The boreal forest is a vital carbon sink. Using 250 year simulations for Canada’s Taiga Plains, a priority of the 2 billion trees program, we tested afforestation and reforestation (A/R) strategies that combine species mix, planting density and surface albedo. Medium-density (600–1400 trees ha−1) mixed stands with ∼25%–40% deciduous trees stored 15%–30% more net ecosystem carbon than conifer monocultures by coupling rapid early growth with long-term retention and greater disturbance resilience. Replanting under-stocked stands with these mixtures raised long-term storage by 18%–30% over business-as-usual. Accounting for albedo showed pure evergreen or deciduous stands lost 6%–20% of their climate benefit, whereas mixed stands yielded net cooling and the highest sequestration (≈ 4.6–4.7 tCO₂e ha−1 yr−1). Partial harvesting plus replanting preserved, and sometimes increased, ecosystem carbon (≈ 300–340 tC ha−1) and productivity (≈ 1.6–2.0 tC ha−1 yr−1) without raising risk. Blending fast-growing deciduous trees with long-lived conifers at intermediate density maximizes boreal A/R climate value and informs reforestation policy elsewhere.

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