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.

受气候变化影响，植物的分布区预计将发生偏移，而其分布区扩张的动态将由定植前沿（colonizing front）的植株个体生长状况决定。分布于现有分布区边缘（range edges）之外的植株将遭遇全新的生物群落（biotic communities），此类物种与新群落交互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交互（biotic interactions）的变化规律知之甚少，这阻碍了我们预测气候变化下物种分布变化的研究进展。

与之相对，已有大量文献证实，物种分布区内部的生物交互会随与同种个体（conspecific individuals）的距离以及同种密度发生变化。本研究旨在验证：这一理论框架能否被拓展，用以解释分布区边缘之外（此处基本无成年同种个体）的生物交互差异。

为此，我们沿一条横跨其中5个物种现有分布区的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）与无脊椎动物植食作用（invertebrate herbivory）。我们从多个空间尺度出发，以与同种及同属（congeneric）树木的距离和密度为自变量，分析叶部病害与植食作用情况。

研究发现，在物种分布区内部，红花槭（Acer rubrum）、光滑山核桃（Carya glabra）、白栎（Quercus alba）以及刺槐（Robinia pseudoacacia）的幼苗，距成年同种个体越远，叶部病害程度越低。而在分布区边缘之外，光滑山核桃、白栎与刺槐的叶部病害程度进一步降低（红花槭未被种植于其分布区之外）。鹅掌楸（Liriodendron tulipifera）无论在分布区内部还是外部，均未出现叶部病害随距离变化而降低的情况。与之相对，黑栎（Quercus velutina）的叶部病害随与同种个体距离变化呈现出显著但特异性的模式。不同空间尺度下，植食作用的距离依赖模式整体较弱，部分甚至呈负相关（即距成年同种个体越远的幼苗，植食作用程度越高）。

综上，我们认为：分布区边缘间的生物交互差异，可被视为距离依赖型生物交互（distance dependent biotic interactions）概念的空间拓展。该理论框架同时为预测其他系统中，距离依赖型生物交互随分布区边缘的变化规律提供了基础。