Multiple competitive mechanisms underlie the effects of a strong invader on early- to late-seral tree seedlings

1. Certain non-native invaders reduce the species diversity and alter the structure of natural communities by displacing native species with differing life histories, successional roles or resource requirements. Few studies have tested the potential for these strong invaders to exert multiple mechanisms of control on natives that differ in these traits. 2. We assessed the mechanisms by which bohemian knotweed (Polygonum x bohemicum) regulates seedling growth and survival among early-, mid- and late-seral tree species in a riparian forest ecosystem in western North America. We used general linear mixed models to compare seedling performance (survival, height and diameter growth, biomass allocation and ectomycorrhizal colonization of root systems) over two growing seasons in paired experimental plots from which knotweed was either removed or retained (controls). Seedling performance was assessed relative to the effects of knotweed on light and soil resources and the traits of the native species. 3. Results from paired t-tests suggest that knotweed had a significant effect on light availability (>85% reduction), but small, mostly non-significant effects on measured soil properties. Knotweed imposed strong controls on growth and survival of all three tree species. The apparent mechanisms of interaction varied in a manner consistent with species ecophysiologies. Reduced survival of early- and mid-seral species was correlated with light limitation beneath knotweed (<= 7% of ambient levels): light transmittance was significantly higher (79%) above surviving seedlings. Knotweed also exerted strong controls on the late-seral species, reducing survival by 24% and height and diameter growth by 91-122% and 37-55%. These effects were not correlated with reductions in light. Instead, in the presence of knotweed, ectomycorrhizal colonization was significantly reduced (64%) and root/shoot ratio was significantly increased, suggesting a disruption of soil mutualisms. 4. Synthesis. We demonstrate that strong invaders can displace co-occurring native species through multiple mechanisms that are consistent with the functional traits of native species. To our knowledge, this is the first study to relate community-level impacts of an invader to the combined effects of resource exploitation and interference of below-ground mutualisms. Where invaders have the ability to displace early- to late-seral dominants, the consequences for community structure and ecosystem functioning can be profound.