Data from: Aboveground biomass is driven by mass-ratio effects and stand structural attributes in a temperate deciduous forest

1.Forest ecosystems are critical for the global regulation of carbon (C), a substantial portion of which is stored in aboveground biomass (AGB). While it is well understood that taxonomic and functional composition, stand structure, and environmental gradients influence spatial variation in AGB, the relative strengths of these drivers at landscape-scales has not been investigated in temperate forests. Furthermore, when biodiversity enhances C storage, it is unclear whether it is through mass-ratio effects (i.e., the dominant trait in communities regulates AGB) or through niche complementarity (i.e., increased AGB due to interspecific resource partitioning).

2.To address these mechanisms, we analyzed data from a census of 28,262 adult trees sampled across 900 ha of temperate deciduous forest in southwestern Pennsylvania. We used data on four key plant functional traits to determine if (1) there is a positive relationship between species diversity and AGB and (2) whether this is due to mass-ratio effects or niche complementarity. We also sought to (3) identify the physical stand structural attributes and topographic variables that influence AGB across this landscape.

3.We found AGB was positively related to species richness and negatively related to species evenness, albeit weakly, while functional diversity indices had neutral effects. AGB was enhanced in communities dominated by traits related to greater maximum tree height, deeper minimum rooting depths and larger seeds. Most importantly, areas with high AGB were dominated by Acer saccharum and Liriodendron tulipifera. Overall, these results support mass-ratio effects, with little evidence for niche complementarity.

4.Synthesis: Stand structure, topography, and species and functional composition, but not taxonomic or functional diversity, were found to be key drivers of AGB at landscape-scales (<900 ha) in this temperate deciduous forest. Our findings suggest that simultaneously managing for both high diversity and for aboveground C storage may prove challenging in some forest systems. Our results further indicate that the impact of tree biodiversity loss on aboveground C stocks will depend greatly on the identity of the species that are lost.

1. 森林生态系统对全球碳（C）调控至关重要，其中相当一部分碳储存在地上生物量（aboveground biomass, AGB）中。目前学界已明确，物种分类与功能组成、林分结构以及环境梯度会影响地上生物量的空间变异，但在景观尺度下，这些驱动因子的相对强度尚未在温带森林中得到研究。此外，当生物多样性提升碳储量时，其作用机制尚不明确：是通过质量比效应（mass-ratio effects，即群落中的优势性状调控地上生物量），还是通过生态位互补（niche complementarity，即种间资源划分导致地上生物量增加）。
2. 为厘清上述作用机制，我们分析了美国宾夕法尼亚州西南部900公顷温带落叶林中的28262棵成年树木普查数据。我们基于4种关键植物功能性状（plant functional traits），开展了两项研究：(1) 物种多样性与地上生物量是否存在正相关关系；(2) 该相关关系是否由质量比效应或生态位互补所驱动。此外，我们还试图(3) 识别影响该景观尺度下地上生物量的林分物理结构属性与地形变量（topographic variables）。
3. 研究结果显示，地上生物量与物种丰富度呈正相关，与物种均匀度呈弱负相关，而功能多样性指数（functional diversity indices）则无显著影响。群落中优势性状若与更高的最大树高、更深的最低生根深度以及更大的种子相关，则地上生物量会得到提升。最为关键的是，地上生物量较高的区域以糖槭（Acer saccharum）和鹅掌楸（Liriodendron tulipifera）为优势物种。综上，本研究结果支持质量比效应，几乎未发现生态位互补的相关证据。
4. 综合分析：在该温带落叶林的景观尺度（<900公顷）下，林分结构、地形以及物种与功能组成是地上生物量的关键驱动因子，而物种分类多样性与功能多样性则并非核心驱动因素。本研究结果表明，在部分森林生态系统中，同时兼顾高生物多样性与地上碳储量管理可能颇具挑战。此外，树木生物多样性丧失对地上碳库的影响，将在很大程度上取决于所丧失物种的具体种类。