Invasive Solidago canadensis versus its new and old neighbors: Their competitive tolerance depends on soil microbial guilds

An important hypothesis in invasion ecology is that new neighbors in an invaded range should be more susceptible (i.e., low tolerance to competitors) to invaders than old neighbors in a home range. To test this hypothesis in the context of different soil microbes, we conducted a pairwise competition experiment with five Chinese native species, five North American native species, and Solidago canadensis (an invader from North America into China), in which all the species combinations were subject to regular soil, fungicided soil, and autoclaved soil. In the absence of S. canadensis, the average growth of Chinese natives was similar to that of North American natives; in the presence of S. canadensis, Chinese natives grew bigger and had a lower competitive response than North American natives. Solidago canadensis plants grown alone grew larger from China than from North America; in contrast, S. canadensis plants grew smaller when they were grown with Chinese natives than when they were grown with North American natives. Fungicide decreased the growth of species from China and North America whereas autoclaving increased their growth in the presence of S. canadensis; fungicide and autoclaving decreased the growth of S. canadensis grown with neighbors. These findings suggest that new neighbors may have a stronger competitive tolerance to S. canadensis than old neighbors and this tolerance may depend on soil microbial guilds, thereby altering the interactions between S. canadensis and its neighbors.