Leaf senescence, a pivotal phenological event, signifies the aging of vegetation canopies and triggers abrupt shifts in various biogeochemical processes. However, the spatiotemporal pattern of leaf senescence age and its primary driving factors across northern lands remains unclear. In this study, we introduced a concept termed leaf critical senescence age (CSA) to characterize the initiation of senescence stage, which quantifies the time span between the onset dates of vegetation growth and senescence. Then, utilizing long-term remote sensing vegetation index data, we investigated the spatiotemporal variations of leaf CSA over northern lands (>30°N). Spatially, leaf CSA displayed extensive variability (ranging from 42 to 263 days), with an average of 146 ± 32 days. Deciduous broadleaf forests exhibited the longest CSA (177 ± 28 days), while shrublands demonstrated the shortest (121 ± 22 days). Temporally, most plant functional types experienced a reversal in leaf CSA trends around 2010, leading to the contrasting trends between 1982–2010 (+0.21 days/year) and 2010–2015 (−2.36 days/year) across northern lands. Further random-forest regression and partial correlation analysis together indicated that temperature was the dominant factor driving spatiotemporal variations in leaf CSA. These findings suggest that climate warming is reshaping the geographical pattern of leaf senescence age, posing great uncertainty to future projections of terrestrial feedback to climate change.

中文翻译：

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北方地区叶片临界衰老年龄的时空变异性及其主要驱动因素


叶片衰老是一个关键的物候事件，标志着植被冠层的老化，并引发各种生物地球化学过程的突然变化。然而，北方地区叶片衰老年龄的时空模式及其主要驱动因素仍不清楚。在这项研究中，我们引入了一个称为叶片临界衰老年龄 （CSA） 的概念来表征衰老阶段的开始，它量化了植被生长开始日期和衰老之间的时间跨度。然后，利用长期遥感植被指数数据，我们研究了北方地区 （>30°N） 叶片 CSA 的时空变化。在空间上，叶片 CSA 表现出广泛的变异性（范围从 42 到 263 天不等），平均为 146 ± 32 天。落叶阔叶林的 CSA 最长 （177 ± 28 d），而灌木林的 CSA 最短 （121 ± 22 d）。从时间上讲，大多数植物功能类型在 2010 年左右经历了叶片 CSA 趋势的逆转，导致北方地区在 1982-2010 年（+0.21 天/年）和 2010-2015 年（-2.36 天/年）之间出现了截然不同的趋势。进一步的随机森林回归和偏相关分析共同表明，温度是驱动叶片 CSA 时空变化的主导因素。这些发现表明，气候变暖正在重塑叶片衰老年龄的地理模式，对未来陆地对气候变化反馈的预测构成了极大的不确定性。