Figure 3.

Schematic of how belowground enemy release and root traits may be interrelated with allelopathy or enhanced mutualism as mechanisms driving invasion success. Increased competitive ability as a result of enemy release could be direct (1), or indirect via root‐trait changes that increase soil resource uptake in the alien range (2). Increased competitive ability can result independently from allelopathy (3), or as a result of changes in root traits due to enemy release that result in increased concentrations of allelopathic compounds in the soil, and suppressive effects on natives in the alien range (4). If allelopathic compounds affect naïve pathogenic soil biota, then allelopathy will lead to effective enemy release in the alien range, and potentially shifts in root traits allowing greater resource uptake (5). Enhanced mutualism in the alien range could be of direct benefit through increased resource uptake (6), or indirect if alien plants invest less in costly, finer root growth; reinvestment into aboveground growth would then enhance competitive ability (7). All effects are positive (black), except the negative effect of mutualist enhancement on resource uptake‐related root traits (gray). Direct effects of belowground mechanisms on competitive ability are shown by solid lines; indirect effects are dashed lines.