Axillary bud banks of two semiarid perennial grasses: occurrence, longevity, and contribution to population persistence |
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Authors: | J R Hendrickson D D Briske |
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Institution: | (1) Department of Rangeland Ecology and Management, Texas A&M University, College Station, TX 77843-2126, USA Fax: (409) 845-6430; e-mail: dbriske@rlem.tamu.edu, US |
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Abstract: | The occurrence, longevity, and contribution of axillary bud banks to population maintenance were investigated in a late-seral
perennial grass, Bouteloua curtipendula, and a mid-seral perennial grass, Hilaria belangeri, in a semiarid oak-juniper savanna. Axillary buds of both species were evaluated over a 2-year period in communities with
contrasting histories of grazing by domestic herbivores. A double staining procedure utilizing triphenyl tetrazolium chloride
and Evan's blue indicated that both viable and dormant axillary buds remained attached to the base of reproductive parental
tillers for 18–24 months which exceeded parental tiller longevity by approximately 12 months. Bud longevity of the late-seral
species, B. curtipendula, exceeded bud longevity of the mid-seral species, H. belangeri, by approximately 6 months. Younger buds located on the distal portion of the tiller base were 3.2 and 1.4 times more likely
to grow out than older proximal buds of B. curtipendula and H. belangeri, respectively. The percentage of older proximal buds, which included comparable portions of viable and dormant buds, that
grew out to produce tillers following mortality of parental tillers was 6.0% for B. curtipendula and 8.4% for H. belangeri. In spite of the occurrence of relative large axillary bud banks for both species, the magnitude of proximal bud growth did
not appear sufficient to maintain viable tiller populations. We found no evidence to support the hypothesis of compensatory
bud growth on an individual tiller basis for either species. Grazing history of the communities from which the buds were collected
did not substantially affect the number, status, longevity, or outgrowth of axillary buds on an individual tiller basis for
either species. However, long-term grazing by domestic herbivores influenced axillary bud availability by modifying population
structure of these two species. Bud number per square meter for B. curtipendula was 25% lower in the long-term grazed compared to the long-term ungrazed community based on a reduction in both tiller number
per plant and plant number per square meter. In contrast, bud number per square meter for H. belangeri was 190% greater in the long-term grazed than in the long-term ungrazed community based on a large increase in plant density
per square meter. Minimal contributions of axillary bud banks to annual maintenance of tiller populations in this mid- and
late-seral species underscores the ecological importance of consistent tiller recruitment from recently developed axillary
buds. Consistent tiller recruitment in grasslands and savannas characterized by intensive grazing and periodic drought implies
that (1) bud differentiation and maturation must be remarkably tolerant of adverse environmental conditions and/or (2) tiller
recruitment may resume from buds that mature following the cessation of severe drought and/or grazing, rather than from mature
buds that survive these disturbances. These scenarios warrant additional research emphasis given the critical importance of
this demographic process to tiller replacement in species populations and the maintenance of relative species abundance in
grasslands and savannas.
Received: 12 August 1996 / Accepted: 30 December 1996 |
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Keywords: | Axillary buds Bud demography Population persistence Population structure Tiller recruitment |
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