Urban Tree Phenology Sensitivity to Precipitation and Temperature in a Mediterranean Climate City

Urban forests are an essential component to livable cities and provide important services to residents. Many of these benefits are dependent on when and how long leaves are on the trees which is determined by their leaf phenology. The transition between phenological phases, including new leaf growth and senescence, is triggered by climatic cues such as light, temperature, and water. However, the specific combination of drivers can vary by species and climate region. The high species diversity and surface heterogeneity of urban areas has made city-wide phenology research challenging. In water-limited Mediterranean climates, precipitation has been shown to be an important driver in natural ecosystems; however, it is unclear whether this is also true in urban landscapes. To understand how climate change may influence phenology in a Mediterranean climate city, we used high resolution PlanetScope satellite imagery from 2018 to 2024 to monitor more than 35,000 species-identified trees in Santa Barbara, California, USA. We found the 3m resolution suitable to study individual trees and successfully detected seasonal NDVI fluctuations in evergreen species, which were validated with field observations. In our image time series, 81.8% of the deciduous tree population and, notably, 69.1% of the evergreen tree population had at least one annual phenology cycle that was detected. We found that trees were sensitive to precipitation, with spring rain delaying the end of the season by 2.25 days/cm on average across species. Warm fall temperatures delayed the green-up date whereas warm winter temperatures led to an earlier green-up. These results build upon methods to study urban forests of semi-arid climates and deepen our understanding of how changing climatic patterns may affect the leaf phenology and productivity of these species.