Problems of regeneration of the mature Eucalyptus regnans F. Muell, (The Big Ash) forest, in the absence of fire at Wallaby Creek, Victoria, Australia

Abstract

Summary This paper is based on observations on the regeneration of a mature E. regnans forest begun in 1949 and extending over the next 50 years. Any regeneration of E. regnans in the unburnt, mature forest is confronted by a phalanx of obstacles. The formation of large gaps which synchronously involve the overstorey, shrub understorey and fern stratum are an essential prerequisite for establishment, although success may not be assured unless the seedbed has been favourably prepared by mechanical disturbance or fire. In this study, regeneration, which was initiated experimentally by a heavy sowing of seed, scarification of soil, addition of nitrogenous fertilizer, creation of moderately good light conditions and the removal of competition by seedlings of understorey species in the first season, failed completely after 10–11 years even though understorey species which germinated from the soil seed bank continued to thrive. Processes of seed germination and seedling development are explored in terms of seed harvesting by ants, insect attack, heating and desiccation of the soil surface, mammal browsing, light intensity and dimorphic leaf orientation, mycorrhizal associations, root competition, allelopathic factors, leaf spot fungi and chewing and gall-forming insects. In the tall forest lateral light is considered critical for continued vigour of advanced seedlings with pendulous foliage. Thus, gaps which permit satisfactory development of young seedlings are inadequate for saplings, especially if they are in competition with vigorous regeneration of understorey species. Under light-limiting conditions, survival is severely jeopardized by any factor affecting leaf area or leaf function. It is concluded that while E. regnans appears to be physiologically tolerant of moderate shade it lacks the wider ecological tolerance to it. Without fire, the regeneration of E. regnans will almost certainly fail in the mature forest unless accompanied by exceptional circumstances. Group tree falls are needed to create regeneration but only on sites incompletely covered by dense ground fern. Synchronous gaps may be needed to disperse herbivore activity in the critical early years. Crown fire eliminates most of the difficulties encountered by E. regnans regeneration by causing mass seedfall, creating a suitable seed bed and by temporarily changing the microclimate, soil moisture regime, soil nutrient status, microbial populations and ant foraging activity. Regeneration may occur in groups following surface fires if large canopy gaps have been produced. In the unlikely event of a very long period without fire, succession to a form of rainforest is likely to occur, although the species may not be eliminated entirely from the area due to the possibility of chance establishment.