Exploring phosphate effects on leaf flammability using a physical chemistry model

Some plants have traits that cause them to be more flammable than others, influencing wildfire spread and fire regimes. Some of these plant traits have been identified through laboratory-scale experiments. We built a numerical model that could quantify the extent of these effects on flammability. Here we present that model and use it to investigate the effect of phosphate content on the flammability of leaves. The model used finite-element methods and was based on heat transfer and thermal decomposition kinetics. Predictions were compared with three laboratory experiments involving ignition of leaf or cellulose samples. We then ran simulations of two situations through which leaf phosphate could influence wildfire spread: horizontal fire spread and crowning. The ignition time and maximum fuel gap that could be bridged by a flame front was predicted. Two key results emerged. (1) The importance of leaf phosphate in laboratory studies of ignition depends on the rate of sample heating, with the strongest effect under slow heating. (2) In the context of wildfires, phosphate was predicted to have modest effects compared with other plant traits influencing moisture content, leaf construction and angle of display.