Does selective defoliation mediate competitive interactions in a semiarid savanna? A demographic evaluation

Three patterns of target-neighbor plant defoliation were imposed on a late- seral, perennial, C-4-grass, Bouteloua curtipendula, in three long-term gra zing regimes to determine the influence of selective defoliation on competi tive interactions and species replacement in a semiarid savanna on the Edwa rds Plateau, Texas, USA. Short-term (3-yr) target plant defoliation did not significantly affect either tiller or plant responses in any of the three grazing regimes. Neighbor plant defoliation, either alone or in combination with target plants, produced a significant defoliation interaction with ti me for tiller number and basal area per plant, but not for tiller recruitme nt or mortality. The minimal effect of selective defoliation on the intensi ty of competitive interactions in this semiarid community indicates that se lective grazing has a less definitive role in mediating herbivore-induced s pecies replacement than it does in mesic grasslands and savannas. This inte rpretation is discussed within the context of long-term (45-yr) change in h erbaceous vegetation associated with grazing in this community. Cumulative tiller recruitment in the intensively grazed regime was only 44 % of that in the ungrazed regime because of greater plant mortality and few er surviving plants that recruited; tillers. Target plant mortality (50 %) only occurred in the intensively grazed regime and the proportion of target plants that initiated tillers decreased by 70, 48 and 32 % in the ungrazed , moderately and intensively grazed regimes, respectively, during the final two years of the investigation. The decrease in cumulative tiller recruitm ent in all grazing regimes was probably mediated by a drought-induced incre ase in median tiller age the second year of the study. However, tiller per tiller recruitment rate among plants that recruited at least one tiller rem ained relatively constant among grazing regimes and years. Intensive, long- term grazing has modified the population structure of this late-seral peren nial grass to the extent that population responses to both herbivory and pe riodic drought have been altered in comparison with those of ungrazed and m oderately grazed populations. Ecological consequences of a herbivore-induce d transition in population structure may be to minimize the effect of selec tive herbivory on competitive interactions and to function as an avoidance mechanism to reduce the probability of localized population extinction in r esponse to intensive long-term herbivory.