Nontarget Tree Mortality after Tree-of-Heaven (Ailanthus altissima) Injection with Imazapyr

Tree-of-heaven (Ailanthus altissima Miller [Swingle]) can be managed easily with herbicide injection. However, the potential herbicide translocation to neighboring trees must be evaluated before widespread recommendations for herbicide injections. We assessed the nontargettranslocation of imazapyr (Arsenal), an herbicide commonly used to manage woody vegetation in forests, from injected tree-of-heaven to neighboring noninjected stems. Targeted imazapyr injections not only killed all injected tree-of-heaven, but also killed 17.5% of neighboring (within 3 m) noninjected tree-of-heaven and eight other tree species 62 weeks after treatment. Nontarget mortality from herbicide translocation decreased as the distance from injected tree-of-heaven increased (up to 3 m) and as stem diameter of noninjected plants increased. The plausible modes ofinter- and intraspecific herbicide translocation include root grafts, mutually shared mycorrhizal fungi, root exudation and absorption, and/or leaf senescence. Because tree-of-heaven is clonal, patch size and vegetation heterogeneity will be an important determinant of herbicide injectionprotocols. In forest environments with many small patches (i.e., high edge to interior ratio) or mixed species stands, nontarget hardwoods are at an increased risk of mortality. In isolated large patches (with lower edge to interior ratio) or dense monospecific clones, injection risk to nontarget species will be relatively low.