New Paper: Decoupled Carbon Assimilation and Growth in Oaks
A new paper co-authored by Dr. Pierrat — led by Mukund Palat Rao and a large team of collaborators — is out now in Science Advances. “Decoupled carbon assimilation and growth responses to aridity in temperate deciduous oaks” shows that photosynthesis and wood formation are not tightly linked across the season in eight North American oak species. Combining satellite and tower-based estimates of gross primary productivity (GPP) with 137 tree-ring chronologies, the team found that current-year radial growth becomes insensitive to climate variability after midsummer, even though 26 to 36% of annual GPP is still assimilated after that point. Hourly flux and stem-growth measurements at four sites confirm that wood formation shuts down earlier than photosynthesis and is restricted to periods of low atmospheric aridity and temperature. Because earth system models typically assume photosynthesis and biomass accumulation are tightly coupled, this decoupling suggests they may be overestimating how much carbon forests can sequester as growth.
Fig. 2. Inferring temporal decoupling between photosynthesis and growth by comparing the annual fraction of cumulative GPP with the climate sensitivity of annual tree RW for 137 North American oak (Quercus spp.) chronologies and seven species. (A) Locations of 74 tree-ring series of the white oak group Quercus sect. Quercus, represented by QUAL (n = 59) and QUMO (n = 15) and their distribution ranges (in gray and blue, respectively). (B) Phenology of the fraction of annual GPP in 2021 as a function of the Day of year (DOY) across all tree-ring sites and species in part (A). Bold black line and green shading represent the median, 5th, and 95th percentiles of cumulative GPP, and light green lines describe GPP at each site. The number (0.36) indicates the median fraction of annual GPP (i.e., 36% annual GPP) assimilated after July (i.e., August to December). Dotted red horizontal lines indicate GPP fractions of 0.05 (start of photosynthetic season) and 0.95 (end of photosynthetic season). Purple solid line describes the fraction of tree-ring chronologies of 74 total that have a significant correlation (P < 0.01, two-sided Student’s t test) between annual RW and 1-month SPEI for the same year between 1950 and the last year of each tree-ring series (also see figs. S2 and S3). Filled circles in black represent months where a minimum of 5% of all 74 series showed a significant correlation (P < 0.01, two-sided Student’s t test) between SPEI and annual radial growth. Subplots (C) and (D) are similar to (A) and (B) but for 40 tree-ring chronologies of the red oak group, Quercus sect. Lobatae, represented by four species QURU, QUCO, QUPA, and QUFA with 0.36 as median post-July annual GPP median fraction and (E) and (F) are for 23 QUDG tree-ring chronologies in California with 0.26 as median post-July annual GPP fraction. Triangles on maps show locations of high-resolution monitoring sites and described in fig. S1 along with species codes. Tree-ring metadata is provided in table S1. From Rao, M. P., et al. 2026, Science Advances 12(24), eady7139. doi.org/10.1126/sciadv.ady7139
Rao, M. P., Pacheco-Solana, A., Li, R., Oryan, B., Jensen, J. E., Rodriguez-Caton, M., Klinek, L., Pierrat, Z. A., Ruehr, S., Oelkers, R., Boeschoten, L. E., Griffin, K. L., McCormack, M. L., Yang, X., Verfaillie, J., Baldocchi, D., Hise, J., Turner, A. J., Scanlon, T. M., Andreu-Hayles, L., Eitel, J. U. H., Pederson, N., Griffin, D., Stahle, D., Maxwell, J. T., Voelker, S., Kannenberg, S. A., Peñuelas, J., & Magney, T. S., 2026. Decoupled carbon assimilation and growth responses to aridity in temperate deciduous oaks. Science Advances, 12(24), eady7139. doi.org/10.1126/sciadv.ady7139
