Reducing dispersal limitation via seed addition increases species richness but not aboveground biomass

We searched for seed addition studies where the experiment included: 1) an unmanipulated control and a treatment of diverse seed mixtures selected from the regional species pool, (2) the number and identity of every species added as seed, and (3) measured species richness and the relative contribution of each species to community biomass (that were not weeded or otherwise altered). Because these criteria were relatively stringent, we did not restrict ourselves to formally standardized literature searches, but rather attempted to find as many datasets as possible that met our search criteria.<br>We began by consulting published meta-analyses , and other experimental data sources (e.g., U.S. Long Term Ecological Research [LTER] websites). We then used different combinations of standardized search strings including terms related to biodiversity (‘richness’, ‘diversity’), biomass (‘ecosystem function’, ‘above-ground biomass’) and seed addition (‘propagule arrival’, ‘BEF experiment’, ‘seeding’). For each experiment identified, we scanned the paper to determine whether it met our inclusion criteria. For some studies, we were able to download or extract data directly. For the rest, we contacted authors to request species-level data and other needed information (i.e. seeded richness).<br>We found a total of 12 studies that tested the effect of seed addition on species richness and biomass in grassland habitats, where the identity and number of seeds per species added was known ( see Table 1, Table S1, Figure S1 in paper). In all studies, species added via the seed addition treatments were part of the broader regional species pool, and were added to each community through a high density of seed mixes added to treatment (but not control) plots .<br>Several experiments included treatments other than the primary focus of this study (e.g., disturbance, nutrient addition), which we omitted from all analyses. We also removed treatments with unknown richness of added seeds (e.g., hay). This led to some experiments having an unbalanced design between controls and treatments. Community level biomass was sometimes measured directly within plots, and other times estimated from percent cover or coarser samples (e.g., biomass strip) (Table S1). Where biomass was measured indirectly (sampled on smaller subplots), the percent cover of each species was divided by the plot percent cover and multiplied by the community biomass (g/m2) sample from a neighboring plot or biomass strip with the same treatment, and per species biomass estimates were then summed to the plot level . We standardized data for each experiment in a hierarchical experimental design: plots nested in blocks, nested in sites. The experiments varied in the number of species added (hereafter seeded richness), the density of seeds added, and length of experiment . Combined, the study-level treatments resulted in a ‘seeded richness gradient’ ranging from one to sixty species .<br>These data were derived from existing studies cited in the paper in Table 1, but as methods indicate these data posted here contain only control and treated plots. Please contact original authors for use of these data, or for entire datasets relating to these studies. Cite original papers when using these data.

我们针对种子添加实验开展了文献检索，筛选标准如下：1）设置未施加任何处理的对照组，以及从区域物种库（regional species pool）中选取的多样种子混合物处理组；2）明确每个被添加物种的数量与身份；3）测定了物种丰富度及各物种对群落生物量的相对贡献，且受试群落未经过除草或其他人为改造。由于本次筛选标准较为严格，我们并未局限于正式标准化的文献检索流程，而是尽力搜寻所有符合检索条件的数据集。
我们首先查阅了已发表的元分析（meta-analysis）及其他实验数据来源（如美国长期生态研究（Long Term Ecological Research, LTER）网站）。随后，我们使用多组标准化检索式进行组合检索，检索词涵盖与生物多样性相关的“丰富度（richness）”“多样性（diversity）”、与生物量相关的“生态系统功能（ecosystem function）”“地上生物量（above-ground biomass）”以及与种子添加相关的“繁殖体引入（propagule arrival）”“BEF实验（Biodiversity-Ecosystem Function experiment）”“播种（seeding）”等术语。针对每一个筛选出的实验，我们通读其论文以确认是否符合纳入标准。部分研究的数据可直接下载或提取；对于其余研究，我们联系原作者以获取物种水平的数据及其他所需信息（即播种物种丰富度（seeded richness））。
我们最终共筛选得到12项符合要求的研究，这些研究均在草地生境中探究了种子添加对物种丰富度及生物量的影响，且已知每个添加物种的身份与种子数量（详见论文中的表1、表S1及图S1）。所有研究中，通过种子添加处理引入的物种均来自更广泛的区域物种库，且仅向处理组（而非对照组）样地施加高密度的混合种子。
部分实验包含了本研究核心关注点之外的处理（如干扰、养分添加），我们将此类处理从所有分析中剔除。同时，我们也剔除了添加种子丰富度未知的处理（如干草处理），这导致部分实验的对照组与处理组间出现设计不平衡的情况。群落水平的生物量有时会直接在样地内测定，有时则通过盖度或更粗粒度的采样（如生物量样带）进行估算（详见表S1）。当生物量通过间接方式测定（即于更小的亚样地采样）时，我们将每个物种的盖度除以样地总盖度，再乘以相同处理下相邻样地或生物量样带测得的群落生物量（单位：克/平方米），随后将各物种的生物量估算值汇总至样地水平。我们针对每个实验的等级试验设计对数据进行标准化处理：样地嵌套于区组，区组嵌套于样点。各实验在添加物种数量（后文简称播种物种丰富度（seeded richness））、种子添加密度及实验时长上均存在差异。综合来看，各研究的处理设置形成了一条覆盖1至60个物种的“播种物种丰富度梯度”。
本数据集源自论文表1中引用的已有研究，但正如方法部分所述，此处发布的数据仅包含对照组与处理组样地的数据。若需使用本数据集或获取对应研究的完整数据集，请联系原作者。使用本数据集时，请引用原研究论文。