Consumption of the organic layer in southern Sweden during fire events and correlations with the Canadian Forest Fire Weather Index (FWI) risk ratings

Abstract: The occurrence and thickness of organic layers in forests can significantly influence fire behavior. Complete understanding of the consumption of these layers during fire events is a knowledge gap that exists in southern Sweden. In this region, sixteen burned sites were measured for fermentation and humus layer thickness. These measurements were compared to those collected on an adjacent non-burned control site. Consumption was calculated to correlate with the numerical rating components of Duff Moisture Code (DMC), Drought Code (DC), Build-Up Index (BUI), and Forest Fire Weather Index (FWI) of the overall Canadian Forest Fire Danger Rating System (CFFDRS). The FWI numerical ratings focused upon in this study were found to be appropriate indicators for relative amounts of fermentation and humus organic layer consumption attributed to fire events in southern Sweden. In particular, variation of fermentation layer consumption was most clearly associated with the DMC, humus layer consumption with the DC, and total (fermentation and humus) organic layer consumption with both the DC and the BUI. On sites where root exposure and tree mortality were noted, consumption of the organic layer was relatively high and the DMC and DC numerical ratings were categorized as extreme or high risks. Site characteristics, in particular microtopography and vegetation, were significant factors in accounting for the amount of consumption of these organic layers. The efficacy of the FWI fire risk ratings for indicating organic layer consumption was bolstered when coupled with these site characteristics. Additionally, planning smoldering fires for forest ecological or management goals is facilitated by the FWI values.