Temporal patterns of vegetation recovery after wildfire in two obligate seeder ash forests

Abstract

In recent years, novel, high-severity wildfire regimes have driven major changes in the structure and function of forests globally. Indeed, many forests are vulnerable to recruitment failures and collapse in the event of decreased intervals between fires, including forests dominated by the Australian obligate seeders, Alpine Ash (Eucalyptus delegatensis) and Mountain Ash (Eucalyptus regnans). Disturbance responses in these forests are often described collectively, with limited understanding of their potential differences. Despite this, management recommendations and ecological implications are often applied broadly across both forest types. Here, using an empirical dataset collected over an eleven-year period (2009–2020), we compare plant communities in south-eastern Australian Alpine Ash and Mountain Ash forests in mature stands (last burned in 1926/1939), and young stands (burned by high-severity wildfire in 2009). To do this, we used measures of forest structure (basal area of dominant plant life-forms) collected over eight years, and the abundance (projective foliage cover) of vascular plant species. We provide evidence of similar, positive-linear trends in the recovery of forest structure in both Alpine Ash and Mountain Ash forests in the first eight years post-wildfire in 2009. Mature Alpine Ash and Mountain Ash forests were compositionally different but had a similar structure and abundance of plant life-forms. Moreover, controlling for the influence of forest type, young stands were compositionally different from mature stands, and were associated with an increase in the abundance of Acacia, herb, ground-fern and graminoid species, and a decline in tree-fern and tree species. Further investigation in young stands revealed marked differences in plant community composition between forest types, with young Mountain Ash forests characterized by a lower abundance of shrub, Acacia and graminoid species, relative to Alpine Ash forests of the same age. Overall, our findings indicate that Alpine Ash and Mountain Ash forest plant communities may have similar structural and functional responses to predicted increases in wildfire. This suggests that when the broad structure and function of forest vegetation is a consideration, ecological implications and management recommendations may be broadly applicable to both Alpine Ash and Mountain Ash forests. However, further consideration is required in early successional stands where differences in species composition between forest types may have implications for management (e.g. increases in flammability). In a period of uncertainty in the future of ash-type forests, our study provides a timely insight into the comparative successional trajectories of their respective plant communities and provides a baseline for future studies.