The regeneration niche of white spruce following fire in the mixedwood boreal forest

Abstract

Early establishment of white spruce (Picea glauca) in mixedwood boreal forest stands following fi re was examined at several times-since-fi re (1-, 2-, 4-, 6-, 14-years). Abiotic and biotic conditions in the stands were assessed at two scales, tree plot (5 m × 5 m) and microsite (1 m × 1 m), along with presence, density and height of white spruce seedlings. Germination and survival of seed sown 1and 4-years post fi re were quantifi ed. Survival and growth of nursery-grown seedlings, and mycorrhizal colonization, survival and growth of sterile seedlings, planted 1-year post-fi re were assessed. At the tree plot scale, presence of white spruce seedlings 1-year post-fi re could be reliably predicted by organic layer depth and distance to and strength of seed source. In contrast, none of the biotic or abiotic factors measured was strongly correlated with occurrence or density of white spruce seedlings 6and 14years post-fi re. At the microsite scale, seedling recruitment immediately post-fi re was limited to a distinct subset of available microsites (greater % cover of downed wood and moss, lower % cover of litter and herbaceous vegetation). Likewise, seedling occurrence in older burns was associated with distinct microsite conditions; although this was more likely an effect of white spruce presence, rather than the cause. Less than 3% of seed sown 1 yr post-fi re survived to become 1 yr old germinants, survival was 41% over the next three years. Availability of suitable regeneration microsites declines rapidly with time-since-fi re; less than 0.3% of seed sown 4 yrs post-fi re survived one year. High rates of mycorrhizal colonization were found on white spruce seedlings planted 1-year post-fi re, including early and late stage fungal species. Microsite characteristics were only weakly correlated with mycorrhizal fungal communities. We propose that immediate post-fi re recruitment of white spruce is a key process in mixedwood boreal succession.