Landscape-scale experimental proof that tall wet <i>Eucalyptus</i> regrowth burns more severely than mature forest

Abstract We conducted a natural experiment using remote sensing and field monitoring data collected pre- and post-wildfire, contrasting mature and regrowth tall wet Eucalyptus forests, at a study site in southeastern Tasmania burned by a wildfire under moderate fire weather conditions. Satellite analyses showed that regrowth patches initiated by logging burned at significantly higher severity than mature forests. Regrowth forests had warmer and drier fire season (October–March) microclimates than mature forests before the fire (16.4 °C vs 14.6 °C), with this effect larger after the fire (20.3 °C vs 16.8 °C). Fine fuels were abundant in both unburned forest types, with mature forest having significantly higher loads of fine fuels (leaf litter and twigs &lt;6 mm in diameter; P &lt; 0.01) and coarse fuels (sticks and twigs between 0.6 and 2.5 cm diameter; P &lt; 0.001). Comparison of pre- and post-fire air borne lidar measurements revealed high consumption of regrowth canopies, especially in the lower half of the 30 m profile, compared to the more modest effects on the 40–60 m tall mature forests. Resprouting enabled rapid recovery of the vegetation vertical profile in both mature and regrowth forests. The perimeter of the fire was nearly exclusively (98%) mature forest, underscoring this was inherently less flammable than regrowth. Collectively these findings suggest that regrowth is more flammable than mature forests because of combustible dense lower canopies and a drier microclimate, which makes dead and live fuel more frequently available to burn. Fire exacerbates these differences, potentially leading to amplifying feedbacks. Managing fire risk in regrowth forests under a rapidly warming climate represents a profound forest management challenge.