Soil biota facilitate exotic Acer invasions in Europe and North America

The primary hypothesis for successful exotic plant invasions is that the invaders have escaped the specialist consumers that control them (Enemy Release Hypothesis). However, few studies have rigorously tested this assertion with biogeographical experiments or considered the effects of soil organisms. We tested the Enemy Release Hypothesis and the enhanced role of mutualisms by comparing density patterns of the North American Acer negundo and European A. platanoides trees in their native and normative ranges. Invaders that have escaped their natural enemies are predicted to attain greater densities in normative than native ranges. To determine whether interactions with soil biota could explain the population distributions observed in the field, we compared the effects of sterilized and nonsterilized soil associated with Acer and non-Acer trees in native and normative ranges on the growth of seedlings in the greenhouse. In the field study, distances from focal trees to the nearest Acer conspecifics were 56-77% less in their normative ranges than in their native ranges. In the greenhouse experiment, the effect of soil biota also differed between native and normative ranges of Acers. Relative to sterilized controls, soil associated with conspecifics and heterospecifics from the native ranges decreased the total biomass and relative change in height of Acer seedlings by 35% and 40%, respectively. Soil associated with conspecifics in the normative ranges decreased the biomass and relative change in height of Acer seedlings by 112% and 64%, respectively; but the soil associated with heterospecifics in the normative ranges increased biomass and relative change in height of Acer seedlings by 13% and 37%, respectively. Our results suggest that invasion of Acers is enhanced by soil biota associated with dominant native species and that the soil biota effect becomes more inhibitory as the Acers establish. The relative difference in soil biota effects between ranges supports the Enemy Release Hypothesis but also suggests that mutualists are relatively more beneficial to Acers in their normative ranges than in their native ranges. Mutualisms may be relatively more beneficial in normative ranges because the invader has escaped from the negative effect of natural enemies that may attenuate the positive effect of mutualists.