Post-grazing and post-fire vegetation dynamics: long-term changes in mountain bogs reveal community resilience

Abstract

Questions: Are Australian 'Sphagnum' bogs compositionally stable or undergoing long-termchange in response to grazing legacies or environmental change along a climatic gradient? Are these 'Sphagnum' bogs resilient to discrete fire events, and over what time scales does recovery from disturbance take place? How does fire disturbance influence species composition in the assembly of fire-prone Australian bog communities? Location: Alpine and subalpine bogs in mainland eastern Australia (Kosciuszko National Park). Method: Full floristic sampling over ca. 50 yr (1960s, 1990, 2005, 2007, 2013) at 11 sites; each site sampled with 25 quadrats (0.1 m2) haphazardly placed during each successive survey. Sites were stratified over alpine and subalpine elevations, in burned and unburned areas. Changes in species composition over space and time were examined withmultivariate and univariate analyses. Results: The 'Sphagnum' bogs of the subalpine and alpine regions show progressive increases in cover of 'Sphagnum' over the last 40-50 yr. Overall species richness and frequency of dominant woody species declined. These trends were not strongly related to the climate gradient. Fire temporarily reduced the frequency of most species but initial floristic composition was regained a decade after fire. There was fire-dependent variation related to regeneration of hygrophyllous woody species through seed germination and seedling growth in open ground. Conclusion: Our results show a degree of community resilience to both grazing and fire, although some observed changes appear directional and the recovery time for grazing was much longer than that for fire. The increase in 'Sphagnum' frequency across subalpine and alpine bogs is likely to reflect progressive recovery of 'Sphagnum' from the grazing era, possibly enhanced by the changing atmosphere. Concurrently, there have been declines in species richness and woody species frequency. The bogs exhibited resilience to infrequent pulse disturbance related to fires, which appear to drive community assembly through cycles of compositional change.