Spectral Responses of Urban and Peri-Urban Woody Canopy to Recurrent Drought: A Matched Three-City Pilot

Urban conditions may increase woody-vegetation drought stress, although management and canopy composition can modify urban–peri-urban differences. We compared spectral anomalies of persistent woody canopy in Berlin, Madrid, and Vienna using Harmonized Landsat–Sentinel-2 observations from 2013–2024. Seventy urban–peri-urban pairs were matched on elevation, long-term temperature, precipitation, and retained woody area. Of 26 predefined candidate recurrent-drought events, 12 met the 80% observation-support criterion. Outcomes were event medians of within-pair urban-minus-peri-urban anomalies in the two-band enhanced vegetation index (EVI2) and normalized difference moisture index (NDMI). Each index produced seven positive and five negative contrasts; medians were 0.00047 for EVI2 and 0.00065 for NDMI. Event-level models adjusted for city and current drought severity examined antecedent severity and inter-event interval. Standardized-predictor coefficients ranged from −0.00225 to 0.00252 index units, and all four heteroskedasticity-consistent 95% confidence intervals included zero. Pair-within-event resampling, spatial buffering, and growing-season-aligned severity analyses also yielded intervals spanning zero. Climate antecedents were identifiable for all 26 candidates; vegetation observation support determined analytical inclusion. The retained cases did not consistently support a uniform urban spectral disadvantage or a directional drought-sequence association. This pilot provides a reproducible comparative design, but its small, observation-selected sample and restriction to persistent dense canopy limit inference about wider urban forests and physiological drought responses.