AILANTHUS ALTISSIMA STIMULATES LEGUME NODULATION IN TRIFOLIUM PRATENSE VIA ROOT EXUDATES: A NOVEL MECHANISM FACILITATING INVASION?

Ailanthus altissima, an invasive Asiatic tree, is notable for its rapid growth rate (particularly in nutrient-poor soils), root-based cloning, and allelopathy. We investigated whether root exudates from A. altissima stimulate nodulation of red clover, Trifolium pratense, as a mechanism by which the former species may increase nitrogen availability and facilitate its own rapid growth and reproduction. To test this possibility and account for the effects of nutrient availability, we conducted a greenhouse experiment that included a full factorial design with 30 T. pratense in each of the following treatments: (1) presence versus absence of A. altissima in leachate source pots, (2) fertilization versus nonfertilization of leachate source pots, and (3) fertilization versus nonfertilization of pots containing T. pratense. Soil leachates acquired from source pots (half of which contained 8-wk-old A. altissima) were applied to pots containing 1-wk-old T. pratense for 7 wk and harvested 1 wk thereafter. (1) Total biomass and shoot:root ratio of T. pratense were unaffected by the presence of A. altissima in leachate source pots. (2) Presence of A. altissima resulted in a 2.57-fold increase in the proportion of T. pratense possessing nodules, an effect that was greatest when the leachate source pots were also fertilized. (3) Presence of A. altissima was associated with a 261% increase in the number of nodules per unit total (Trifolium) biomass and a 171% increase in nodule biomass per unit total biomass; these traits were otherwise unaffected. Root exudates from A. altissima stimulate nodulation by T. pratense, regardless of the nutrient status (fertilization) experienced by either species. The nodule-stimulating mechanism reported here may be responsible -- at least in part -- for previous reports of increased soil nitrogen levels associated with invasion by A. altissima, a nutrient increase that likely facilitates its growth, allelopathy, and reproduction.