Analysing Changes in NDVI: A Long-Term Remote Sensing Approach to Monitor Trends in Plant Phenology of Urban Green Infrastructure

Climate adaptation strategies for many cities include urban green infrastructure as nature-based solutions due to their potential to mitigate urban heat island effects and reduce surface runoff. However, rising temperatures, drought events and altered precipitation patterns are expected to impact plant phenology by shortening dormancy, resulting in earlier flowering and extended growing seasons. These changes can increase irrigation demand and susceptibility to damage, posing a risk to urban green infrastructure and its ecosystem functions.

In this study, we investigated changes in the growing cycles of urban green infrastructure in four European cities (Birmingham, Paris, Graz, Barcelona) from 1984 to 2024.

The approach is based on monitoring the Normalized Difference Vegetation Index (NDVI) from satellite images to assess long-term trends and analyze the potential effects of climate change on plant phenology in an urban environment. Although analyzing NDVI in urban environments is still relatively new, it is becoming more feasible due to the increased availability of long-term, high-resolution satellite images.

Monthly NDVI values were derived from pre-processed Landsat satellite images to analyze changes in urban green infrastructure and plant phenology. Raster-based pixel counts with an NDVI value above 0.3 were normalized to highlight intra-annual vegetation peaks and seasonal shifts. Temporal trends in vegetation activity were assessed using the non-parametric Mann-Kendall trend test to identify upward or downward trends in the time series. The Theil-Sen Slope Estimator was subsequently applied to determine the magnitude and direction of the detected trends.

The results showed that all four cities expanded their urban green spaces over the past 40 years, with Barcelona exhibiting a particularly substantial increase. Normalized NDVI values revealed an earlier occurrence of peak NDVI and decreases during summer months in certain years, indicating a possible link to drought events. Statistically significant increases in NDVI were observed in March, April, October, and November, indicating both an earlier onset and later offset of the growing season.

Overall the study shows the current changes in plant phenology and developments of urban green infrastructure under climate change. Integrating remote sensing of vegetation with urban water management can support more efficient and adaptive management strategies for irrigating urban green spaces.