Potential biomass production and carbon sequestration were studied in relation to different site index conditions, stand densities and silvicultural treatments for the main forest species in northwest Spain. Site quality, stand density and silvicultural treatments affected biomass production and carbon sequestration and, therefore, optimal rotation length. Potential biomass production and carbon sequestration were highest and optimal rotations were shortest for the best site indexes, highest initial stand densities and the silvicultural treatments that involved least disturbance. Fast-growing species can be used as both energy crops (biomass) and carbon sinks, which contribute to mitigating climate change. However, as slow-growing species act as sinks for longer, they are preferable for planting in environments that are less suitable for fast-growing species.