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）循环调控至关重要，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，分类与功能组成、林分结构以及环境梯度均会影响AGB的空间变异，但在温带森林中，尚未有研究探讨这些驱动因子在景观尺度下的相对影响强度。此外，当生物多样性提升碳储量时，其作用途径究竟是通过质量比效应（mass-ratio effects，即群落中的优势性状调控AGB），还是生态位互补（niche complementarity，即因种间资源分化而提升AGB），目前仍不明确。
2. 为阐明上述作用机制，我们对宾夕法尼亚州西南部900公顷温带落叶阔叶林内采样得到的28262株成年树木的普查数据进行了分析。我们依托4种关键植物功能性状数据，旨在解答两个核心问题：（1）物种多样性与AGB之间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，我们还尝试（3）识别该景观尺度下影响AGB的林分物理结构属性与地形变量。
3. 研究结果显示，AGB与物种丰富度呈正相关，与物种均匀度呈弱负相关，而功能多样性指数对AGB无显著影响。在具备更高最大树高、更深最低扎根深度以及更大种子相关性状的群落中，AGB水平更高。尤为关键的是，AGB较高的样区以糖枫（Acer saccharum）和北美鹅掌楸（Liriodendron tulipifera）为优势种。总体而言，上述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：在该温带落叶阔叶林的景观尺度（<900公顷）下，林分结构、地形以及物种与功能组成是影响AGB的关键驱动因子，而分类多样性与功能多样性并非核心驱动因素。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的经营目标可能颇具挑战。此外，本研究结果还显示，树木生物多样性丧失对地上碳库的影响，在很大程度上取决于所丧失的物种的种类。