Plant neighbors differentially alter a focal species’ biotic interactions through changes to resource allocation

Abstract Plant resource allocation strategies are thought to be largely a consequence of changing abiotic conditions and evolutionary history. However, biotic interactions also influence how a plant allocates resources. As a result, plants mediate indirect interactions between organisms above- and belowground through resource allocation. Neighboring plants can influence plant fitness directly through competition for resources, and indirectly by altering associated community interactions (associational effects). Given the importance of community interactions for plant success, and the known ability for plant neighbors to change these interactions, the goal of this “pandemic project” was to separate inter- and intraspecific plant associations, above- and belowground, to understand how different plant neighbors alter plant resource allocation, and if this in turn alters biotic interactions. We specifically investigated associational effects on herbivory and soil microbial community interactions. To do so, we established a common garden experiment, manipulating plant neighbors and extent of interactions (aboveground only versus above- and belowground interactions, using customized pot types), and measured changes to a focal plant and its biotic interactions over two growing seasons. We found evidence of both neighbor effects and pot type, showing that neighbor interactions affect a focal plant through both above- and belowground processes, and how the focal plant is affected depends on neighbor identity. Though neighbors did not directly alter herbivory or most soil microbial interactions, they did alter the relationship between belowground microbial communities and plant function. Resource allocation responses were reduced with time, showing the importance of extending experiments beyond a single growing season, and is an important consideration when making predictions about plant responses to changing conditions. This study contributes to a growing body of work showing how the community context affects the above- and belowground interactions of a plant through plant resource allocation strategies.