Predator Satiation and Site Alteration Following Fire: Mass Reproduction of Alpine Ash (Eucalyptus Delegatensis) in Southeastern Australia

Abstract

This study examines seedfall, seed predation, and seedling recruitment in a relatively fire—sensitive tree, the alpine ash (Eucalyptus delegatensis), in the Brindabella Range, Australian Capital Territory (A.C.T.), Australia. An experimental high—intensity fire was used at Bushrangers Creek (BC) to determine whether massive, synchronized seedfall and subsequent seedling recruitment are linked to (1) satiation of postdispersal seed predators and/or (2) reduction of the physical and biotic resistance to seedling growth and survivorship. Results were contrasted with findings at Moonlight Hollow (MH), a site which served as a natural, unburned control. Overall results wee consistent with the predator—satiation hypothesis. High—intensity fire killed 93% of the alpine ash trees at BC and caused a massive, synchronized release of seeds, averaging 405 viable seeds/m 2 (4021 total ovular structures/m 2) in the 3 wk after fire. Fire precipitated an immediate drop in weekly removal of seeds from trays, from an average of 65.3% prefire to 13.6% postfire. Postfire seed removal at BC dropped to significantly lower levels than corresponding measures at unburned MH. Caging experiments demonstrated that ants were the only important removal agents of alpine ash seed at BC and MH. Although abundance and species richness of ants trapped on the seedbed increased following fire, the number of ants trapped per available seed significantly declined due to the massive input of seed. Fire and subsequent seedfall resulted not only in a greater absolute number of emergent seedlings (66.1 seedlings/m 2 at BC; 0.8 seedlings/m 2 at MH) but also in a significantly greater proportion of emergence from seed input from the canopy (14.7% at BC; 2.7% at MH). Seedling mortality was great, and after 72 wk mean density at BC was 1.9 seedlings/m 2. Successful recruitment did not occur at unburned MH. A phytometer experiment demonstrated that fire neither enhanced the physical environment for early growth and survivorship of seedlings nor reduced the effect of natural enemies on E. Delegatensis seedlings. Greater frequency of grazing on seedlings growing in burned soil and a more rigorous physical environment at BC appeared to account for lack of enhanced growth and survivorship at postfire BC. Massive, synchronized release of stored seed from shrub and tree species following fire is a common feature in many fire—prone environments. Parallels in seed storage, postdispersal seed predation, and regeneration following fire in Eucalyptus and Pinus suggest that postdispersal seed predators are important selective agents in the evolution of seed storage and induction of mass seed release by fire.