Data from: Foliar damage beyond species distributions is partly explained by distance dependent interactions with natural enemies

Plant distributions are expected to shift in response to climate change, and range expansion dynamics will be shaped by the performance of individuals at the colonizing front. These plants will encounter new biotic communities beyond their range edges, and the net outcome of these encounters could profoundly affect colonization success. However, little is known about how biotic interactions vary across range edges and this has hindered efforts to predict changes in species distributions in response to climate change. In contrast, a rich literature documents how biotic interactions within species ranges vary according to distance to and density of conspecific individuals. Here, we test whether this framework can be extended to explain how biotic interactions differ beyond range edges, where conspecific adults are basically absent. To do so, we planted seven species of trees along a 450 km latitudinal gradient that crossed the current distributional range of five of these species and monitored foliar disease and invertebrate herbivory over five years. Foliar disease and herbivory were analyzed as a function of distance to and density of conspecific and congeneric trees at several spatial scales. We found that within species ranges foliar disease was lower for seedlings that were farther from conspecific adults for Acer rubrum, Carya glabra, Quercus alba, and Robinia pseudoacacia. Beyond range edges there was even less foliar disease for C. glabra, Q. alba and R. pseudoacacia (A. rubrum was not planted outside its range). Liriodendron tulipifera did not experience reduced disease within or beyond its range. In contrast, Quercus velutina displayed significant but idiosyncratic patterns in disease at varying distances from conspecifics. Patterns of distance dependent herbivory across spatial scales was generally weak and in some cases negative (i.e., seedlings further from conspecific adults had more herbivory). Overall, we conclude that differences in biotic interactions across range edges can be thought of as a spatial extension to the concept of distance dependent biotic interactions. This framework also provides the basis for general predictions of how distance dependent biotic interactions will change across range edges in other systems.植物分布预计将随气候变化发生偏移，分布区扩张动态将由定植前沿（colonizing front）的个体表现所塑造。此类植物会在分布区边缘（range edges）之外遭遇全新的生物群落（biotic communities），此类相遇的净效应可能深刻影响定植成功率（colonization success）。然而，目前学界对分布区边缘间生物相互作用（biotic interactions）的变化规律知之甚少，这阻碍了针对气候变化下物种分布变化的预测工作。与之形成对照的是，已有大量文献记录了物种分布区内的生物相互作用如何随与同种个体（conspecific individuals）的距离及种群密度发生变化。本研究旨在检验该理论框架能否被拓展，用以解释分布区边缘之外（此处同种成年个体基本缺失）的生物相互作用差异。为此，我们沿一条跨越其中5个物种种群当前分布范围的450千米纬度梯度（latitudinal gradient），种植了7个树种，并连续5年监测其叶部病害（foliar disease）与无脊椎动物植食危害（invertebrate herbivory）情况。我们以多个空间尺度下与同种及同属树木（congeneric trees）的距离和密度为自变量，分析叶部病害与植食危害的变化规律。研究结果显示：在物种分布区内，红花槭（Acer rubrum）、光滑山核桃（Carya glabra）、白栎（Quercus alba）与刺槐（Robinia pseudoacacia）的幼苗（seedlings）距同种成年个体越远，叶部病害程度越低；在分布区边缘之外，光滑山核桃、白栎与刺槐的叶部病害程度进一步降低（红花槭未被种植在其分布区外）。鹅掌楸（Liriodendron tulipifera）在其分布区内外均未表现出病害随距离降低的规律。与之相反，毛脉栎（Quercus velutina）在与同种个体的不同距离下，展现出显著但具特异性的病害变化模式。不同空间尺度下依赖距离的生物相互作用（distance-dependent biotic interactions）模式整体较弱，部分甚至呈负相关（即幼苗距同种成年个体越远，植食危害程度越高）。综上，我们认为分布区边缘间的生物相互作用差异，可被视为依赖距离的生物相互作用概念的空间延伸。该框架同时为预测其他系统中依赖距离的生物相互作用如何随分布区边缘发生变化提供了理论基础。