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，二者相互作用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作用沿分布范围边缘的变化规律尚所知甚少，这一短板也阻碍了针对气候变化下物种分布变化的预测研究。
与之相对，已有大量文献阐明了物种分布范围内的生物交互作用如何随同种个体的距离与密度发生变化。本研究旨在检验这一研究框架是否可被推广，用以解释在同种成体基本缺失的分布范围边缘之外，生物交互作用存在的差异。
为此，我们沿一条横跨其中5个物种种群当前分布范围的450公里纬度梯度，种植了7种树木，并连续5年监测其叶部病害与无脊椎动物植食情况。我们以多个空间尺度下同种、同属树木的距离与密度为自变量，分析叶部病害与植食程度的变化规律。
研究结果显示：在物种分布范围内，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，离同种成体越远，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种植）。鹅掌楸（Liriodendron tulipifera）在其分布范围内外均未表现出病害程度随距离降低的现象。与之相对，黑栎（Quercus velutina）的叶部病害与同种个体距离间呈现出显著但特异性的变化模式。不同空间尺度下，植食程度的距离依赖模式普遍较弱，部分甚至呈现负相关（即幼苗离同种成体越远，植食程度越高）。
综合来看，我们认为分布范围边缘间的生物交互作用差异，可视为距离依赖型生物交互作用概念的空间延伸。该研究框架同时为预测其他生态系统中，距离依赖型生物交互作用沿分布范围边缘的变化规律提供了通用基础。