Wood is the main terrestrial biotic reservoir for long-term carbon sequestration1, and its formation in trees consumes around 15% of anthropogenic carbon dioxide emissions each year2. However, the seasonal dynamics of woody biomass production cannot be quantified from eddy covariance or satellite observations. As such, our understanding of this key carbon cycle component, and its sensitivity to climate, remains limited. Here, we present high-resolution cellular based measurements of wood formation dynamics in three coniferous forest sites in northeastern France, performed over a period of 3 years. We show that stem woody biomass production lags behind stem-girth increase by over 1 month. We also analyse more general phenological observations of xylem tissue formation in Northern Hemisphere forests and find similar time lags in boreal, temperate, subalpine and Mediterranean forests. These time lags question the extension of the equivalence between stem size increase and woody biomass production to intra-annual time scales3, 4, 5, 6. They also suggest that these two growth processes exhibit differential sensitivities to local environmental conditions. Indeed, in the well-watered French sites the seasonal dynamics of stem-girth increase matched the photoperiod cycle, whereas those of woody biomass production closely followed the seasonal course of temperature. We suggest that forecasted changes in the annual cycle of climatic factors7 may shift the phase timing of stem size increase and woody biomass production in the future.
H.E.C. and C.B.K.R. thank E. Cornu, E. Farré, C. Freyburger, P. Gelhaye and A. Mercanti for fieldwork at the French sites; M. Harroué for sample preparation in the laboratory; B. Longdoz of the forest ecology and ecophysiology (EEF) team of the French national institute for agronomy research (INRA), and the association for the study and monitoring of air pollution in Alsace (ASPA), for the meteorological data. M. Nicolas of the French permanent plot network for the monitoring of forest ecosystems (RENECOFOR) for the meteorological data and the description of the soil profiles. L. Kulmala and J. Guiot for comments on an early version of this manuscript. H.M., T.J. and P.N. thank T. Kalliokoski for the Solböle data. J.G.H. thanks L.H. Zhai for laboratory work and Y. Bergeron for project supervision. H.V. and V.G. thank J. Hacurová and G. Vichrová for laboratory work. H.E.C. was supported by a grant overseen by the French National Research Agency (ANR) as part of the ‘Investissements d'Avenir’ programme (ANR-11-LABX-0002-01, Lab of Excellence ARBRE). D.F. and P.F. acknowledge the SNF (INTEGRAL-121859 and LOTFOR-150205), and the WSL SwissTree project. J.G.H. was funded by 100 Talents Program of the Chinese Academy of Sciences (Y421081001). I.S. and A.G. were funded by the Austrian Science Fund (FWF P19563-B16 and P22280-B16). H.M., T.J. and P.N. were supported by grants from the Academy of Finland (Nos. 250299, 257641 and 265504). P.P., J.G. and K.C. were supported by the Slovenian Research Agency, young researchers' programme and programmes P4-0015 and P4-0107. H.V. and V.G. were supported by the European Social Fund and the state budget of the Czech Republic, Project Indicators of trees vitality Reg. No. CZ.1.07/2.3.00/20.0265. M.V.B. and A.V.K. were supported by Russian Science Foundation project 14-14-00295. The Canadian boreal forest dataset collected by H.M, C.K, A.D. and S.R. were supported by the CRSNG, the FRQNT, the FCI and the Consortium de Recherche sur la Forêt Boréale Commerciale. S.R. was also supported by the CAS President's International Fellowship Initiative (GRANT No. 2015VBB032). The dataset on xylem phenology was generated by the GLOBOXYLO initiative (http://www6.nancy.inra.fr/foret-bois-lerfob_eng/Projects/Current/GLOBOXYLO), which was developed in the framework of the FPS COST Action STReESS (FP1106).