.