Revealing intra-annual carbon sequestration patterns through xylem anatomy and eddy covariance fluxes in eastern white pine

Forests are major terrestrial carbon sinks, playing a crucial role in climate change mitigation. Nonetheless, the long-term seasonal carbon sequestration dynamics are scarcely understood. Here, we investigated the relationships between climate variability, carbon fluxes, and the xylem biomass in an 80-year-old plantation of Pinus strobus in Ontario, Canada. From Eddy Covariance tower, we obtained daily Gross Primary Production (GPP), precipitation, VPD and air temperature for the period 2003-2018. To estimate inter-and intra-annual xylem biomass accumulation we selected 12 trees and built wood anatomical chronologies of cell number (CN), cell wall area (CWA) and overall wall area per ring (RWA). We used moving windows correlations of daily climate data and GPP with anatomical chronologies to analyse their associations at intra-annual scale. Temperature in early spring and precipitation in mid-spring and summer strongly positively affected GPP, while summer VPD had a negative effect. For the first time, we observed strong positive correlations between GPP in the growing season and CWA (proxy for carbon quantity in each xylem cell) both in earlywood (May 10 – Aug 1, r = 0.652) and latewood (Jul 3 – Sep 3, r = 0.885). Strong positive correlations were also found between GPP and CN and RWA (r ≥ 0.724). Our results suggest a direct influence of CO2 fluxes on the accumulation of carbon in woody biomass. This work will help to reduce uncertainties in woody carbon accumulation dynamics, opening new perspectives in the study of forest carbon cycle.