Nitrogen Availability and Intrinsic Water‐Use Efficiency Differently Drive Broadleaf and Conifer Growth Near an Expanding City

Abstract Urban and peri‐urban forests are vital for ecological services and urban quality of life, making tree growth responses to environmental stressors key for management. In the northern Qinling Mountains near Xi'an, the largest city in northwest China, rising levels of NO x and PM 2.5 emissions from urban development have led to elevated nitrogen deposition. However, the impact of climate, pollution, and nitrogen deposition on nearby forests remains unclear. To address this, we used tree‐ring width measurements (basal area increment, wood carbon isotopes (δ 13 C) and nitrogen isotopes (δ 15 N), and C/N ratios to examine the effects of climate and nitrogen emissions on tree growth and intrinsic water‐use efficiency (iWUE) over the 20th and early 21st centuries. By comparing two broadleaf species (chestnut, Castanea mollissima ; oak, Quercus aliena ) with a conifer species (pine, Pinus tabulaeformis ), we examined their long‐term (1922–2018 for oak and pine; 1947–2018 for chestnut) growth patterns and physiological responses. We found radial growth increased in recent decades, especially in broadleaves. Chestnut and oak growth was driven by CO 2 and nitrogen fertilization effects as expressed by increasing iWUE and decreasing δ 15 N over the past century. Pine showed a continuous decline in δ 15 N and a weaker growth response to CO 2 and nitrogen fertilization. NO x emissions significantly promoted the growth of all three species. Our results provide new insights into species‐specific long‐term growth responses of broadleaves and conifers to urban expansion, elucidating the complex carbon‐nitrogen interactions under rapid environmental change.