Low-severity fires in the boreal region: reproductive implications for black spruce stands in between stand-replacing fire events

Stand-replacing crown fires are the most prevalent type of fire regime in boreal forests in North America. However, a substantial proportion of low-severity fires are found within fire perimeters. Here, we aimed to investigate the effects of low-severity fires on the reproductive potential and seedling recruitment in boreal forest stands in between stand-replacing fire events. We recorded site and tree characteristics from 149 trees within 12 sites dominated by mature black spruce Picea mariana (Mill.) B.S.P. trees in the Northwest Territories, Canada. The presence of fire-scarred trees supported classification of sites as unburned or affected by low-severity fires in recent history. We used non-parametric tests to evaluate differences in site conditions between unburned and low-severity sites. We used linear and additive statistical models to evaluate differences in tree age, size and reproductive traits among unburned trees and trees from low-severity sites. The results showed a significantly higher density of dead black spruce trees in low-severity sites and marginally significant higher presence of permafrost. Trees from low-severity fire sites were significantly older, exhibited significantly lower tree growth and showed a tendency towards a higher probability of cone presence and percentage of open cones compared with trees from unburned sites. Surviving fire-scarred trees affected by more recent low-severity fires showed a tendency towards a higher probability of cone presence and cone production. The density of black spruce seedlings increased significantly with recent low-severity fires. Trees in low-severity sites appeared to have escaped mortality from up to three fires, as indicated by fire-scar records and their older ages. Shallow permafrost at low-severity sites might cause lower flammability, allowing areas to act as fire refugia. Low-severity surface fires temporarily enhanced the reproductive capacity of surviving trees and the density of seedlings, probably as a stress response to fire events.