Abstract: Atmospheric nitrogen (N) deposition profoundly influences plant growth in forest ecosystems. While numerous studies have reported the effects of N addition on tree radial growth, the response patterns across trees with different stem diameters and the underlying anatomical mechanisms associated within their wood growth rings remain unclear. Here, we examined the effects of a 14-year N addition experiment on tree radial growth and anatomical structure in a temperate birch (Betula platyphylla) forest in Dongling Mountain, Beijing. Using steel-band dendrometers, micro-core method, and micrography, we assessed how N additions affect the radial growth of birch trees across different diameter classes through changes in wood anatomical structure. The relative growth rate (RGR) of birch was positively correlated with stem diameter. Consistent with this trend, key anatomical parameters, including ring width, cell number, maximum vessel diameter, and average ring cell diameter, increased significantly with tree size. N addition further amplified this growth disparity between larger and smaller trees. This amplification was primarily mediated through earlywood anatomy: N addition promoted the expansion of earlywood lumen diameter (a key component of average ring cell of earlywood). Importantly, the enlargement of average ring cell directly translated into increased earlywood width, thereby accelerating radial growth, with larger trees exhibiting disproportionately greater responses. Our study demonstrates that nitrogen addition exacerbates size-dependent growth inequalities in temperate forests through earlywood anatomical adjustments, providing a mechanistic basis for understanding how atmospheric nitrogen inputs affect forest stand dynamics.

Key words: nitrogen addition, birch, stem diameter, radial growth, wood anatomical structure