Understanding forest cover change is of increasing importance to ecologists and forest managers for developing long-term management strategies. In areas of North America’s boreal forest where average time between stand replacing fire is less than the life span of dominant tree species, post-fire stand composition often resembles pre-fire conditions, suggesting boreal stands may not undergo succession. Alternatively, studies indicate that long-term absence of catastrophic fire results in a compositional shift from shade-intolerant species to mixed coniferous-deciduous or coniferous stands dominated largely by shade-tolerant species.

Using repeatedly measured plot data, our aim was to address the following questions: (1) Will boreal stands undergo succession in the long-term absence of catastrophic fire? (2) Do boreal stands exhibit multiple successional pathways? (3) Can successional pathways of boreal stands be inferred from time since fire (TSF), site characteristics, or non-stand replacing disturbances?

We combined repeatedly measured sample plots with sequential aerial photographs to re-construct historic changes in species composition for 361 boreal stands located in northern Ontario, Canada. Plots were measured in 8-15 year intervals for a 60-year period and covered a wide range of species composition, site characteristics, and stand ages. Time since fire was used to age all plots. Plots were classified into 8 cover type classes based on percentage of species basal area. Multinomial logistic regression was used to analyze stand transitions and to test if cover type change was significantly related to TSF, site characteristics, and minor disturbance.

Results/Conclusions

Preliminary results show a transition from post-fire stands dominated by shade-intolerant species such as jack pine (Pinus banksiana Ait.), and trembling aspen (Populus tremuloides Michx.) to mixed-stands of increased shade-tolerant species (e.g., balsam fir Abies balsamea (L.) Mill., and cedar Thuja occidentalis L.). All cover types demonstrated multiple successional pathways related to TSF except for balsam fir and cedar stands. Cover type transition in balsam fir stands was most related to minor disturbance (e.g., insect damage - Choristoneura fumiferana). Cedar stands remained relatively stable overtime transitioning only to mixed-conifer stands on upland sites.

Results from this study show boreal forest stands in central Canada will undergo succession in the long-term absence of stand replacing fire. Also, that boreal forest stands may exhibit multiple successional pathways related to TSF, site conditions, and minor disturbance. Observed successional patterns suggest cedar dominated stands may form a successional climax, challenging contemporary research that implies climax associations do not exist.