Effect of moisture content and canopy height on flammability of gorse (Ulex europaeus) shrubs measured in a large-scale oxygen consumption calorimeter

Background Vegetation flammability is strongly influenced by fuel load, structure, and moisture content (MC). Previous studies often tested individual plants or plant parts, limiting their relevance to landscape-scale fire modelling. Aims This study aimed to assess how MC and canopy height affect gorse shrub flammability using a new area-based approach. Methods Combustibility, sustainability and consumability of 36 gorse shrubs of three heights (0.9, 1.3 and 1.7 m) and two MC conditions (dried and fresh), each representing 2 m² of gorse canopy, were tested in a large-scale oxygen consumption calorimeter using a novel sample preparation protocol and a custom 0.91 MW ignition system. Key results The reduction in energy release between fresh and dried shrubs aligned with the energy absorbed by water contained in fine particles (0–5 mm). Most flammability metrics were affected by shrub height and MC, although heat release per unit of consumed dry mass did not change with MC. Conclusions Water in fine fuels limits total heat release by reducing the combustion of larger particles. Taller gorse canopies generate more heat due to higher fuel loads. Implications The proposed area-based flammability testing approach supports integration with remote sensing data, enabling improved physics-based fire behaviour models and risk assessments in shrubland ecosystems.