Carbon allocation in Euphorbia esula and neighbours after defoliation

Weeds increase their dominance in a grazed plant community by avoiding herbivory and (or) by tolerating herbivory more than neighbouring plants. After defoliation, allocating carbon to shoots at the expense of roots may confer tolerance. We determined carbon allocation patterns of undefoliated and recently defoliated (75% clipping level) plants of the invasive leafy spurge (Euphorbia esula L.) growing with alfalfa (Medicago sativa L.), Kentucky bluegrass (Poa pratensis L.), or Idaho fescue (Festuca idahoensis Elmer). Plants were labeled with 13CO2 24 h after clipping to determine allocation patterns; all plants had equal access to the 13CO2. Based on relative distribution of 13C, defoliation did not affect the amount of carbon allocated 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 the other species. Based on relative enrichment of 13C, defoliation increased sink strength of remaining shoots on defoliated E. esula plants. Conversely, 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 allocation of carbon to its extensive root system, not increased allocation to its shoot system, confers grazing tolerance.