Biodiversity and climate determine the functioning of Neotropical forests

Tropical forests account for a quarter of the global carbon storage and a third of the terrestrial productivity. Few studies have teased apart the relative importance of environmental factors and forest attributes for ecosystem functioning, especially for the tropics. This study aims to relate aboveground biomass (AGB), biomass dynamics (i.e., net biomass productivity and its underlying demographic drivers: biomass recruitment, growth and mortality) to forest attributes (tree diversity, community-mean traits, and stand basal area) and environmental conditions (water availability, soil fertility and disturbance).We used data from 26 sites, 201 one-ha plots and 92,000 trees distributed across the Neotropics. We quantified for each site water availability and soil total exchangeable bases and for each plot three key community-weighted mean functional traits that are important for biomass stocks and productivity. We used structural equation models to test the hypothesis that all drivers have independent, positive effects on biomass stocks and dynamics.Of the relationships analysed, vegetation attributes were more frequently significantly associated with biomass stocks and dynamics than environmental conditions (in 67% versus 33% of the relationships). High climatic water availability increased biomass growth and stocks, light disturbance increased biomass growth, and soil bases had no effect. Rarefied tree species richness had consistent positive relationships with biomass stocks and dynamics, probably because of niche complementarity, but was not related to net biomass productivity. Community-mean traits were good predictors of biomass stocks and dynamics.Water availability has a strong positive effect on biomass stocks and growth, and a future predicted increase in (atmospheric) drought might, therefore, potentially reduce carbon storage. Forest attributes – including species diversity and community-weighted mean traits – have independent and important relationships with AGB stocks, dynamics, and ecosystem functioning, not only in relatively simple temperate systems, but also in structurally complex hyper-diverse tropical forests.热带森林占全球碳储量的四分之一，贡献了全球三分之一的陆地生产力。此前鲜有研究厘清环境因子与森林属性对生态系统功能的相对重要性，针对热带地区的相关研究尤为匮乏。本研究旨在探究地上生物量（AGB）、生物量动态（即净生物量生产力及其核心种群驱动因子：生物量补充、生长与死亡率）与森林属性（树种多样性、群落平均性状及林分断面积）及环境条件（水分可获得性、土壤肥力与干扰）之间的关联。本研究使用了分布于新热带区的26个样点、201块1公顷样方以及92000株树木的调查数据。我们针对每个样点量化了水分可获得性与土壤总交换性碱基含量，并针对每个样方测定了3项与生物量储量及生产力密切相关的关键群落加权平均功能性状（community-weighted mean functional traits）。我们采用结构方程模型（structural equation models）验证了如下假说：所有驱动因子均对生物量储量及动态产生独立的正向影响。在本次分析的关联关系中，植被属性与生物量储量及动态的显著关联频率高于环境条件（占比分别为67%与33%）。较高的气候水分可获得性提升了生物量生长与储量，轻度干扰促进了生物量生长，而土壤碱基含量未产生显著影响。稀疏化树种丰富度与生物量储量及动态呈现稳定的正向关联，这一现象可能源于生态位互补效应，但与净生物量生产力并无关联。群落平均性状可较好预测生物量储量与动态。水分可获得性对生物量储量及生长具有显著正向影响，因此未来预测的（大气）干旱加剧或会降低区域碳储量。森林属性——包括物种多样性与群落加权平均性状——对AGB储量、动态及生态系统功能均存在独立且重要的关联，这一规律不仅存在于结构相对简单的温带生态系统，同样适用于结构复杂、物种高度丰富的热带森林。