AXILLARY BUD BANKS OF 2 SEMIARID PERENNIAL GRASSES - OCCURRENCE, LONGEVITY, AND CONTRIBUTION TO POPULATION PERSISTENCE

The occurrence. longevity, and contribution of axillary bud banks to p opulation maintenance were investigated in a late-seral perennial gras s, Bouteloua curtipendula, and a mid-seral perennial grass, Hilaria be langeri, in a semiarid oak-juniper savanna. Axillary buds of both spec ies were evaluated over a 2-year period in communities with contrastin g histories of grazing by domestic herbivores. A double staining proce dure utilizing triphenyl tetrazolium chloride and Evan's blue indicate d 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 b uds 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. curtipend ula and H. belangeri, respectively. The percentage of older proximal b uds, which included comparable portions of viable and dormant buds, th at 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 t he occurrence of relative large axillary bud banks for both species, t he magnitude of proximal bud growth did not appear sufficient to maint ain viable tiller populations. We found no evidence to support the hyp othesis of compensatory bud growth on an individual tiller basis for e ither species. Grazing history of the communities from which the buds were collected did not substantially affect the number, status, longev ity, or outgrowth of axillary buds on an individual tiller basis for e ither species. However, long-term grazing by domestic herbivores influ enced axillary bud availability by modifying population structure of t hese two species. Bud number per square meter for B. curtipendula was 25% lower in the long-term grazed compared to the long-term ungrazed c ommunity based on a reduction in both tiller number per plant and plan t number per square meter. In contrast, bud number per square meter fo r H. belangeri was 190% greater in the long-term grazed than in the lo ng-term ungrazed community based on a large increase in plant density per square meter. Minimal contributions of axillary bud banks to annua l maintenance of tiller populations in this mid- and late-seral specie s underscores the ecological importance of consistent tiller recruitme nt from recently developed axillary buds. Consistent tiller recruitmen t in grasslands and savannas characterized by intensive grazing and pe riodic drought implies that (I) bud differentiation and maturation mus t be remarkably tolerant of adverse environmental conditions and/or (2 ) tiller recruitment may resume from buds that mature following the ce ssation of severe drought and/or grazing, rather than from mature buds that survive these disturbances. These scenarios warrant additional r esearch emphasis given the critical importance of this demographic pro cess to tiller replacement in species populations and the maintenance of relative species abundance in grasslands and savannas.