Accelerating anthropogenic influence on annual peak wood density and associated carbon accumulation across northern conifers

Northern coniferous forests hold significant climatic and ecological importance, yet the optimal air temperature (OAT) governing their annual peak wood density remains unknown. Here, using tree-ring maximum latewood density (MXD) from 14,000 samples at 551 sites, we reveal an OAT range of 15–21 °C governing annual MXD variability, a more sensitive thermal metric than seasonal temperature. Reconstructions of large-scale (40°–73°N) weighted average OAT hours since 1690 show unprecedented increases over the past 30 years (~10% above the 1850–1900 baseline), driven by anthropogenic warming. Higher OAT hours are associated with an estimated average annual increase of 0.23 mg biomass (0.12 mg carbon) per 1 cm² stem surface area within the latewood increment during 1921–1994. Projections show OAT hours during 2050–2100 reaching up to 23.3% above the 1850–1900 baseline, with stronger northern increases reducing the latitudinal disparity. These results imply that assuming static density may underestimate the density-related carbon contribution in northern conifers under warming. Reconstructions based on tree-ring and hourly temperature data identify an optimal thermal range across northern conifers and link rising optimal-temperature hours to anthropogenic warming and biomass and carbon gains, with further increases projected