An index for tracking sheltered forest floor moisture within the Canadian Forest Fire Weather Index System

The Duff Moisture Code (DMC) component of the Canadian Forest Fire Weather Index (FWI) System is used by fire management agencies across Canada as an indicator of the susceptibility of the forest floor to lightning fire ignition. However, this model was developed for the moisture content of the forest floor away from the sheltering influences of overstory trees, an area where lightning strikes usually ignite the forest floor. Through destructive sampling of the forest floor in a mature jack pine stand in northern Ontario over several summers, the moisture content of the forest floor in sheltered areas close to the boles of dominant overstory trees was found to be significantly lower than in other less heavily sheltered areas of the stand. Observations from a network of in-stand rain gauges revealed that rainfall penetration through the canopy (throughfall) was correlated with both open rainfall amount and the duration of a storm. Observed throughfall amounts were used to develop a throughfall relationship for extremely sheltered locations (within ∼ 0.5 m of the boles) in a mature jack pine stand. This throughfall model was used, along with differences in forest floor drying rate, to develop a new duff moisture index for strongly sheltered areas of the forest floor. Calculated values of this new moisture model, which has the same daily weather observation requirements as the FWI System's DMC model, were found to match observed moisture contents quite well.