Data from: Effects of species diversity on fine root productivity increase with stand development and associated mechanisms in a boreal forest

There is a growing interest in understanding the relationship between diversity and below-ground productivity due to the critical contribution of below-ground systems to overall terrestrial productivity. Yet, the temporal (seasonal and developmental) changes in diversity effects on below-ground productivity and their underlying mechanisms remain unclear. We hypothesized that (i) diversity effects on fine root productivity increase with stand development, and (ii) increased diversity effects associated with stand development result from augmented horizontal soil space utilization, increased forest floor depth for rooting, enhanced effects in nutrient-poor soil layers and/or foraging towards high nutrient availability. We investigated the effects of tree species diversity on fine root productivity by sampling 18 stands dominated by single species and their mixtures in post-fire boreal forests of two stand ages (8 and 34 years following stand-replacing fire). Species evenness was significantly higher in species mixtures than in single-species-dominated stands at both age classes, while species richness did not differ across stand types and age classes. We found that the annual fine root production was higher in mixtures than the mean of single-species-dominated stands in both stand ages, with a significantly higher magnitude of effects in the 34-year-old than 8-year-old stands. Mixtures had higher horizontal soil volume filling than single-species-dominated stands with a more pronounced increase in the 34-year-old than 8-year-old stands. Compared with the 8-year-old stands, the 34-year-old stands had increased forest floor depth and greater overyielding with soil depth, and their fine root productivity was more responsive to the vertical variation in soil phosphorus concentrations among soil layers. Synthesis. Our results provide evidence for increasing positive diversity effects on fine root productivity with stand development in heterogeneous natural forests. Moreover, our results indicate that the increased positive diversity effects with stand development was the result of multiple mechanisms, including higher horizontal soil volume filling, a thicker forest floor layer for rooting, a higher magnitude of complementarity in nutrient-poor deep soil layers and stronger nutrient foraging towards soil layers with high nutrient concentrations in older than younger stands.由于地下生态系统对陆地生态系统整体生产力的关键支撑作用，学界对植物多样性与地下生产力之间关联的研究兴趣与日俱增。然而，多样性对地下生产力的影响随时间（季节与林分发育（stand development）进程）的变化规律及其内在机制仍未明晰。 本研究提出两项核心假设：其一，植物多样性对细根生产力（fine root productivity）的正向效应随林分发育进程而增强；其二，伴随林分发育产生的多样性效应提升，源于以下多重机制：土壤水平空间利用程度提高、可扎根的枯落物层（forest floor）厚度增加、贫瘠土层中的互补效应增强，以及对高养分有效性土壤区域的觅食能力提升。 本研究以两场火后北方针叶林（boreal forests）的两个林分年龄（林分更替火灾（stand-replacing fire）后8年和34年）为研究对象，共设置18个单优林分及其混交林样地，以此探究树木物种多样性对细根生产力的影响。在两个年龄组的样地中，混交林的物种均匀度均显著高于单优林分，但物种丰富度在不同林分类型与年龄组间均无显著差异。 研究结果显示，两个林分年龄的混交林年细根生产量均高于对应单优林分的平均值，且34年生林分中的多样性效应强度显著高于8年生林分。混交林的土壤水平空间填充量高于单优林分，且34年生林分的该指标提升幅度较8年生林分更为显著。与8年生林分相比，34年生林分的枯落物层厚度更大，且随土壤深度增加的超产效应（overyielding）更显著；同时其细根生产力对不同土层间土壤磷浓度的垂直变化响应更强。 综合分析表明，本研究结果证实，在异质性天然林中，植物多样性对细根生产力的正向效应随林分发育进程而增强。进一步研究发现，伴随林分发育产生的多样性正向效应提升源于多重机制：包括更高的土壤水平空间填充量、更厚的可扎根枯落物层、贫瘠深层土壤中更强的互补效应，以及相较于幼龄林，老龄林对高养分浓度土层更强的养分觅食能力。