The effects of experimental fires on black spruce forest floors in interior Alaska

Abstract

Seven units (about 2 ha each) of black spruce – feather moss forest were experimentally burned over a range of fuel moisture conditions during the summer of 1978. Surface woody fuels were sparse and the principal carrier fuel was the forest floor (largely mosses and their decomposition products). Forest floors after burning comprised a small-scale mosaic of unburned, scorched, lightly burned, moderately burned, and heavily burned (organic materials entirely consumed) conditions. Percentage of the unit area in the moderately and heavily burned condition ranged from 11.2 to 77.2% and percent decrease in forest-floor thickness varied from 27.4 to 63.1% in the seven units. Forest-floor consumption was most closely correlated with the moisture content of lower moss (01 horizon) and lower duff layers (022 horizon) at the time of burning. For the first 3 years after fire, biomass production was greater on heavily burned than on lightly burned sites (58 vs. 37 g/m2 on an annual basis). Heavily burned sites were completely dominated by the invading species Epilobiumangustifolium L., Ceratodonpurpureus (Hedw.) Brid., and Marchantiapolymorpha L., whereas lightly burned plots were occupied by sprouting species such as Calamagrostiscanadensis (Michx.) Beauv., Vacciniumuliginosum L., and Ledumgroenlandicum Oeder. Soil pH and amounts of total P and available P in the forest floor increased significantly as a result of burning; and in all cases, increases reached a maximum in moderately and heavily burned areas. Total N in the forest floor increased significantly in moderately burned, but decreased slightly in heavily burned areas. Total N and total P showed smaller increases in the surface mineral soil as a result of burning. Supplies of available P in the mineral soil increased almost 4-fold in moderately burned and over 16-fold in heavily burned areas.