Impacts of the scale effect on quantifying the response of spring vegetation phenology to urban intensity

Urban vegetation phenology is essential for understanding climate change impacts on urban ecosystems, offering insights into ecological and health implications. Although previous studies have explored the response patterns of vegetation phenology along the urban-rural gradient, quantitative analysis considering the neighborhood and scale effects is still insufficient. In this study, we comprehensively assessed the responses of the start of season (SOS) on urban-rural gradients depicted by both impervious surface percentage (ISP) and its kernel density values (ISPKD) at different spatial scales in 32 large cities in the north temperate zone. Our results revealed that the ISPKD can moderately amplify the sensitivity of spring phenology responses to urban intensity compared to the direct ISP indicator, with a mean difference of 0.02 days per ISP or ISPKD unit increase among cities with a significant linear response. These changes are associated with the magnified urban-rural phenological difference when characterizing the urban-rural gradient using ISPKD compared to ISP, with an increased average urbanrural difference of 1.37 days across all study cities. Observations at coarser spatial resolutions made the overall advancing trends of spring phenology on the urban-rural gradient more pronounced. Specifically, the average trends across all study cities showing significant response trends across all spatial resolutions shifted from -0.07 days/% at 250 m resolution to -0.11 days/% at 2 km. In addition, coarser resolutions of 500 m, 1 km, and 2 km amplified the average urban-rural phenological differences by 0.69 days, 0.68 days, and 1.37 days, respectively, compared to 250 m. These findings provide quantitative evidence for understanding the impact of urban intensity on spring phenology, deserving attention when drawing conclusions from vegetation phenology products at different resolutions.