Data from: Does a facultative mutualism limit species range expansion?

The availability and quality of mutualists beyond a species’ range edge may limit range expansion. With the legume Chamaecrista fasciculata, we asked to what extent the availability and quality of rhizobia beyond the range edge limits host range expansion. We tested the effect of rhizobia availability on plant growth by transplanting seed from three locations into five sites spanning C. fasciculata’s range (interior, at the northern and western range edges, and beyond the range edges), and inoculating half the seeds with rhizobia. We recorded growth of all surviving plants, and for the uninoculated plants, whether they had formed nodules or not. We isolated rhizobia from nodules collected on the uninoculated plants, and cross-inoculated seed from four populations (both range edge and interior populations) in the greenhouse to determine whether the quality of rhizobia differed between regions. We found that seeds transplanted beyond the range edge were less likely to be nodulated when they were not experimentally inoculated, and there was benefit to inoculation at all sites. In the greenhouse, the three inocula that formed nodules on plants, from the range interior, northern edge and beyond the northern edge, did not detectably differ in their effect on plant growth. These results suggest that low densities of suitable rhizobia beyond the range edge may limit range expansion of legume species.物种分布范围边缘以外的互惠共生生物的可获得性与质量，可能会限制该物种的分布范围扩张。本研究以豆科植物显花决明（Chamaecrista fasciculata）为研究对象，探讨了分布范围边缘以外的根瘤菌（rhizobia）可获得性与质量，在多大程度上限制了宿主的分布范围扩张。我们通过将三个地理种群的种子移栽至覆盖显花决明分布范围的五个样地中（包括分布区内部、北部与西部分布边缘，以及分布范围以外区域），并对半数种子进行根瘤菌接种处理，以此测试根瘤菌可获得性对植物生长的影响。我们记录了所有存活植株的生长状况，并针对未接种处理的植株，统计其是否形成根瘤。我们从未接种植株所结的根瘤中分离得到根瘤菌，并在温室中对四个种群（涵盖分布边缘与分布内部种群）的种子进行交叉接种实验，以明确不同区域根瘤菌的质量是否存在差异。研究发现，未经过人工接种、移栽至分布范围以外区域的种子，其形成根瘤的概率更低，且在所有样地中，人工接种根瘤菌均能为植株带来生长益处。温室实验结果显示，从分布区内部、北部边缘以及北部边缘以外区域分离得到的、可成功形成根瘤的三株根瘤菌接种物，其对植物生长的影响未检测到显著差异。上述结果表明，分布范围以外区域适宜根瘤菌的低密度水平，可能会限制豆科植物的分布范围扩张。