Increasing atmospheric CO₂ stimulates photosynthesis which can increase net primary production (NPP), but at longer timescales may not necessarily increase plant biomass. Here we analyse the four decade-long CO₂-enrichment experiments in woody ecosystems that measured total NPP and biomass. CO₂ enrichment increased biomass increment by 1.05 ± 0.26 kg C m^-2 over a full decade, a 29.1 ± 11.7% stimulation of biomass gain in these early-secondary-succession temperate ecosystems. This response is predictable by combining the CO₂ response of NPP (0.16 ± 0.03 kg C m^-2 y^-1) and the CO₂-independent, linear slope between biomass increment and cumulative NPP (0.55 ± 0.17). An ensemble of terrestrial ecosystem models fail to predict both terms correctly. Allocation to wood was a driver of across-site, and across-model, response variability and together with CO₂-independence of biomass retention highlights the value of understanding drivers of wood allocation under ambient conditions to correctly interpret and predict CO₂ responses.