Dual drivers of plant invasions: Enemy release and enhanced mutualisms

Abstract Invasive plants often express above‐ground traits, such as higher growth than native plants, which promote their success. This may reflect low levels of invertebrate herbivory and/or high rates of arbuscular mycorrhizal fungi (AMF) association. However, the root traits that contribute to invasive success are less well known. Moreover, the combined roles of above‐ground herbivory, AMF, and root traits in the invasion process are poorly understood. We conducted field surveys at 17 sites along a latitudinal gradient in China (22.77° N to 42.48° N) to investigate the relationships among above‐ground herbivory, AMF colonization, and root traits for five pairs of closely related invasive and native Asteraceae plant species. We experimentally manipulated above‐ground insect feeding for two of these pairs of plant species in a middle latitude (34.79°N) common garden. We measured above‐ground invertebrate abundance, leaf damage, AMF colonization, root morphological traits associated with nutrient uptake, and root soluble sugar concentrations. In the field survey, invasive plants had lower leaf damage and Hemiptera abundances plus higher AMF colonization, thinner roots with more surface area and higher concentrations of root soluble sugars than native plants. Leaf damage decreased with increasing latitude for native plants. In the common garden, invasive plants had lower leaf damage and Hemiptera abundances plus higher AMF and greater surface area of fine roots than native plants. Leaf damage and Hemiptera reduced AMF colonization via a phenotypic effect of reduced fine root soluble sugars. Synthesis : Our results indicate that low above‐ground invertebrate herbivory on invasive plants contributes to their success directly by increasing their growth and indirectly via root soluble sugars that increase their AMF colonization. Invasive plants appear to benefit from greater root volume and surface area, but this did not vary with latitude or above‐ground invertebrate herbivory. These results highlight the importance of considering above‐ and below‐ground processes simultaneously to understand how they interact to determine plant invasion success.