Abstract

--- - Background Fine fuels, characterized by high surface-area-to-volume ratios and rapid moisture exchange, significantly influence fire behavior; yet, cryptogamic organisms, such as lichens, are often overlooked in flammability studies, despite their prevalence in temperate and boreal ecosystems. Lichens, covering similar to 8% of terrestrial surfaces, can act as ladder fuels, potentially enhancing fire spread in forests. Their flammability is thought to depend on moisture content, chemical composition, and physical traits such as thallus morphology; yet empirical evidence remains limited. This study assessed the flammability of four abundant lichen species from southern Chile, using laboratory epiradiator assays. It evaluated the influence of moisture content, growth form (foliose vs. fruticose), and secondary metabolites on ignition and burning behavior, with implications for fire dynamics under climate change. - Results Moisture content varied widely across species, with foliose lichens generally absorbing more water when hydrated, but fruticose lichens retained slightly more moisture when dry. Ignition frequency was high across all species and humidity conditions (98-100%), comparable to that of native Chilean vascular plants. Ignition and burning times were significantly influenced by moisture content, and to a lesser extent by morphology. In particular, ignition time was significantly longer (1072%) in wet state, while burning time lasted 110% longer in dry conditions. Notably, Nephroma antarcticum retained high flammability even when hydrated, despite longer ignition times, suggesting other traits may offset the dampening effect of water content. Chemical analyses revealed a diverse array of secondary metabolites, particularly terpenoids and phenolics, although their relationship with flammability appeared complex and species-dependent. - Conclusions Despite their poikilohydric nature, lichens ignite readily, acting as effective ladder fuels under both dry and moist conditions, similar to the most flammable species of the surrounding Chilean vegetation. Lichen flammability was primarily driven by its high water-holding capacity, which, similarly to other common forest fuels, influences ignition and burning times, while thallus morphology played a less direct role. Terpenoids may enhance flammability, though N. antarcticum's longer ignition times suggest complex interactions. These findings highlight the ecological relevance of lichens in fire-prone ecosystems and underscore the need to integrate them into fire behavior models and management strategies.