Growth is not just about wood : investigating the link between ecosystem carbon uptake and net primary productivity in three European forests

A body of work published over the past two decades shows that there is a partial decoupling between the fixation of carbon by photosynthesis and the formation of wood in forests. Years of high photosynthesis are not necessarily accompanied by large wood production. In this contribution, we explore the link between photosynthesis, estimated using eddy covariance fluxes and the productivity of the various forest compartments (leaves, wood, fruit, roots) measured over 12 to 21 years on three forest sites in France belonging to the European ICOS network (two temperate sites, one oak forest and one beech forest, and one Mediterranean oak forest). Annual wood productivity was not significantly correlated with carbon fixation, neither GPP nor NEP, at any of the three sites. On the other hand, annual above-ground net primary productivity was significantly correlated with GPP at all sites and with NEP at two of the three sites, i.e. the Mediterranean and the temperate oak forests. In these oak forests, years of high productivity are masting years, supporting the hypothesis that reproduction is limited by the availability of recent photo-assimilates. These results remind us that wood growth is not necessarily representative of tree productivity, and invites us to investigate the causes of the inter-annual variability of carbon allocation to the different organs and tissues of trees.