Woody thickening in the grassy ecosystems of eastern Australia is caused by, then changes, fire regimes: a review

Open grassy eucalypt-dominated ecosystems are significant habitat for a broad range of fauna and flora, including threatened species. Across eastern Australia, many grassy ecosystems have thickened with shrubs and saplings over the last two centuries, following a reduction in fire frequency. While CO2 fertilisation and increased grazing pressure can also encourage woody thickening, here we focus on the central role of fire. Healthy grassy ecosystems contain scattered shrubs and saplings which do not excessively impact the grass layer. Unhealthy woody thickening, which typically involves elevated densities of only a few ‘expander’ woody species, changes ecosystem structure and thins the grass layer, reducing the abundance and diversity of understorey plants that need light to thrive. In this review of the interaction between fire regimes and woody thickening, we challenge the assumption that thickening primarily occurs from regular seed germination spread evenly throughout inter-fire intervals. We highlight the role of fires that are severe and/or implemented in dry conditions, in promoting dense woody recruitment in the first year or two after fire. Thus severe fires concentrate recruitment in the initial years after fire, followed by sapling growth and irregular germination during inter-fire intervals. The characteristics of recent fires, species available and the duration of inter-fire intervals therefore combine to influence woody plant abundance and size. Occasional, high severity fires in the presence of seed banks of expander woody species, such as wattles, promote rapid woody thickening. Woody thickening changes fire regimes in two critical ways. It reduces the capacity of fires to ignite and spread under mild conditions, while increasing the potential severity of fires once they do burn with more severe fire weather. Grassy communities carry fires under milder conditions with greater soil moisture than do communities where woody thickening has reduced the abundance of grasses. Excessive woody thickening makes it considerably more difficult to implement low intensity, patchy management fires, including cultural and ecological burns. Woody thickening increases the potential upper severity of fires due to increased continuity of elevated fuels. Plant traits that promote woody thickening include long-lived seed banks with fire-cued dormancy triggers, and the capacity to recruit during inter-fire intervals. Grassy, open ecosystems should be prioritised for regular, low intensity fires with good soil moisture to maintain their healthy condition. Regular burning can restore the structure of open ecosystems in the early stages of woody thickening. Follow up burns within a few years of a severe fire that recruited dense woody seedlings can interrupt the progression to woody thickening. Maintaining and restoring grassy eucalypt ecosystems provides healthy habitats which are less likely than woody thickened ecosystems to carry dangerous, high severity fires.