| Wildfires reinitiate the process of forest succession in boreal mixedwood forests; however, the timing of white spruce (Picea glauca Moench Voss) regeneration post-fire, which greatly influences the successional development of stands, varies greatly between fires. I investigated whether fire history attributes in conjunction with masting (the intermittent and synchronized production of abundant seed crops over large areas) of white spruce affected the timing and density of spruce regeneration, and ultimately the successional pathways of mixedwoods. I also determined whether: (1) aging errors occur, (2) hare browsing causes missing rings, (3) a delayed regeneration pulse occurs, and if so, when? (4) what factors affect the successional pathway of mixedwood stands post-fire, and (5) the concordance between pre- and post-fire density of white spruce. I studied 17 fires in northeastern Alberta that occurred during mast years (high seed production) as well as non-mast years (low seed production).; Burn timing relative to mast years had a long-lasting effect on regeneration. Significantly more spruce regeneration occurred in mast year fires than non-mast year fires. The greatest decline in density occurred between a mast year fire and fires with a one year delay before a mast year. Crossdating verified that a large mast cohort occurred on all mast year fires, and that most initial regeneration occurred within 5 years of the fire on recent fires. Ground-level age significantly underestimated the true age of spruce (up to 27 years in 40-year-old trees), and aging errors increased as stands aged. Clipping experiments indicated that browsing like that inflicted by snowshoe hares could lead to substantial age underestimation in white spruce. Delayed regeneration peaked 38 to 44 years after fire, and occurred primarily on decayed logs. Out of 20 stands from 1941 and 1942 fires, 7 stands were dominated by initial regeneration, 7 had “even” mixtures of initial and delayed regeneration, and 6 were dominated by delayed regeneration. Significant variability in spruce age structure occurred between the oldest fires despite similar masting history, burn dates, and seed availability. Finally, the likelihood of concordance between pre- and post-fire populations of white spruce was greater on mast and one-year-delay fires, than fires with longer delays. Our results indicate that masting is a keystone process that interacts with fire history to affect the persistence of white spruce in mixedwood landscapes. |
