Decadal biomass increment is stimulated by CO2-enrichment in early-succession woody ecosystems

Predictive understanding of the future terrestrial carbon sink remains elusive and forest responses to increasing CO ₂ are a large contributor to uncertainty in this understanding. To date many experiments and studies have shown a stimulation of Net Primary Production (NPP) by elevated CO ₂ . However, it is unclear whether a stimulation of NPP leads to longer term increases in biomass. To study this we synthesized data from the only four, decade long, woody ecosystem CO ₂ enrichment experiments that measured biomass and NPP. We used mixed-effects statistical models to analyse the data and ran an ensemble of terrestrial biosphere models to assess their ability to reproduce the observed decadal responses. The criteria of decade long experiments that measure biomass and NPP necessarily restricts the analysis to young and regrowing woody ecosystems. In three of these young woody ecosystems, decadal-scale biomass increment was stimulated by elevated CO ₂ . A linear relationship between decadal biomass increment and cumulative NPP was preserved across both ambient and elevated CO ₂ treatments, i.e. CO ₂ did not affect the relationship between biomass increment and cumulative NPP. However, because wood allocation increased as NPP increased , a higher proportion of NPP was retained as biomass under increased CO ₂ . Each forest showed strong within-treatment variability in NPP, thought to be caused by within site heterogeneity in nutrient availability. This variability coupled with the preserved linear relationship across treatments suggests that the factors governing the retention of NPP as biomass across a range of natural climatic and edaphic variability also govern the retention of CO ₂ stimulated NPP. At the two sites where state-of-the-art ecosystem models were expected to perform better, models under-predicted the biomass stimulation by CO ₂ . This under-prediction was caused by an under-prediction of both the NPP response to CO ₂ and the increase in the wood allocation fraction in response to CO ₂ . These data clearly demonstrate a sustained long-term stimulation of biomass in young woody ecosystems as a response to mid-century CO ₂ concentrations.