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，植物的分布范围预计将发生迁移，其扩张动态将由定植前沿个体的生存表现所决定。这些植物在分布范围边缘之外会遭遇全新的生物群落，这类交互的净效应将深刻影响其定植成功率。然而，目前学界对生物交互如何随分布范围边缘变化仍知之甚少，这阻碍了针对气候变化下物种分布变化的预测工作。与之相对，已有大量研究文献揭示了物种分布范围内的生物交互如何随同种个体的距离与密度变化。本研究旨在验证这一理论框架是否可推广至解释分布范围边缘之外（此处基本不存在同种成年个体）的生物交互差异。为此，我们沿着横跨其中5个物种现有分布范围的450公里纬度梯度种植了7种树木，并对其为期5年的叶部病害与无脊椎动物取食情况进行监测。我们以多个空间尺度下同种、同属树木的距离与密度为影响因子，对叶部病害与无脊椎动物取食情况展开分析。我们发现，在物种分布范围内，红花槭（Acer rubrum）、光滑山核桃（Carya glabra）、白栎（Quercus alba）与刺槐（Robinia pseudoacacia）的幼苗，若距离同种成年树木越远，其叶部病害程度越低。在分布范围边缘之外，光滑山核桃、白栎与刺槐的叶部病害程度进一步降低（红花槭未在其分布范围外进行种植）。鹅掌楸（Liriodendron tulipifera）的叶部病害程度在其分布范围内外均未出现随距离同种个体距离增加而降低的情况。与之相对，绒毛栎（Quercus velutina）则表现出与同种个体距离不同时，叶部病害呈现显著但特异性的变化模式。在多个空间尺度下，距离依赖型植食取食模式整体较弱，部分情况下甚至呈现负相关（即幼苗距离同种成年树木越远，其被取食程度越高）。综上，我们认为分布范围边缘间的生物交互差异，可视为距离依赖型生物交互概念的空间延伸。该理论框架同时为预测其他系统中，距离依赖型生物交互如何随分布范围边缘变化提供了通用基础。