Canadian Forest Service Publications

The impact of root exclusion on duff moisture and fire danger. 2015. Thompson, D.K.; Studens, J.; Krezek-Hanes, C.; Wotton, B.M. Canadian Journal of Forest Research 45(8):978-986.

Year: 2015

Issued by: Northern Forestry Centre

Catalog ID: 36083

Language: English

Availability: PDF (request by e-mail)

Available from the Journal's Web site.
DOI: 10.1139/cjfr-2014-0397

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The impact of root water uptake on duff moisture content was investigated at mature deciduous, mixedwood, and conifer stands on the southern edge of the boreal forest. 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 point the root exclusion plots remained at 15‒20% volumetric water content while intact plots declined to as low as 5% volumetric water content during rain-free periods. Only in the root-intact plots did 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 cumulative growing season water loss in the duff by 19‒31%, depending on stand type. Root exclusion also decreased the number of days with a high probability of duff smouldering by 87‒100%, suggesting that fuel thinning treatments for community fire protection involving reduction in basal area may depress duff ignitions in the short-term after treatment.

Plain Language Summary

The current forest fire danger rating system in Canada accounts for drying of forest floor duff only through water loss to the atmosphere directly. There is question as to the amount of water that is directly taken up by trees, as opposed to being evaporated by the atmosphere. A manipulation experiment was conducted in three forest stands near Sault Ste. Marie, Ontario, in order to quantify the role of direct water uptake by tree roots in duff. Manipulated plots had a layer of water-permeable geotextile laid around the duff in order to allow free water movement, but to exclude tree roots from taking up water in the duff block. Prior to leaf-out in the spring, manipulated plots were the same moisture content as control plots, but after leaf-out a 30% decrease in the rate of duff drying after rainfall was observed in the manipulated plots where roots were intact. The probability of smouldering combustion in the duff was estimated using existing mathematical models based on moisture content and density. Control plots were modelled as having a high smouldering probability for an average of 72 days per year, while the wetter manipulated plots with roots excluded were only modelled to be at high risk of smouldering for 14 days. This study has implications to model duff ignition patterns in FireSmart stands, where the manipulation experiment performed here simulates the impact of tree thinning that occurs in FireSmart stands.