Code for Decreasing rainfall frequency contributes to earlier leaf onset in northern ecosystems

<strong>C</strong><strong>limate change substantially advances the leaf onset date (LOD) and regulates carbon uptake by plants. Unlike temperature, the effect of precipitation remains largely elusive. Here we use carbon flux measurements, <em>in situ</em> records of leaf unfolding, and satellite greenness observations to examine the role of precipitation frequency (<em>P</em>_freq, number of rainy days) in controlling the LOD in northern ecosystems (&gt;30°N). Widespread decreases in <em>P</em>_freq during the last three decades positively contributed to the advance in LOD, possibly due to increased exposure to radiation, exhibiting a dominant control of LOD over ~10% of the area. Lower <em>P</em>_freq may also enhance chilling at night and warming at daytime, leading to earlier LOD consequently. We further develop a weighted precipitation growing-degree-day algorithm that projected a generally earlier LOD than currently predicted. These results highlight the need for a comprehensive understanding of the impacts of precipitation on LOD, which is necessary for improved projections.</strong>