Carbon dynamics following variable retention harvesting in boreal mixedwood forests

Retention harvests are promoted as an alternative to clearcuts to enhance ecological values in managed forests. Understanding how retention affects carbon (C) dynamics over time and in various forest types is important for balancing objectives like timber production and C storage. This is particularly crucial now, as the climate mitigating effects of boreal forests are weakening due to both forest harvests and natural disturbances. Using data from a relatively long-term experiment (pre-harvest to 18-years post-harvest) in previously unharvested boreal mixedwood forest, we compared C pools (mature trees, regenerating trees and shrubs, deadwood, and soil) among harvest levels (clearcuts, 10%, 20%, 50%, 75% retention, and unharvested reference). Soil C appeared to be invariant at the scale of this study, so we focused our analyses on biomass in living and dead vegetation. Total pre-harvest C storage was greater in conifer-dominated and mixed stands than in deciduous (broadleaf)-dominated stands, reflecting mainly greater biomass in live trees but also in downed deadwood. Net loss of C from the forest up to 3-years post-harvest scaled with harvest intensity in all forest types. At 3- and 18-years post-harvest in deciduous and 3-years post-harvest in conifer stands, all retention harvests resulted in larger C stocks than clearcuts; only higher retention levels provided this benefit at 3- and 18-years post-harvest in mixed (75% retention) and at 18 years in conifer stands (50%, 75% retention). In some forest types, the highest retention levels (75% for deciduous and mixed stands, 50% and 75% for conifer stands) maintained total C stocks statistically equivalent to unharvested forest at both 3- and 18-years post-harvest. Deciduous stands became net C sinks by 3–7 years post-harvest, likely due to prolific aspen regeneration and growth. Mixed and conifer stands, however, were nearly C-neutral or were C sources until 12–18 years post-harvest. This reflected persistent effects of pre-harvest forest type, including less aspen regeneration, slower growth of conifer seedlings, and mortality of retained conifers. Our results suggest that strategic retention harvesting could serve as a practical option to couple C storage options to other management considerations.