Urbanization delays plant leaf senescence and extends growing season length in cold but not in warm areas of the Northern Hemisphere

Aim Urbanization is becoming one of the most important drivers of global environmental change as human population and economic development rapidly increase. However, the effects of urbanization on plant phenology are still poorly understood, especially for leaf senescence and growing season length across large spatial scales. We aimed to fill this knowledge gap by combining in situ observations and remote sensing phenological data. Location The United States and Europe. Time period 2009-2018. Major taxa studied Vascular plants. Methods We divided the United States and Europe into 10 km by 10 km grid cells. We estimated leaf senescence dates for 93 species, and growing season length for a subset of 54 of these species, for grid cells with enough data using a database with >22 million in situ phenology observations. We also estimated growing season lengths at the community level for the Eastern Temperate Forest ecoregion in the US using remote sensing data. We then investigated effects of urbanization (using human population density as a proxy), temperature, and their interactions on leaf senescence and growing season lengths using linear mixed models. Results Urbanization and warmer regional temperature both delayed plant leaf senescence. In addition, the effects of urbanization on leaf senescence and growing season lengths depended on climate context: urbanization delayed leaf senescence and extended growing season length in cold regions; however, urbanization advanced leaf senescence and shortened growing season length in warm regions, implying the positive effects of urbanization on growing season length in cold regions may be weaker in a warmer future. Main conclusions Our study provides strong empirical evidence that the influence of urbanization on plant phenology and growing season length varies with regional temperature. Our results have important implications for predicting plant phenology and growing season length in a warmer and more urbanized future.