Modelling silvicultural alternatives for conifer regeneration in boreal mixedwood stands (aspen/white spruce/balsam fir)

We model and compare the biological and financial constraints of four prescriptions that serve as alternatives to conventional clearcutting followed by planting in eastern and western boreal mixedwood stands. These alternative prescriptions for full or p artial conifer stocking are (1) reliance on advance regeneration with or without augmentation by fill-planting; (2) understory scarification d uring a mast year; (3) direct seeding either aerially or with a scarifier-seeder; and (4) underplanting. Our main conclusions concerning the biological constraints are that (1) advance regeneration, mainly of balsam fir in the east and white spruce in the west, requires >26 000 a nd > 4000 trees/ha (because of different distributions), respectively, to achieve full conifer stocking; (2) reliance on a mast year requ ires at least 6 m 2 /ha of mature conifer basal area, but much less if some advance regeneration is present or only moderate stocking is desired; ( 3) aerial seeding with 35% scarification requires about a half-million seeds/ha to achieve full conifer stocking, while a scarifier-se eder would require only a third of this application rate; and (4) underplanting is constrained to aspen stands with >25% incident light at planting height. In all cases, alternative prescriptions become more feasible if only moderate or minimal stocking is the silvicultural objectiv e. A costing exercise for the four prescriptions in comparison with a clearcut followed by planting shows that reliance on advance regenerat ion or understory planting are the cheapest alternatives to achieve full or partial conifer stocking. With the exception of full conifer st ocking in situations where there is little advance regeneration (and where herbicides can be used), conventional plantations are never the ch eapest approach. In such cases, fill planting and use of a scarifier-seeder become viable options. Aerial seeding and reliance on a mast year ar e the most expensive of the alternatives. We conclude, tentatively, that there is enough conifer basal area in most of the eastern boreal mixedwood of Canada to allow for the use of either or both a mast year and advance regeneration to achieve full or partial conifer stocki ng. By contrast, in the west conifer basal area will seldom be sufficient for natural seeding, and the density of advance regeneration is likewise often too low. Finally, because of light constraints, understory planting appears to have a much wider applicability in the west than in the east.