Mangrove diversity enhances plant biomass production and carbon storage in Hainan Island, China

Mangrove forests, one of the highest carbon density ecosystems, are very different from other forests as they occupy saline and tidal habitats. Although previous studies in forests, shrublands, and grasslands have shown a positive effect of biodiversity on plant biomass and carbon storage, it remains unclear whether this relation to biodiversity also exists in mangrove forests. Here, we evaluate the possible effects of mangrove species diversity, structural characteristics, and environmental factors on mangrove biomass production and carbon storage, using survey data from 234 field plots of 30 transects in the mangrove forests along the coastlines of Hainan Island, China, during 2017 and 2018.We found that mangrove species diversity had a positive effect, not only on mangrove biomass production, but also on soil carbon storage. This positive effect was more strongly evident in the forest communities than in either the shrub communities or forest-shrub mixed communities, with the forests type having the biggest mangrove biodiversity and carbon storage. In addition, the diversity effect was affected by structural characteristics, namely, mangrove biomass increased exponentially with tree stem diameter and decreased with tree density. Furthermore, we observed a resource-dependent mediation of the mangrove ecosystem when linking diversity to biomass. The areas with high soil Nitrogen content and Mean annul precipitation (MAP) showed higher mangrove biomass and carbon storage. This suggests that the spatial pattern of mangrove carbon storage and diversity was driven by both climate factors (MAP) and soil fertility (soil N).Our findings suggest that mangrove forests with greater diversity also have higher carbon storage capacities and conservation potential. Thus, biodiversity conservation is crucial for mangrove to mitigate the greenhouse effect. Our findings strengthen the understanding of the diversity effects on mangrove ecosystem services and have important implications for mangrove restoration and conservation.

Methods
We collected data based on an intensive field survey from 234 field plots of 30 transects in the mangrove forests along the coastlines of Hainan Island, China, during 2017 and 2018.

We calculated the mangrove species diversity, structural parameters, mangrove biomass production, and carbon storage based on the breast-height diameter, species, height, tree density from field plots.

红树林（Mangrove forests）是全球碳密度最高的生态系统之一，与其他森林类型迥异，因其栖息于盐渍化潮汐生境中。尽管此前针对森林、灌丛与草原的研究已证实生物多样性对植物生物量及碳储量存在正向效应，但该类生物多样性关联是否同样适用于红树林仍未明确。本研究依托2017-2018年中国海南岛沿岸红树林30条样带共234个样地的调查数据，评估红树林物种多样性、结构特征及环境因子对红树林生物量生产与碳储量的潜在影响。

研究结果表明，红树林物种多样性不仅对红树林生物量生产存在正向作用，同时也能提升土壤碳储量。该正向效应在乔木群落中表现得更为显著，优于灌丛群落与乔灌混合群落，其中纯乔木群落的红树林生物多样性与碳储量均为最高。此外，多样性效应受群落结构特征调控：红树林生物量随树木胸径呈指数增长，却随树木密度升高而降低。进一步研究发现，在将多样性与生物量关联分析时，红树林生态系统存在资源依赖型中介效应。土壤全氮含量与年平均降水量（Mean Annual Precipitation, MAP）较高的区域，其红树林生物量与碳储量也更高。这表明红树林碳储量与多样性的空间格局同时受气候因子（MAP）与土壤肥力（土壤全氮）调控。

本研究结果显示，物种多样性更高的红树林具备更强的碳储能力与保护潜力。因此，生物多样性保护对于红树林缓解温室效应至关重要。本研究深化了对生物多样性效应影响红树林生态系统服务功能的认知，可为红树林修复与保护工作提供重要理论参考。

研究方法
本研究依托2017-2018年对中国海南岛沿岸红树林开展的密集野外调查，共设置30条样带、234个样地，并据此收集相关数据。研究人员基于样地实测的胸径、物种组成、树高及树木密度，计算得到红树林物种多样性、群落结构参数、生物量生产及碳储量。