Unraveling the chemistry of plant flammability: Exploring the role of volatile secondary metabolites beyond terpenes

Plant flammability research has proven pivotal in comprehending the contribution of vegetation to the flammability of forest ecosystems. Yet, the relationship between many leaf chemical traits and plant flammability is poorly understood. While terpenes and some leaf nutrients in plants have been extensively studied for their role in flammability, a wide array of other secondary metabolites remain unexplored in this context. Here, we present the volatile secondary metabolites composition of fresh leaves from nine dominant species from central Chile, both native and exotic, and determine whether there is variability within and among species in chemical composition and flammability. Moreover, we investigate how these compounds influence various leaf flammability traits. The Chilean Mediterranean ecosystem emerges as a useful study site given its unique endemic flora, increased frequency of forest fires, proliferation of invasive plants and extensive land conversion that favors the spread of fire-prone exotic species, and significant scarcity of phytochemical research dedicated to this ecosystem. A total of 118 volatile chemical compounds were quantified, belonging to over ten groups of volatile secondary metabolites. Terpenes, ketones, and hydrocarbons comprised 75 % of these compounds, and each species displayed a unique phytochemical profile. Surprisingly, some native species (Citronella mucronata, Cryptocarya alba) exhibited equivalent or higher leaf flammability than the well-known flammable exotics Eucalyptus globulus and Pinus radiata, respectively. Leaf flammability was best explained by the concentration of aldehydes, ketones, green leaf volatiles, and aromatic compounds. Interestingly, terpenes as well as moisture content were not significantly correlated with flammability. In conclusion, our results highlight the importance of considering a broader range of phytochemicals, beyond terpenes, to fully understand leaf flammability among species. Consequently, a deeper understanding -within and across ecosystems- of the influence exerted by diverse groups of phytochemicals on flammability is an urgent need for forest management planning in an increasingly flammable world.