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及其关键驱动因素


叶子衰老是一个关键的物候事件，意味着植被冠层的老化，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日期之间的时间跨度。然后，利用长期遥感植被指数数据，研究了北方地区（>30°N）叶片CSA的时空变化。在空间上，叶CSA表现出广泛的变异性（范围从42天到263天），平均为146±32天。落叶阔叶林表现出最长的CSA（177±28天），而灌木丛表现出最短（121±22天）。从时间上看，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，导致北部地区 1982-2010 年（+0.21 天/年）和 2010-2015 年（-2.36 天/年）之间的趋势形成鲜明对比。进一步的随机森林回归和偏相关分析表明，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子衰老年龄的地理格局，给未来陆地对气候变化的反馈预测带来很大的不确定性。