Bc. Hendon et Dd. Briske, DEMOGRAPHIC-EVALUATION OF A HERBIVORY-SENSITIVE PERENNIAL BUNCHGRASS - DOES IT POSSESS AN ACHILLES-HEEL, Oikos, 80(1), 1997, pp. 8-17
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.