Process and patterns of duff consumption in the mixedwood boreal forest

Postfire tree recruitment in the boreal forest is restricted to patches from which the duff (organic layer) has been removed by fire. Duff consumption occurs by smoldering combustion, propagation of which is determined by bulk density, moisture content, and depth. This study investigated interactions among these factors, their spatial distribution, and spatial patterns of duff consumption in two wildfires. A hypothesized positive relationship between moisture content and depth was supported by a laboratory study. Duff characteristic data were collected from two burns and comparable unburned areas of mixedwood forest in western Canada to describe and explain patterns of duff consumption within and between top-slope Pinus banksiana Lamb.-Picea mariana (Mill.) BSP stands and lower slope pure stands of Picea mariana on glaciofluvial hillslopes. In unburned stands, bulk density did not differ significantly between stands, while depths were significantly greater in Picea stands. With moist duff, consumption was significantly greater in Picea than in Pinus-Picea stands as depth limited smoldering in the thin duff of Pinus-Picea stands. With dry duff, smoldering was propagated regardless of depth resulting in no differences between stand types. The spatial correlation between burned patches and fire-killed trees was explained by within-stand spatial variation in duff moisture due to precipitation interception by tree crowns.