Forest age class structures as indicators of sustainability in boreal forest: Are we measuring them correctly?

Forest age-class structures are one of the principal indicators of economic and ecological sustainability of forest management. These structures are the aggregation of stand level ages. The question we address is how the latter should be measured. We compared measures based on the mean age of canopy trees (MTA) and on time since last stand initiating fire (TSF). We conducted a timber supply modelling exercise using the two age measures in an area of black-spruce dominated boreal forest in Quebec, Canada, which has only recently come under forest management. The area is characterized by a long fire return interval, and for most of the forest, TSF exceeds mean tree longevity. We found that MTA underestimated TSF by 16% up to age 171 years. After this age, MTA remained constant at approximately 140 years. Eighty percent of the study area's commercial forest has been misclassified in terms of age class. Clearcut harvesting is the dominant silvicultural treatment in the study area but partial cutting could be applied in an effort to better retain old-forest characteristics in managed stands. To sequence these treatments requires the correct definition of stand eligibility in terms of age and structure. We found that using ecological constraints based on MTA rather than TSF may fail to achieve intended abundance or quality of old growth forest. We conclude that TSF is the more adequate indicator of forest age structure. Age structures derived from stand level MTA are unreliable and misleading indicators of ecological and economic sustainability.