Fuel moisture and micro-scale combustion calorimetry data for Calluna vulgaris across the UK, April-May 2023

Volunteers were requested to collect samples on 19 April 2023, or as close as possible to this date, based on forecasts of dry, fire-prone weather for the UK. At each sampling location, the approximate height of the heather canopy was measured, and live C. vulgaris was clipped from c. 10 plants along a 20 m transect. Samples from each transect were combined in an aluminium tin, sealed with tape to prevent moisture loss, and sent for analysis by priority mail. Samples for fuel moisture analysis only (n = 700) were weighed, dried at 80°C for 48 h, and then reweighed.

Samples for calorimetric analysis (n = 130) were analysed using an FAA Micro Calorimeter (Fire Testing Technology Ltd, East Grinstead, UK). Two subsamples of 6.2‒31.1 mg were analysed from each sample. The heating rate of the pyrolysis chamber was set to 3°C s-1, the maximum pyrolysis temperature to 750°C, and the combustion chamber temperature to 900°C. The N₂ flow rate was set to 80 cm³ min-1 and the O₂ flow rate to 20 cm³ min-1. For each flammability metric (HRC, PHRR, THR and Tmax) the mean of the tests for the two subsamples was taken. The remainder of each sample was used to obtain FMC: the sample was weighed, then dried at 50 °C until it reached constant weight.

Fuel moisture was defined as: Fuel Moisture = ((wet mass - dry mass) / dry mass) × 100