Photosynthetic carbon assimilation drives leaf area expansion but not growth duration across northern ecosystems

Vegetation spring phenology signals the start of vegetation functional recovery in ecosystems and plays a vital role in shaping the global carbon cycle. Among key spring phenological events, leaf growth directly influences carbon dioxide exchange on a global scale. However, the extent to which photosynthetic carbon assimilation in one year influences leaf growth in the subsequent year remains difficult to quantify. Using satellite-derived leaf area index (LAI) data, we estimated the start of the growing season (SOS) and the peak of the growing season (POS), dividing the leaf growth process into two components: leaf area increment (∆LAI) and leaf growth duration (∆DOY). Our results show that ∆LAI in northern ecosystems is experiencing a significant upward trend, with over 10.7% of the region exhibiting ∆LAI growth rates exceeding 0.2 m2 m–2 year–1. This trend is primarily driven by solar radiation and photosynthetic carbon assimilation. In contrast, ∆DOY shows only a slight shortening trend, with most changes ranging from –0.2 to 0.2 days year–1. Moreover, the influence of photosynthetic carbon assimilation on ∆DOY appears minimal, likely because photosynthesis affects the timing of both SOS and POS simultaneously. Our findings demonstrate that photosynthetic carbon assimilation primarily drives leaf area expansion, with limited effects on leaf growth duration in northern ecosystems. This study contributes to advancing vegetation phenology theories, emphasizing that accurately quantifying the impacts of photosynthetic carbon assimilation is crucial for improving future predictions of vegetation seasonality and the carbon cycle.