Fire Severity and Vegetation Response in the Boreal Swedish Forest

We studied plant survival and colonization over an experimental gradient, from fire lightly scorching the soil to fire consuming most of the organic soil layer, at two forest sites in northern Sweden. The gradient was achieved by adding different amounts of fuel to small plots that were burned in 1988 and 1989. Temperature was recorded at four soil strata during burning. We analyzed survival of seeds and rhizomes in the soil immediately after fire, and followed vegetation cover and seedling establishment until 1993. During fire, there was a steep decline in maximum temperature with increasing depth below the char, irrespective of the depth of burn in the mor layer, indicating that burn depth can be used as a general indicator of heat impact below ground. Lethal temperature was not recorded deeper than 20—30 mm under the burn boundary. Plant survival was determined both by depth of burn and by depth distribution of regenerative structures in the soil. Three rhizomatous species, the dwarf shrubs Vaccinium myrtillus and Vaccinium vitis—idaea and the grass Deschampsia flexuosa, were dominant in the prefire vegetation. For all three species, the bulk of the soil bud bank was located within the mor layer, but was more superficial for D. flexuosa. Initial mortality in the bud bank was progressively higher with increasing depth of burn, and this determined the regrowth over the following years. After fires that consumed only the moss layer, cover of the Vaccinium species returned to prefire levels within 2—4 yr, and D. flexuosa showed a dramatic increase in cover as well as in fruiting. Fires that burned slightly deeper nearly eliminated D. flexuosa, and the deepest burning fires also eliminated Vaccinium spp. In contrast to regrowth from rhizomes, colonization from seed was better after relatively deep—burning fire, both for species with a soil seed bank and for species dispersing seed onto the burnt soil. However, after fires consuming most of the organic soil layer, seed bank species were also badly affected, whereas dispersers showed progressively better establishment with increasing depth of burn. Differences between treatments were still great after 5 yr, indicating that variation in depth of burn will have a long—lasting impact on the vegetation. These results from experimentally burned plots were corroborated by an analysis of depth distribution of viable plant rhizomes and seeds, and the initial colonization at a site newly burned in a wildfire. The precise response patterns of boreal vegetation to variation in burn depth will depend on characteristics of the species present. However, we assume that these results have a high degree of generality, since, in podzolized soils, most rhizomatous species are predominantly located in the mor layer, since the dormant seed bank typically is concentrated at the interface of mor and mineral soil, and also since a thick organic soil layer is a poor seedbed for incoming seeds. The results indicate that in boreal forest, depth of burn is a more important variable than fire front intensity for the understory vegetation, in contrast to the situation in ecosystems with little accumulation of organic material on the mineral soil.