Managing forest harvesting to maintain old growth in boreal and sub-boreal forests

Old-growth stands can be rare in northern coniferous forests, and hence are worthy of protection and special management. We describe some quantitative guidelines for recognizing old-growth stands and options for maintaining a long-term supply of old-growth values in landscapes managed for timber production. In the Sub-Boreal Spruce forests of central British Columbia, attributes most indicative of old-growth status include stand age, the density of large (>1.0 m(3)) snags and downed logs, stand basal area and volume. It is suggested that partial cutting could occur in some old-growth stands, while still maintaining their structural and functional attributes, if large logs, snags and trees are retained at the threshold densities necessary to recognise old-growth status. At the landscape level, the use of extended timber crop rotations is advocated. Planning for a tapered forest age class distribution (with decreasing areas of forest allowed to persist to successively older ages) is suggested as a means of sustainably generating true old-growth, and as an alternative to the use of partial cutting and patch retention. Arithmetic formulas are developed which provide guidelines for the proportion of the forest land base to be kept in each successive age class. This model for regulating human disturbance in commercial forests holds promise as a mechanism for allowing continued timber harvest and even-aged stand management while retaining a near-natural proportion of old-growth forest in northern landscapes.