Fuel moisture and flammability of leaf litter in British forest plantations and their implications for wildfire risk

The UK faces increased wildfire risk due to climate change, but efforts to improve forecasting of this risk are limited by a lack of data on fuel properties. We assessed tree leaf litter flammability and ground fuel (litter and duff) moisture in samples from six managed forest plantations across Great Britain over the course of one year. We sampled litter and duff monthly, from Scots & Corsican pine [Pinus sylvestris L. and Pinus nigra subsp. laricio (Poir.) Maire], Sitka & Norway spruce [Picea sitchensis (Bong.) Carrière and Picea abies (L.) H.Karst.], and Silver birch (Betula pendula Roth) plantations (representative of long-needled, short-needled, and broadleaved trees), measured moisture content, and used cone calorimetry to obtain data on leaf litter flammability. We examined relationships between rainfall, litter moisture, and flammability, and showed that cumulative rainfall over the previous 20–25 days most strongly influenced litter moisture, which in turn accounted for much of the variation in flammability. Silver birch maintained higher litter moisture than coniferous species throughout the year, which may be attributed to differences in leaf morphology, decomposition and phenology between broadleaves and conifers. All species reached minima of both litter moisture and duff moisture low enough to sustain burning during the year. While previous studies have often used artificially composed litter samples to demonstrate the effects of leaf properties (especially leaf shape) on flammability, our results using unmodified litter only partially confirm these effects. Interspecies differences in the peak heat release rate may be attributable to both structural properties and volatile content of the litter. However, differences in total heat release, previously attributed to leaf morphology, were evident only when measured on an areal basis, and not on a mass basis. Our data do not support the hypothesis that the morphology of broadleaves results in more ignitable litter beds. This study presents forest litter flammability data through an annual cycle for the first time, and the first extensive litter flammability data from the British Isles. It provides fuel moisture and heat content data that will contribute to the development of fuel models for UK forest environments, enabling forest management that is better informed about fire risk.