This study investigated Acer campestre physiological responses to heat stress using a two-phase approach across 2022-2023 seasons. Initially, controlled laboratory measurements established baseline data on chlorophyll content, non-photochemical quenching (NPQ), and electron transport rate (ETR max) isolated from environmental influences. The following year, field data collection from tree canopies captured natural dynamics including fluctuations in leaf temperature and responses to weather variations. Analysis revealed significant time-lag effects between leaf temperature and physiological parameters. Morning measurements showed NPQ lags at multiple time points, with a negative correlation (β = -0.03, p = 0.026) between same-day leaf temperature and NPQ indicating compromised heat dissipation under thermal stress. ETR max exhibited a negative two days’ lag (p = 0.046), suggesting anticipatory downregulation of photosynthesis. Evening measurements demonstrated synchronized responses without significant lag. Seasonal comparisons revealed a five-day lag in chlorophyll fluorescence recovery following extreme heat events in 2022, while 2023 responses remained more stable under moderate conditions. These findings emphasize both immediate and delayed physiological responses in urban trees and highlight the importance of integrated forestry strategies for enhancing climate resilience in urban ecosystems.