Effects of even-aged and uneven-aged management on carbon dynamics and timber yield in boreal Norway spruce stands: a forest ecosystem model approach

We used a gap-type forest ecosystem model to study how even- and uneven-aged management affected the carbon dynamics and timber production in boreal Norway spruce stands. In business-as-usual management, the intensity of thinnings (from below) and single-tree selective cuttings followed those recommended for even-aged (BT) and uneven-aged management (BSC) in practical forestry in Finland. Moreover, higher or lower basal area thresholds, and shorter or longer production cycles, were used in simulations. We found that, the mean annual carbon uptake, volume growth, and carbon stock in trees and harvested timber, were nearly the same under even-aged (BT) and uneven-aged (BSC) management, when assuming full seed crop in latter one. However, the carbon stock in the soil and ecosystem and the mean annual net ecosystem exchange were slightly smaller under BT. The carbon retention time was longer under BSC. The net present value (NPV with interest rate of 3 per cent) of timber production was clearly lower under BT, when the calculation was initiated at planting on clear-cutting area, in opposite to when initiating calculation a few years before the second thinning. Higher basal area thresholds and longer production cycles increased carbon stocks, carbon retention and timber yield, regardless of management system. On the other hand, the results of uneven-aged management (BSC) were very sensitive to the success of natural regeneration and ingrowth of seedlings, as a reduction of the seed crop by 25–75 per cent from the full seed crop decreases the volume growth by 44–74 per cent and timber yield up to 46 per cent.