Stability in the patterns of long-term development and growth of the Canadian spruce-moss forest

Aim The spruce-moss forest is the main forest ecosystem of the North American boreal forest. We used stand structure and fire data to examine the long-term development and growth of the spruce-moss ecosystem. We evaluate the stability of the forest with time and the conditions needed for the continuing regeneration, growth and re-establishment of black spruce (Picea mariana) trees. Location The study area occurs in Quebec, Canada, and extends from 70 degrees 00' to 72 degrees 00' W and 47 degrees 30' to 56 degrees 00' N. Methods A spatial inventory of spruce-moss forest stands was performed along 34 transects. Nineteen spruce-moss forests were selected. A 500 m2 quadrat at each site was used for radiocarbon and tree-ring dating of time since last fire (TSLF). Size structure and tree regeneration in each stand were described based on diameter distribution of the dominant and co-dominant tree species [black spruce and balsam fir (Abies balsamea)]. Results The TSLF of the studied forests ranges from 118 to 4870 cal. yr bp. Forests < 325 cal. yr bp are dominated by trees of the first post-fire cohort and are not yet at equilibrium, whereas older forests show a reverse-J diameter distribution typical of mature, old-growth stands. The younger forests display faster height and radial growth-rate patterns than the older forests, due to factors associated with long-term forest development. Each of the stands examined established after severe fires that consumed all the soil organic material. Main conclusions Spruce-moss forests are able to self-regenerate after fires that consume the organic layer, thus allowing seed regeneration at the soil surface. In the absence of fire the forests can remain in an equilibrium state. Once the forests mature, tree productivity eventually levels off and becomes stable. Further proof of the enduring stability of these forests, in between fire periods, lies in the ages of the stands. Stands with a TSLF of 325-4870 cal. yr bp all exhibited the same stand structure, tree growth rates and species characteristics. In the absence of fire, the spruce-moss forests are able to maintain themselves for thousands of years with no apparent degradation or change in forest type.