Carbon allocation in Euphorbia esula and neighbours after defoliation

Weeds increase their dominance in a grazed plant community by avoiding herb ivory and (or) by tolerating herbivory more than neighbouring plants. After defoliation, allocating carbon to shoots at the expense of roots may confe r tolerance. We determined carbon allocation patterns of undefoliated and r ecently defoliated (75% clipping level) plants of the invasive leafy spurge (Euphorbia esula L.) growing with alfalfa (Medicago sativa L.), Kentucky b luegrass (Poa pratensis L.), or Idaho fescue (Festuca idahoensis Elmer). Pl ants were labeled with (CO2)-C-13 24 h after clipping to determine allocati on patterns; all plants had equal access to the (CO2)-C-13. Based on relati ve distribution of C-13, defoliation did not affect the amount of carbon al located to roots of E. esula. The amount of carbon allocated to shoots of E . esula was higher when growing with P. pratensis than when growing with th e other species. Based on relative enrichment of C-13, defoliation increase d sink strength of remaining shoots on defoliated E. esula plants. Converse ly, roots of unclipped E. esula plants were stronger sinks for carbon than roots of clipped plants. Even though defoliation increased "sink strength" of remaining shoots of E. esula, the amount of carbon allocated to the root system was unaffected by defoliation, suggesting that uninterrupted alloca tion of carbon to its extensive root system, not increased allocation to it s shoot system, confers grazing tolerance.