As major terrestrial carbon sinks, forests play an important role in mitigating climate change. The relationship between the seasonal uptake of carbon and its allocation to woody biomass remains poorly understood, leaving a significant gap in our capacity to predict carbon sequestration by forests. Here, we compare the intra-annual dynamics of carbon fluxes and wood formation across the Northern hemisphere, from carbon assimilation and the formation of non-structural carbon compounds to their incorporation in woody tissues. We show temporally coupled seasonal peaks of carbon assimilation (GPP) and wood cell differentiation, while the two processes are substantially decoupled during off-peak periods. Peaks of cambial activity occur substantially earlier compared to GPP, suggesting the buffer role of non-structural carbohydrates between the processes of carbon assimilation and allocation to wood. Our findings suggest that high-resolution seasonal data of ecosystem carbon fluxes, wood formation and the associated physiological processes may reduce uncertainties in carbon source-sink relationships at different spatial scales, from stand to ecosystem levels.

5 N Sukachev Institute of Forest SB RAS Federal Research Center 'Krasnoyarsk Science Center SB RAS 660036 Krasnoyarsk Akademgorodok Russia

AMAP University of Montpellier CIRAD CNRS INRAE IRD Montpellier France

Centre for Ecological Sciences Indian Institute of Science Bangalore 560012 India

Centre for Functional Ecology Associate Laboratory TERRA Department of Life Sciences University of Coimbra 3000 456 Coimbra Portugal

CoLAB ForestWISE Collaborative Laboratory for Integrated Forest and Fire Management Quinta de Prados 5000 801 Vila Real Portugal

College of Forestry Guangxi Key Laboratory of Forest Ecology and Conservation Guangxi University Daxue East Road 100 Nanning Guangxi 530004 China

College of Life Sciences Anhui Normal University Wuhu 241000 China

CREAF E08193 Bellaterra Barcelona Catalonia Spain

DendroLab Department of Natural Resources and Environmental Science University of Nevada Reno NV 89557 USA

Department of Agricultural and Food Sciences University of Bologna 40127 Bologna Italy

Department of Agricultural Sciences University of Naples Federico 2 1 80055 Portici Napoli Italy

Department of Biology and Geology Physics and Inorganic Chemistry Rey Juan Carlos University c Tulipán s n 28933 Móstoles Spain

Department of Botany Leopold Franzens University of Innsbruck 6020 Innsbruck Austria

Department of Botany University of Kashmir India 190006 Kashmir Srinagar India

Department of Environmental Sciences Botany University of Basel Schönbeinstrasse 6 CH 4056 Basel Switzerland

Department of Forest Botany Dendrology and Geobiocenology Faculty of Forestry and Wood Technology Mendel University in Brno Zemedelska 1 61300 Brno Czech Republic

Department of Forest Ecology The Silva Tarouca Research Institute for Landscape and Ornamental Gardening Brno Czechia

Department of Forest Sciences University of Helsinki PO Box 27 00014 Helsinki Finland

Department of Geography and Regional Planning Environmental Science Institute University of Zaragoza 50009 Zaragoza Spain

Department of Geography University of Cambridge Cambridge CB2 3EN UK

Department of Physical Geography and Geoecology Charles University CZ 12843 Prague Czech Republic

Department of Plant and Environmental Sciences Weizmann Institute of Science Rehovot 76100 Israel

Department of Wood Science and Wood Technology Mendel University in Brno 61300 Brno Czech Republic

Dipartimento di Agraria Università Mediterranea di Reggio Calabria 89122 Reggio Calabria Italy

Dipartimento di Agricoltura Ambiente e Alimenti Università degli Studi del Molise 86100 Campobasso Italy

Direction de la Recherche Forestière Ministère des Ressources Naturelles et des Forêts du Québec 2700 rue Einstein Québec QC G1P 3W8 Canada

Division of Biological Sciences University of Montana Missoula MT USA

Earth Systems Research Center Institute for the Study of Earth Oceans and Space University of New Hampshire Durham NH USA

EIFAB iuFOR Universidad de Valladolid Campus Duques de Soria E 42004 Soria Spain

Faculty of Agricultural Environmental and Food Sciences Free University of Bozen Bolzano Piazza Università 5 39100 Bozen Bolzano Italy

Forest Research Institute Université du Québec en Abitibi Témiscamingue Rouyn Noranda QC Canada

Global Change Research Institute c Tulipán s n 28933 Móstoles Spain

Guangdong Open Laboratory of Geospatial Information Technology and Application Guangzhou Institute of Geography Guangdong Academy of Sciences Guangzhou 510070 China

Institució Catalana de Recerca i Estudis Avançats Passeig de Lluis Companys 23 08010 Barcelona Spain

Institut National de l'Information Géographique et Forestière 54250 Champigneulles France

Institute for Atmospheric and Earth System Research Forest Sciences Faculty of Agriculture and Forestry P O Box 27 University of Helsinki FI 00014 Helsinki Finland

Institute for Atmospheric and Earth System Research Physics Faculty of Science P O Box 68 University of Helsinki FI 00014 Helsinki Finland

Institute of Geography Johannes Gutenberg University Mainz Mainz Germany

Instituto Pirenaico de Ecología Consejo Superior de Investigaciones Científicas 50192 Zaragoza Spain

Istituto di Ricerca sugli Ecosistemi Terrestri Consiglio Nazionale delle Ricerche 50019 Sesto Fiorentino Italy

Izmir Katip Çelebi University Faculty of Forestry Izmir Türkiye

Key Laboratory of Vegetation Restoration and Management of Degraded Ecosystems Guangdong Provincial Key Laboratory of Applied Botany South China Botanical Garden Chinese Academy of Sciences Guangzhou 510650 China

Laboratoire sur les écosystemes terrestres boreaux Département des Sciences Fondamentales Université du Québec à Chicoutimi 555 boulevard de l'Université Chicoutimi QC G7H2B1 Canada

MOE Key Laboratory of Biosystems Homeostasis and Protection College of Life Sciences Zhejiang University Hangzhou 310058 China

National Resources Institute Helsinki Finland

Natural Resources Institute Finland Latokartanonkaari 9 00790 Helsinki Finland

Oeschger Centre for Climate Change Research University of Bern Hochschulstrasse 4 CH 3012 Bern Switzerland

School of Geography and Ocean Science Nanjing University Nanjing 210093 China

Siberian Federal University 79 Svobodny pr 660041 Krasnoyarsk Russia

Slovenian Forestry Institute 1000 Ljubljana Slovenia

South China National Botanical Garden Guangzhou 510650 China

State Key Laboratory of Tibetan Plateau Earth System Environment and Resources Institute of Tibetan Plateau Research Chinese Academy of Sciences Beijing 100101 China

Swiss Federal Institute for Forest Snow and Landscape Research WSL Zürcherstrasse 111 CH 8903 Birmensdorf Switzerland

Universitat Autònoma de Barcelona Bellaterra E08193 Barcelona Catalonia Spain

Université Clermont Auvergne INRAE PIAF 63000 Clermont Ferrand France

Université de Lorraine AgroParisTech INRAE SILVA F 54000 Nancy France

University of Ljubljana Biotechnical Faculty 1000 Ljubljana Slovenia

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