Extreme fire severity interacts with seed traits to moderate post-fire species assemblages

Premise: Climate change is pushing fire regimes to new extremes, with unprecedented large-scale severe fires observed globally. Persistent soil-stored seed banks are a key mechanism for plant species recovery after fires, but extreme fire severity may generate soil temperatures beyond thresholds seeds are adapted to. Seeds are protected from lethal temperatures through soil burial, with temperatures decreasing with increasing depth. However, smaller seeds, due to their lower mass and corresponding energy stores, are restricted to emerging from shallower depths compared to larger-seeded counterparts. We examined recruitment patterns across a landscape-scale gradient of fire severity, to determine whether seed mass and dormancy class mediate shifts in community assemblages.

Methods: We surveyed 25 sites in wet sclerophyll forests in south-eastern Australia impacted by the 2019-2020 Black Summer Fires, burnt at either moderate, high, or extreme severity. We calculated abundance and density of seedlings from 27 common native shrub species.

Key results: Extreme severity fires caused significant declines in seedling recruitment. Recruitment patterns differed between dormancy class, with steeper declines in seedling emergence for species with physiologically dormant (PD) compared to physically dormant (PY) seeds at extreme fire severity. Relative emergence proportions differed between fire severity and seed size groups for both PY and PD species.

Conclusions: Large-scale extreme severity fires favour larger-seeded species, shifting community composition. Future recurrent extreme fire events could therefore place smaller-seeded species at risk. Seed mass, dormancy type and other novel seed traits should be considered when exploring post-fire responses, to better predict impacts on plant species.