Flammability metrics from cone calorimetry for common wildfire fuels sampled in Great Britain from 2021-2023

The samples were collected from 22 sites across Great Britain (see supporting documentation). The general sampling regime included: (1) fortnightly sampling of lowland heath vegetation from March 2021 to February 2022; (2) monthly sampling of forest plantation litter from July 2022 to July 2023; and (3) monthly sampling of upland heaths and moorlands from July 2022 to July 2023, with sampling occasionally prevented by weather conditions or travel restrictions. As data for low fuel moisture conditions is of particular importance, further sampling of lowland heaths was undertaken on days without precipitation, and additional sampling at one site (Woodbury Common) focused on periods of dry weather from May to October 2023. Whether dead and/or live fuel samples were present varied seasonally according to species, and a small number of collected samples were lost during processing, or contained insufficient material to use two replicates for flammability testing. Weather data were recorded on the majority of sampling occasions; this depended on the equipment available at the time and location and is therefore not consistent across the dataset. Some individual weather data, fuel moisture values, and time-to-ignition values are missing due to human error.
Shrubs were sampled by cutting approximately the top 15 cm of the canopy, and non-woody plants by taking the whole aboveground plant. Each litter sample was collected at a single point, and the duff samples from immediately beneath. Weather data were recorded with a Kestrel 5400FW or 3550FW Fire Weather Meter (Kestrel instruments, USA). Moisture content was preserved by sealing samples in zip-lock polythene bags (litter and vegetation) or polythene tubes (duff). Laboratory processing began typically 24 hours after collection, excepting lowland heath vegetation which was tested on the day of collection.
Each sample was divided to provide two subsamples for flammability testing and one for moisture content measurement. Moisture content was obtained by weighing subsamples (c. 20 g), then placing them in a drying cabinet at 40-50° C until they reached constant weight. The flammability subsamples were analysed in an 'iCone plus' cone calorimeter (Fire Testing Technology Ltd, East Grinstead, UK). The weighed subsample is filled into a perforated steel cylinder of 375 cm3, and placed 25 mm beneath the heating element, set to 750° C, to achieve a heat flux of 50 kW m−2 at the surface of the sample. The apparatus maintains an electric arc 15 mm above the surface to ignite the pyrolysate. Time to ignition (TTI) is recorded manually. The sample remains in place until flaming ceases, and the test time is defined as ending 20 s after cessation of flaming. Non-ignition after 300 s (240 s for lowland heaths) results in the test being discontinued, as the fuel is considered non-ignitable in any realistic scenario. PHRR, THR and EHC are calculated automatically and recorded after the test is complete.