The Duff Moisture Code and the limits of sustainable combustion: examining the evidence for a widely used threshold

Background Canadian fire managers rely on the value of the Duff Moisture Code (DMC) for estimating lightning ignition and sustained smouldering in ground fuels. A simple rule used widely operationally suggests that lightning does not ignite fires and smouldering is not sustainable until the DMC >20. Aims We examine the strength of evidence supporting this simple rule. Methods We used daily lightning, fire and weather data from 2000 to 2019 to estimate the probability of lightning fire ignition across a number of regions in Canada. We also examined datasets of forest floor consumption from experimental burns carried out in pine forests in Canada. Key results Neither the 20 years of lightning fire ignition data nor the observed forest floor consumption data reveal consistent signals of an ignition threshold at or around DMC = 20. Conclusions and Implications Increasing DMC is associated with increasing probability of ignition from lightning and the extent of forest floor consumption, but there is little to no evidence to support the existence of a meaningful threshold across Canada when DMC is ~20. Users of this simple rule, be it for lightning ignition or fire perimeter extinction, should be aware that the data do not support a meaningful threshold around this value.