Spatiotemporal variability of leaf critical senescence age across northern lands and its key drivers

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.