Flammability of tropical grasses: Towards a functional ecology of fire in savannas

Fire is a key ecological process in tropical savannas, yet species-specific contributions to fuel flammability remain poorly understood in West Africa. Here, we present the first experimental assessment of flammability traits in the Lamto humid savanna (Côte d’Ivoire), focusing on five dominant perennial grasses, litter, and realistic mixtures. We quantified four plant flammability traits (ignitibility, combustibility, sustainability, consumability) and six fire behavior parameters (flame height, fuel consumption, and maximum temperatures at three heights). Our results show that ignitibility varied strongly among species, with Hyparrhenia diplandra and Loudetia simplex igniting more rapidly than others. In contrast, combustibility, sustainability, and consumability were relatively consistent across fuel types. Fire behaviour also varied: complete combustion occurred in some mixtures, whereas Andropogon schirensis and litter-containing mixtures left significant unburned material, likely due to lower fuel porosity. Importantly, mixture flammability was non-additive: mixtures did not reflect the sum of their components but instead approximated the average flammability of constituent species. Principal Component Analysis and hierarchical clustering identified three functional flammability groups: (i) highly flammable fuels (Imperata cylindrica, L. simplex, litter), (ii) moderately flammable fuels (A. schirensis, H. diplandra, mixture 3), and (iii) poorly flammable fuels (A. canaliculatus and mixtures 1, 2, 4). These findings highlight the non-additive and species-specific nature of savanna fuel flammability, with direct implications for fire intensity, severity, and management. This trait-based approach provides a foundation for predicting fire behavior in West African savannas and for integrating species-level flammability into conservation-oriented fire management.