DEMOGRAPHIC-EVALUATION OF A HERBIVORY-SENSITIVE PERENNIAL BUNCHGRASS - DOES IT POSSESS AN ACHILLES-HEEL

Plant and tiller demography of a C-4 perennial bunchgrass, Eriochloa s ericea, were evaluated to define specific traits that contribute to it s sensitivity to herbivory by domestic grazers. We tested three hypoth eses: (1) defoliation adversely impacts tiller recruitment to a greate r extent than mortality, (2) tiller recruitment and/or mortality are p articularly sensitive to defoliation at the time of culm elongation, a nd (3) synchronous tiller recruitment contributes to a meristematic li mitation which constrains growth following defoliation. Demographic va riables were monitored on permanently marked plants and tillers that w ere defoliated at various frequencies and stages of phenological devel opment for two successive years. Hypotheses one and three were rejecte d because defoliation adversely impacted per capita tiller mortality t o a greater extent than per capita tiller recruitment and tiller recru itment occurred throughout the spring and summer, rather than synchron ously. Apical meristem elevation in vegetative tillers did not extend beyond 5 mm above the soil surface and the proportion of reproductive tillers did not exceed 10% of the total number of tillers. Low values for both morphological attributes indicate that they did not contribut e to a meristematic limitation constraining leaf growth. Hypothesis tw o was not rejected because cumulative, but not per capita, tiller recr uitment was reduced to a greater extent by defoliation during culm elo ngation than during the pre-culm or post-culm stage. Plants that recei ved multiple defoliations exhibited the greatest decline in basal area and tiller number compared to undefoliated plants, and only the undef oliated plants and plants defoliated during the pre-culm stage recruit ed a sufficient number of tillers to offset tiller mortality. In spite of the adverse effects of two of the four defoliation regimes, we wer e unable to identify a specific trait (i.e., Achilles heel) within thi s species that contributed to a meristematic limitation and herbivory- sensitivity. An alternative interpretation of herbivory-sensitivity is proposed which emphasizes the involvement of more subtle extrinsic me chanisms, including herbivore-mediated competitive interactions and dr ought-herbivory interactions. Assessment of herbivory resistance based on the 'specific trait approach' fails to recognize and address the i mportance of extrinsic mechanisms associated with herbivore-induced pr ocesses at higher ecological scales.