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.

植物分布预计会因气候变化而发生转移，而分布范围扩张的动态将由定植前沿（colonizing front）的个体表现所决定。这些植物会在其分布范围边缘之外遭遇全新的生物群落（biotic communities），这些相互作用的最终净效应可能会深刻影响定植成功率。然而，目前对分布范围边缘处的生物交互差异的了解仍十分有限，这阻碍了我们预测气候变化下物种分布变化的相关研究。与之形成对比的是，已有大量文献阐述了物种分布范围内的生物交互如何随与同种个体（conspecific individuals）的距离及其密度而变化。本研究旨在验证这一理论框架是否可被推广，用以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，我们沿一条450千米的纬度梯度（latitudinal gradient）种植了7个树种，该梯度跨越了其中5个物种当前的分布范围，并对5年间的叶部病害（foliar disease）和无脊椎动物植食危害（invertebrate herbivory）进行了监测。我们以多个空间尺度下的同种、同属（congeneric）树木的距离与密度为自变量，分析了叶部病害与植食危害的发生情况。研究发现，在物种分布范围内，红花槭（Acer rubrum）、光滑山核桃（Carya glabra）、白栎（Quercus alba）和刺槐（Robinia pseudoacacia）的幼苗距离同种成年个体越远，叶部病害程度越低。在分布范围边缘之外，光滑山核桃、白栎和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种植）。北美鹅掌楸（Liriodendron tulipifera）无论在分布范围内还是范围外均未表现出病害程度随距离降低的现象。与之相反，黑栎（Quercus velutina）在不同距离的同种个体附近的病害模式呈现出显著但独特的特征。不同空间尺度下的距离依赖性植食危害模式整体较弱，部分甚至呈现负相关（即幼苗距离同种成年个体越远，植食危害程度越高）。综上，我们认为分布范围边缘处的生物交互差异可被视为距离依赖性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性生物交互如何随分布范围边缘发生变化提供了基础。