Microclimate and species composition shape the contribution of fuel moisture to positive fire-vegetation feedbacks

Abstract

Ecosystems with positive fire-vegetation feedbacks are highly sensitive to extreme fire events, which are expected to increase in frequency with climate change. Understanding mechanisms maintaining flammability differences between alternative vegetation states is key to elucidate possible ecosystems shifts related to altered fire regimes. Here we assessed whether microclimatic differences between pyrophobic forests and pyrophilic shrublands in northwestern Patagonia can affect the fuel moisture content and partially explain their contrasting flammability. We measured dead and live fuel moisture content and recorded air temperature and relative humidity during one fire season in 10 sites corresponding to either forest or shrubland physiognomy. To disentangle effects from microclimate and species composition on live/dead fuel moisture content we used similar-composition fuels (wood sticks and single-species live leaves) and fuel mixtures (litter and a mixture of live leaves). We found that forests’ microclimate is cooler and moister than shrublands’, harbouring dead fuels with higher moisture content, irrespective of species composition. This suggests that dead fuel moisture content is contributing to make forests less flammable than shrublands. Live fuels tended to show higher moisture content under the forest microclimate, but only when single-species samples were considered. Contrarily, the species mixtures showed higher moisture content in shrublands, the most desiccant communities, suggesting that live fuel moisture content does not contribute to make shrublands more flammable.