Comparative Analysis of Diurnal Thermal Stress Responses and Lag Effects in Acer Campestre Using Chlorophyll Fluorescence During UK Summers 2022–2023

Urban trees face increasing extreme heat, yet the timing of their short-term physiological responses remains unclear. This study investigated thermal stress dynamics in Acer campestre during two contrasting UK summers (2022 and 2023) and examined how sampling frequency affects the detection of temporal lag effects. Chlorophyll fluorescence parameters (NPQ and ETRmax) were measured daily during the 2022 high heat days and weekly under the milder conditions of 2023. Generalised linear mixed models and cross-correlation analyses explored the relationships between leaf temperature and photosynthetic performance. In 2022, NPQ showed increased morning thermal sensitivity under extreme heat, but detrended analyses did not reveal strong multi-day lag effects. ETRmax responded in synchrony with temperature, showing no detectable time lag. In 2023, apparent multi-day correlations matched exactly with the weekly sampling interval and were diminished or nullified by detrending, suggesting sampling artefacts rather than physiological memory. Across both years, morning measurements consistently indicated the greatest thermal sensitivity. These findings reveal that Acer campestre exhibits threshold-dependent stress responses and highlight the importance of high-frequency sampling for accurately capturing short-term physiological dynamics. Sparse sampling can produce misleading lag patterns, emphasising the need to match measurement frequency with the biological response timescales in urban tree monitoring. This graphical abstract describes the contrasting methodological approaches and key physiological findings between the two years. The study compared extreme heat conditions in 2022 (depicted by the red thermometer and text) with moderate thermal conditions in 2023 (orange thermometer and text). The left panel displays laboratory measurements from 2022, where a researcher in lab attire collected detached leaf samples for chlorophyll fluorescence analysis using specialised sensors measuring NPQ, ETRmax, and leaf temperature (T). The right panel shows field-based canopy measurements from 2023, featuring a researcher with safety equipment using a ladder to reach tree canopies with the same sensor technology. Diurnal response patterns are illustrated through sun symbols representing morning, midday, and evening measurements, with directional arrows indicating parameter trends. In 2022, extreme heat caused declining NPQ and ETRmax responses (downward red arrows) despite rising temperatures (upward arrows), indicating physiological stress. The year 2023 showed mixed responses with NPQ decreasing in the morning but increasing later, while ETRmax consistently increased throughout the day (upward orange arrows), suggesting normal diurnal optimisation without thermal limitation. The temporal analysis results are summarised at the bottom, with clock and calendar symbols representing lag effects and sampling frequency differences, respectively. Daily sampling in 2022 revealed morning thermal sensitivity under extreme heat, NPQ showing strong morning responsiveness (β = -0.03, p = 0.026) and ETRmax responding synchronously to temperature with no detectable lag (β = -0.08, p = 0.04). Weekly sampling in 2023 detected morning-specific correlations at sample intervals − 6 and − 7 (β = -0.28 and − 0.29, respectively), which represent sampling structure artefacts rather than biological memory, highlighting the critical importance of sampling methodology in detecting genuine temporal lag effects in plant stress responses. Acer campestre shows contrasting heat stress responses across two UK summers. Heat and sampling frequency strongly influence detection of physiological lags. Morning-specific NPQ sensitivity observed in both years, strongest in 2022. Sparse sampling in 2023 produced interval-based correlations, not true memory. Heat intensity and sampling frequency critically shape physiological lag detection.