Fagaceae plants are dominant species in subtropical and temperate forests in China. Studying the geographical pattern of their carbon contents can provide key parameter support for high-precision forest carbon accounting. To investigate the spatial variation and influencing factors of carbon content in bark, sapwood, and heartwood, stem samples from 168 individual trees belonging to 41 species of 5 genera in the Fagaceae family were collected from different regions of China. Carbon was determined with the dry combustion method using an elemental analyzer. The variation in carbon content was partitioned, carbon content among tissues were compared, spatial patterns with latitude and longitude and relative importance of interpreting variables were quantified. The carbon content of stem tissues ranged from 411 to 563 mg·g⁻¹. Variation was primarily driven by geographical location (34%–53%), followed by residuals (26%–40%). The inter-species difference also made significant contributions, ranging from 23% (bark) and 21% (sapwood) to 18% (heartwood). Generally, the carbon content among the three tissues followed the order: bark (494 ± 26 mg g⁻¹) (±SD) < sapwood (503 ± 21 mg g⁻¹) < heartwood (509 ± 23 mg g⁻¹). There was an obvious geographical variation pattern in stem carbon content. The carbon content of different tissues gradually decreased with northward latitude and westward longitude. Aridity index (with a relative importance of 22%), longitude (38%), and solar radiation (27%) were the most important driving factors of bark, sapwood, and heartwood C, while the influence of temperature and precipitation was weak. The results highlight the importance of geographical and environmental gradients over taxonomic differences and provide critical parameters for improving forest carbon storage estimates in China.

Fagaceae trees dominate in the temperate and subtropical forests in East Asia. Understanding the spatial patterns of their carbon contents and the influencing factors can support high-precision forest carbon accounting. A comprehensive understanding of the changes in carbon in multiple organs of trees such as Fagaceae trees is still lacking at a large scale. This study investigated the inter-tissue variation, spatial patterns, and climatic drivers of carbon fraction across nine tissues (leaves, branches, bark, sapwood, heartwood, stump, coarse roots, medium roots, and fine roots) in 41 Fagaceae species (5 genera) from 12 sites across China’s major forest biomes. The sampling sites ranged from northern temperate to northern-tropical and covered an elevation range of 1200 m. The carbon fraction was measured with dry combustion after dried at 60 °C. Variance decomposition revealed that geographical location was the dominant source of variation (16%–55%), outweighing differences at the species and genus levels. Significant disparities in carbon fraction were observed among tissues, following a general pattern of leaves (517 mg g⁻¹) ≈ fine roots (516 mg g⁻¹) > heartwood (510 mg g⁻¹) > sapwood (504 mg g⁻¹) > branches (501 mg g⁻¹) ≈ medium roots (500 mg g⁻¹) > bark (495 mg g⁻¹) > coarse roots (488 mg g⁻¹) ≈ stump (487 mg g⁻¹). This indicated a “high-at-both-ends” arcuate pattern from leaves to fine roots. Spatially, carbon fractions in most tissues exhibited significant declining trends with increasing latitude and eastward longitude. Generalized additive models identified mean annual temperature and precipitation as the most influential factors for most above-ground tissues, while fine roots were primarily regulated by temperature seasonality. These findings help us understand the differences in tree carbon fraction from an organ perspective, highlighting the critical importance of multi-tissue sampling protocol. We recommend integrating the spatial and climatic drivers for refining forest carbon accounting. More species should be included to separate the species and climatic effects in the future.