The conversion of natural forests into managed forests raises concerns about sustained timber production and nature conservation. This study combined long-term inventory projections for some 84,000 ha of productive boreal mixedwood in Saskatchewan with fragmentation analysis. A spatially unrestricted clearcut harvesting regime was simulated for a I 00-year period (1984-2084). The current harvest rates of 1.5 m3ha−1per year could be sustained, but the age class distribution of the forest would markedly shift towards younger stands. Maximizing timber yields could raise harvest rates by 10%. These harvest rates would make it increasingly difficult to set aside permanently productive forest for ecosystem protection and alternative forest uses. A reduction of current harvest rates by at least 30% would support forest conservation. Spatial restrictions of the harvesting regime would be required to prevent forest fragmentation.