Data from: Positive species diversity and above-ground biomass relationships are ubiquitous across forest strata despite interference from overstorey trees

1. There is growing concern over rates of global species diversity loss and its implications on healthy ecosystem functioning. While positive relationships between tree species diversity and forest biomass production have been observed, forests are structurally complex, consisting of understorey vegetation layers that also contribute to ecosystem functioning as they often account for the majority of species richness. However, relationships between understorey vegetation diversity and function are largely unexplored. Further, few studies have simultaneously assessed how both overstorey and understory vegetation interact and contribute to overall ecosystem function. 2. By analysing Canada’s National Forest Inventory database using structural equation modelling, we explored the relationships between species richness and above-ground biomass production across forest vegetation strata while accounting for potentially confounding factors, including climate, physical site characteristics, and forest ageing. 3. We found positive relationships between species richness and biomass production across all forest vegetation layers, but the relationship was strongest for the overstorey layer. Species richness of the understorey tree, shrub, and herb layers was positively related to overstorey species richness. However, overstorey biomass had a negative effect on the biomass production of all understorey layers. 4. Our results suggest that resource filtering by overstorey trees might have reduced the strength of the positive diversity-productivity relationships in the forest understorey, supporting previous hypotheses that the magnitude and direction of diversity-productivity relationships is context specific and dependent on the conditions of the surrounding environment. Further, heterogeneity in understory resources, as affected by the overstorey, may promote niche complementarity as the main mechanism driving diversity-productivity relationships in understorey vegetation.1. 全球物种多样性丧失速率及其对生态系统健康功能的影响，正日益受到学界的广泛关注。尽管已有研究证实乔木物种多样性与森林生物量生产之间存在正向关联，但森林结构极为复杂，包含林下植被层（understorey）——这类植被往往占据了群落绝大多数的物种丰富度（species richness），同样对生态系统功能发挥着重要贡献。然而，林下植被多样性与生态系统功能之间的关联尚未得到充分探索。此外，鲜有研究同时评估乔木层（overstorey）与林下植被的相互作用及其对整体生态系统功能的贡献。 2. 本研究依托加拿大国家森林清查数据库（Canada’s National Forest Inventory database），采用结构方程模型（structural equation modelling）分析方法，在控制气候、立地物理特征、林分老化等潜在混淆因子（confounding factors）的前提下，探究了不同森林植被层（strata）中物种丰富度与地上生物量生产（above-ground biomass production）之间的关联。 3. 研究结果显示，所有森林植被层的物种丰富度与生物量生产均呈正向关联，但乔木层的关联强度最高。林下乔木层、灌木层与草本层的物种丰富度与乔木层物种丰富度呈显著正相关。然而，乔木层生物量对所有林下植被层的生物量生产均存在负向影响。 4. 本研究结果表明，乔木层的资源过滤作用可能削弱了森林林下植被层中多样性-生产力关系（diversity-productivity relationships）的正向强度，这一发现支持了此前的学术假说：多样性-生产力关系的强度与方向具有情境依赖性，取决于周边环境条件。此外，受乔木层调控的林下资源异质性，可能会推动生态位互补（niche complementarity）效应——这正是驱动林下植被多样性-生产力关系的核心机制。