Changing fire patterns alongside the growing role of invasive species in forested landscapes may be interacting to shift post-fire vegetation communities. Identifying some of the mechanisms of change may help target ecosystem management.

Here, we aim to understand some of the relationships between fire and introduced species in a grassy woodland environment in south-eastern Australia, and the role soil seedbanks may be playing in facilitating any changes. We sampled the soil seedbank and extant vegetation shortly before a wildfire, with vegetation surveys repeated thereafter. We used three combinations of data, including a Before-After-Control-Impact (BACI) survey design and planned contrasts, to assess changes in the extant vegetation pre- and post-fire, the role of the soil seedbank in those changes, and how past fire history may have impacted the vegetation community. Analyses were conducted at both the species and plant functional group (PFG) level.

The wildfire modified the vegetation community through significant changes to three PFGs: trees and shrubs, resprouting herbs with persistent seedbanks, and introduced annual seeders with wind dispersed seed. Sites that were burnt typically had higher cover of introduced species, and lower cover of native trees and shrubs. The soil seedbank contributed some new species into the extant vegetation, predominantly introduced species, with the soil seedbank becoming more similar to the extant vegetation after fire. Increasing fire severity typically favoured PFGs with a high proportion of introduced species, with many native PFGs showing negative or negligible relationships with fire severity. We did not detect a consistent influence of time since fire or fire frequency for the majority of PFGs.

Synthesis: Fire events increase the cover of introduced annuals in grassy woodland environments which may be increasing the flammability of the ecosystem, suggesting the potential for a grass or forb fire cycle. Given grass-fire cycles typically reduce the cover of trees and shrubs, a management option is to support tree regeneration either through active restoration, or by reducing the occurrence of fire in these ecosystems until trees are both reproductively mature, and large enough to survive fire. However, the decision making around ecosystem management should be led by the Traditional Owners of the land, the Gunditjmara.