Inter-annual variation in carbon allocation explains partial decoupling between assimilation and growth in six forest sites

Predicting the future carbon balance of forests, and their carbon sequestration capacity, requires a precise understanding of how gross primary productivity (GPP) is partitioned among autotrophic respiration and the different compartments of the net primary productivity (NPP). Studies show an important variability of the NPP:GPP ratio across forest, and a partial decoupling between GPP and wood production in forest ecosystems. This suggests an interannual variation of carbon allocation among tree functions and organs, which is generally not accounted for in most dynamic vegetation models. Using GPP estimated from eddy-covariance measurements and independent above-ground NPP measured over 6 to 20 years on six forest sites (5 sites in France belonging to the ICOS network and one site in Brazil), we explored the interannual variations in GPP partitioning to aboveground growth and its distribution among years and aboveground organs (wood, leaves, fruits, flowers).

The partitioning of GPP to aboveground biomass varied considerably across sites, with Mediterranean evergreen forests showing the lowest values (15% and 18%), temperate forests intermediate values (21-37%), and the tropical eucalypt site showing the highest fraction (48%). At four of the sites, biomass production exhibited a larger inter-annual variability than did GPP, suggesting a greater sensitivity to environmental controls of the carbon sinks than the carbon source. All sites but one exhibited a significant correlation between annual aboveground NPP and annual GPP, but with small R² values between 0.2 and 0.6, thus showing a rather weak coupling between the two productivities. The coupling was generally even weaker for wood production alone (generally considered as the main carbon sequestration in forests) than for total aboveground NPP (which also includes short-lived organs such as leaves that will rapidly decompose). Finally, we observed that the inter-annual correlations between GPP and biomass production varied depending on the onset of the GPP integration time-window, indicating different temporal lags between assimilation and growth according to species and organs.

This works highlights the necessity to take into account inter-annual variations of carbon allocation in forest carbon balances, and to better understand of the climatic drivers of sink activity including potential lags between assimilation, storage and growth.