Decadal biomass increment in early secondary succession woody ecosystems is increased by CO enrichment.

Increasing atmospheric CO 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 CO-enrichment experiments in woody ecosystems that measured total NPP and biomass. CO enrichment increased biomass increment by 1.05 ± 0.26 kg C m 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 CO response of NPP (0.16 ± 0.03 kg C m y) and the CO-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 CO-independence of biomass retention highlights the value of understanding drivers of wood allocation under ambient conditions to correctly interpret and predict CO responses.