The impact of root exclusion on duff moisture and fire danger

The impact of root water uptake on duff (both fibric and humic horizons) moisture was investigated at deciduous, mixedwood, and conifer stands in Ontario, Canada. Roots were actively excluded from the duff layer using geotextiles inserted at the duff-mineral soil interface and along the plot edges; liquid and vapour water flow was otherwise not affected by the geotextiles. Root exclusion caused little difference in duff moisture content prior to early June, after which the root exclusion plots remained at 15%-20% volumetric water content, whereas root-intact plots declined to as low as 5% volumetric water content during rain-free periods. Only in the root-intact plots did the duff water content reach sufficiently low levels that duff evaporation was limited by low water content. The net effect of root exclusion was to reduce the cumulative growing season water loss in the duff by 19%-31%, depending on the stand type. Root exclusion also decreased the number of days with a high probability of duff smouldering from as many as 72 days.year(-1) to as few as 0 days.year(-1). This root exclusion experiment provides a model for short-term duff moisture transitions under thinned forests such as those forests under community wildfire protection.