Predicting the long-term yield trajectory of black spruce stands using time since fire

Yield curves are traditionally constructed with mean age of dominant trees as the temporal variable. However, when tree longevity is shorter than the average period of time between two successive disturbances, mean age of dominant trees becomes a doubtful temporal variable in old-growth stands. We thus used the period of time since the last fire (TSF) to model yield of black spruce (Picea mariana [Mill.] B.S.P.) stands in northeastern Quebec, Canada. For the first 200 years after fire, a system of difference equations closely fits the pattern of change in merchantable volume obtained from repeated measurements of 67 permanent sample plots (PSPs). For TSF > 200 years, the absence of precise TSF constrained us to determining only the general yield trajectory, i.e., whether the volume decreases down to a zero value or is maintained at a constant positive value. A procedure based on the observed distribution of 260 PSPs over volume classes suggested that merchantable volume of black spruce stands is generally maintained around a constant value for a long period of time. Using TSF rather than mean age of dominant trees in yield modelling produced large differences indicating that TSF should be used where available, especially in regions dominated by old-growth forests.