Whether increased photosynthates under elevated atmospheric CO₂ could translate into sustained biomass accumulation in forest trees remains uncertain. Here we demonstrate how tree radial growth is closely linked to litterfall dynamics, which enhances nitrogen recycling to support a sustained effect of CO₂ fertilization on tree-ring growth. Our ten-year observations in two alpine treeline forests indicated that annual (or seasonal) stem radial increments generally had a positive relationship with the previous year’s (or season’s) litterfall and its associated nitrogen return and resorption. Annual tree-ring width, annual litterfall and annual nitrogen return and resorption all showed an increasing trend during 2007–2017, and most of the variations were explained by elevated atmospheric CO₂ rather than climate change. Similar patterns were found in the longer time series of tree-ring width index from 1986–2017. The regional representativeness of our observed patterns was confirmed by the literature data of six other tree species at 11 treeline sites over the Tibetan Plateau. Enhanced nitrogen recycling through increased litterfall under elevated atmospheric CO₂ supports a general increasing trend of tree-ring growth in recent decades, especially in cold and nitrogen-poor environments.

_{大气中 CO 2}升高时光合产物的增加是否会转化为林木中持续的生物量积累仍不确定。在这里，我们展示了树木径向生长如何与凋落物动态密切相关，后者增强了氮循环以支持 CO _{2的持续影响}树轮生长施肥。我们对两个高山林线森林的十年观察表明，每年（或季节性）的茎干径向增量通常与前一年（或季节）的枯枝落叶量及其相关的氮回流和再吸收呈正相关。2007-2017 年年轮宽度、年枯枝落叶量和年氮返还与再吸收均呈增加趋势，大部分变化由大气 CO _{2升高解释}而不是气候变化。在 1986 年至 2017 年的树木年轮宽度指数的较长时间序列中也发现了类似的模式。我们观察到的模式的区域代表性得到了青藏高原 11 个林线地点其他 6 种树种的文献数据的证实。_{在大气 CO 2}升高的情况下，通过增加凋落物来增强氮循环，支持近几十年来树木年轮生长的普遍增长趋势，尤其是在寒冷和缺氮的环境中。