Invading rain forest pioneers initiate positive fire suppression feedbacks that reinforce shifts from open to closed forest in eastern Australia

Abstract

Question Suppression of Aboriginal burning and wildfire from forests in which fires were historically frequent may trigger environmental changes that further suppress fire frequency and intensity. In high‐rainfall regions of eastern Australia, long‐unburnt open forests are frequently invaded by rain forest pioneer trees, which in turn modify open‐forest understorey structure and composition. We examine altered understorey fuel properties as potential fire suppression feedback mechanisms reinforcing switches from fire‐dependent eucalypt open forest to fire‐resistant closed forest. Location Northeastern New South Wales, Australia. Methods We characterized forest understorey leaf traits, plant crown architecture and vegetation structure to test for differences in flammability across a mosaic of contrasting times since fire (recently burnt — 4 years; long unburnt — 16 years) and midstorey composition (open, sclerophyll, rain forest) in subtropical shrubby dry sclerophyll eucalypt forest. Results Drivers of flammability were lower in long‐unburnt open forest with a rain forest pioneer midstorey at multiple scales (leaf, foliage clump, crown and stratum) than either long‐unburnt open forests with a sclerophyll midstorey or recently burnt open forest with an open midstorey. Grassy (graminoid) and sclerophyllous shrub fuels declined sharply beneath a rain forest pioneer midstorey, consistent with the fire suppression feedbacks regulating tropical savanna to closed forest transitions. Rain forest pioneers also reduced understorey flammability drivers through floristic compositional changes that increased leaf and foliage clump separation in shrub and midstorey fuel strata, and increased leaf moisture in the shrub stratum. Conclusion Our results suggest that rain forest pioneers are ecosystem engineers that modify open‐forest fuel properties, initiating a positive fire suppression feedback that facilitates their persistence and continued transition to a fire‐resistant closed forest. The fire suppression feedbacks we identify may help explain paleoecological and modern observations of rain forest expansion under high CO2 atmospheres, despite increased temperatures and seasonal drought severity.

Rainforest persistence and recruitment after Australia’s 2019–2020 fires in subtropical, temperate, dry and littoral rainforests

“…However, fire-resilient rainforest plants may quickly reassert competitive pressure postfire (e.g. Williams et al 2012), potentially limiting long-term re-establishment of typical sclerophyll understorey plant communities, fauna habitat (Laurance 1997;Tasker et al 2017) and understorey fuels that underpin open-forest flammability (Hoffmann et al 2012a;Baker et al 2021). Our results demonstrate resprouting potential among a wide diversity of rainforest taxa known to colonise long-unburnt open forests adjacent to the major rainforest classes of eastern Australia.…”

“…Finally, regeneration of flammable ground-layer plants (e.g. graminoids) was negligible in all plots, and any minor regeneration is likely to be quickly eliminated as regenerating rainforest plants achieve canopy closure (Williams et al 2012;Hoffmann et al 2012a;Baker et al 2021).…”

“…Other potential climate drivers include increased regional rainfall (Bowman et al 2010), increased atmospheric humidity (Willett et al 2007), and improved forest water-use efficiency in elevated CO 2 atmospheres (Keenan 2015). While there is increasing evidence that invading rainforest pioneers displace open forest flora and fauna (Chapman and Harrington 1997;Woinarski et al 2004;Baker et al 2020aBaker et al , 2020b and disrupt ecosystem function (Hoffmann et al 2012a;Baker et al 2021), the threshold at which these invading rainforest plants become resistant to removal by subsequent fires is not well understood (Baker et al 2020a). Resprouting capacity in woody plants often increases from a low level at the seedling and sapling stage, and peaks at the young to mature stage, before declining during late maturity to senescence (Bond and Van Wilgen 1996;Vesk 2006;Clarke et al 2013;Jaureguiberry et al 2020), although there is variation between growth forms and ecosystem type (Clarke et al 2013).…”

Frequently Burnt Subtropical Eucalypt Forest Is More Resilient to Wildfire Than Rarely Burnt Forest

“…Excessive sapling density smothers grasses and herbs in subtropical eucalypt forest (Kington et al, 2021;Lewis & Debuse, 2012;Baker et al, 2020), and it is grasses and herbs that provide the bulk of species diversity in these forests of South East Queensland (Ryan, 2012). The loss of native grass cover reduces the capacity of land managers to implement low-intensity fires under mild conditions, because fire carries through a continuous grass layer while there remains good soil moisture early in the dry season but is less capable of spreading through a ground layer of predominantly leaf litter until soil and fuel dry out (Baker et al, 2021). A forest cluttered with tree saplings has very high to extreme elevated fuel that can increase the intensity of a fire and create a ladder of fuel that burns into and damages the canopy (Barker et al, 2022).…”

Kangaroo grass is a keystone species for ecological fire management

“…Without regular burning, the density of woody plants can increase, and kangaroo grass is replaced by less flammable, shade‐tolerant herbs and leaf litter (Baker et al, 2020, 2021; Lunt, 1998; Stone et al, 2022; Watson et al, 2009; Watson & Morris, 2020). Planned burning then becomes more difficult while the soil is moist because leaf litter, herbs and saplings are less combustible but burn intensely once dry (Prior et al, 2017).…”