Competition in the understory: Rooting out the mechanisms
Traits associated with root morphology and nutrient uptake rate may contribute to the competitive ability of invasive exotic shrubs by influencing access to soil nutrients and ability to extract those resources. An experiment using two native species, Rubus allegheniensis and Viburnum dentatum, and two invasive exotic species, Rubus phoenicolasius and Berberis thunbergii, examined whether exotic and native species differ in root competition for nitrogen, or in morphological and other traits that may influence competitive ability. We grew replicate plants from each species (1) with inter-specific competitors, (2) with intra-specific competitors, and (3) individually without competitors (14 treatment combinations) in a randomized layout in a greenhouse. Each experimental container had a central patch amended with 15N-enriched Lolium multiflorum litter. After six months, we measured above- and belowground growth, root morphology, and N uptake. We used MANOVAs to assess differences among species in plant traits and in the effects of intra- and inter-specific competition on plant growth and N uptake.
Exotic species did not consistently differ from natives in their competitive impacts or root traits. Instead, plant size was a better predictor of competitive effects. Both Rubus species (Rosaceae) were stronger competitors and typically larger plants than B. thunbergii (Berberidaceae) and V. dentatum (Adoxaceae). B. thunbergii and V. dentatum differed from both Rubus species and each other in plant size, above- and belowground biomass, 15N content, and leaf C:N ratio. Both Rubus species exerted measurable competitive effects on other species, resulting in decreased aboveground size of competitors, but did not routinely decrease 15N uptake or root biomass of competitors. However, when competing with Rubus, leaf C:N ratios of all species except R. phoenicolasius were greater than when grown alone, suggesting that large Rubus plants did decrease the total nitrogen available to competitors. Belowground competitive ability in shrubs can be associated with plant size and growth rate, and may be more closely related to phylogeny than to plant origin. A broader comparison of many pairs of related native and exotic species is needed to test whether taxonomic similarity is a better predictor of belowground competitive impacts than native vs. exotic status.