Investigating the Variability of Urban Tree Phenology Using Volunteer-Hosted Phenocams

The timing of vegetative phenology governs annual ecosystem function and is a component of community diversity and productivity. Consequently, phenology is key to monitoring and measuring ecological responses to environmental change. In cities, the timing of tree phenological events such as leaf unfolding and leaf coloring is altered due to regional climate factors like winter chilling length and spring warming temperatures, as well as site-specific variation such as land cover and genus. However, the variability of these changes to urban phenology, as well as the factors that govern variation, are not well explored. Likewise, while many studies use digital time-lapse cameras to study phenology in rural settings, few have used phenological cameras, phenocams, in the urban environment. To investigate the phenology of urban trees using phenocams, this novel study used the city of Washington, D.C. to explore the interannual changes occurring in city trees, as well as the feasibility of using phenocams in combination with volunteer camera hosts. Phenocams were placed throughout the greater D.C. area in the homes of volunteers and on the campuses of two educational institutions. Each phenocam captured images of either a single tree crown or multiple over the course of three study years. From camera imagery, start of season (SOS) and end of season (EOS) were estimated by fitting a spline to each tree’s green chromatic coordinate (Gcc) data, which were extracted at a near-continuous temporal resolution. Statistical analysis revealed that there were significant differences in SOS between years. Further analysis of climatic and site variables found that temperature and impervious surface significantly influenced SOS and EOS respectively. Additional analysis showed that tree genera responded to annual changes in moderate synchrony. Assessment of community science implementation found that inclusion of volunteers increased the educational reach of the study, but likely created noise in the data due to issues such as accidental phenocam disturbance and repositioning. This novel study provides useful information for the methodology of future urban phenocam research and provides a preliminary understanding of phenology in the Washington, D.C. area.