Seasonal and diurnal lag effects in chlorophyll fluorescence response of Acer campestre to heat stress
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This study uses chlorophyll fluorescence metrics to explore the seasonal and diurnal lag effects in the heat stress response of Acer campestre, a common urban tree species in the UK. A two-phase design was employed: a controlled mesocosm experiment in 2022 and in situ field monitoring in 2023. In 2022, time series and lag analysis of NPQ and ETRmax revealed significant delays in physiological response during morning hours. NPQ exhibited lag effects at 0, -1, and − 2 days, with a significant negative association between same-day leaf temperature and NPQ (β = -0.03, p = 0.026), indicating reduced heat dissipation under thermal stress. ETRmax demonstrated a significant anticipatory decline two days before peak temperatures (β = -0.08, p = 0.046), suggesting downregulation of photosynthesis as a pre-emptive protective mechanism. In contrast, evening data in 2022 displayed no significant lag effects, indicating more synchronised physiological responses. During the 2023 season, field-based light response curves showed strong diurnal patterns without lag. ETRmax increased progressively from 14.84 at 09:30 am to 21.92 at 04:30 pm ( p < 0.001), indicating increasing photosynthetic capacity throughout the day. NPQ peaked in early July but showed no statistically significant correlation with leaf temperature at any time point. These findings highlight marked differences in physiological behaviour between morning and evening and across years with different thermal intensities. Morning lag effects were especially pronounced in 2022, suggesting a “thermal memory” where previous heat exposure alters current physiological responses. These insights underscore the importance of accounting for time-of-day and delayed stress effects when evaluating urban tree resilience and developing adaptive urban greening strategies under intensifying climate extremes.