When Heat Meets Smog: Ozone During Heatwaves Shifts Autumn Timing in China’s Forests

A heatwave is rarely just a heatwave. Across much of China, the same stagnant, sun-drenched weather systems that drive temperatures to dangerous highs also cook the atmosphere photochemically, producing ground-level ozone at concentrations harmful to both human lungs and plant leaves. A new nationwide analysis, published in Environmental and Sustainability Indicators, has now untangled these two intertwined stresses and found that when ozone rides along with heatwaves, it leaves a distinct fingerprint on when China’s ecosystems shut down for the winter. The study, led by Wenxing Hou of Beijing Normal University and colleagues, examined nearly a decade of satellite observations spanning 2014 to 2023 and reached a conclusion with important implications for how scientists model the ecological consequences of compound climate and air-quality extremes.

The core insight of the research is deceptively simple but methodologically powerful. An annual count of compound heatwave-ozone events can rise for two very different reasons: either heatwaves as a whole become more frequent, or a larger share of the heatwaves that do occur coincide with elevated ozone. These two scenarios expose vegetation to fundamentally different conditions, yet both inflate the same summary statistic. To separate them, the team constructed a statistical framework in which the total number of heatwave events was held fixed, so that the coefficient associated with compound events isolated the effect of ozone co-occurrence per se rather than general heatwave activity. This fixed-total parameterization, compared against a conventional separate-count model, revealed how much hidden information the raw compound-event count had been carrying.

The data underpinning the analysis were assembled from some of the most advanced environmental datasets available. Daily maximum temperatures came from the ERA5 atmospheric reanalysis produced by the European Centre for Medium-Range Weather Forecasts, with heatwave thresholds calibrated against the 1983 to 2012 baseline at each pixel using the 90th percentile of warm-season temperatures. A heatwave was declared when daily maxima exceeded this local threshold for at least three consecutive days. Ozone exposure was mapped using the CHAP dataset, a machine-learning reconstruction of daily ground-level ozone at one-kilometer resolution that blends ground stations, satellite retrievals, and reanalysis fields, with a cross-validation coefficient of determination of 0.87. Days were classified as ozone-stressed when the eight-hour daily maximum concentration exceeded 100 micrograms per cubic meter, a widely used operational cutoff in Chinese compound-exposure studies.

For the biological response, the researchers turned to the MODIS MCD12Q2 land-surface phenology product, which tracks the timing of seasonal canopy transitions from vegetation-index time series. Their focal variable was the end of season, or EOS, the date at which the canopy moves toward dormancy in autumn. Croplands were deliberately excluded because planting, irrigation, and harvest schedules dominate their seasonal rhythms and would obscure environmental signals. That left forests and grasslands, the two dominant natural vegetation types covering roughly a quarter and nearly thirty percent of China’s landscape respectively. Using a fixed-effects panel regression over more than 48,000 sampled pixels, the team quantified how year-to-year variation in exposure was associated with year-to-year variation in EOS within each pixel, while absorbing persistent local differences and shared annual fluctuations. Uncertainty was estimated with a spatial block bootstrap of 2,000 replicates to guard against the statistical distortion caused by neighboring pixels sharing conditions.

The mapping of compound stress itself produced striking geography. Compound heatwave-ozone events clustered most densely over the North China Plain and the Fenwei Plain, regions where dense industrial activity and vehicle fleets supply abundant nitrogen oxides and volatile organic compounds, the raw ingredients of ozone, and where persistent anticyclonic systems trap both heat and pollutants over the boundary layer. Roughly one in five land pixels fell into the upper compound-frequency classes. Heatwave-only events, by contrast, were concentrated in Southern China and the Tarim Basin, where monsoon cloudiness and stronger atmospheric ventilation suppress photochemical ozone accumulation despite frequent heat. Ozone-only stress days blanketed the mid-latitudes, with more than thirty percent of the land area experiencing over 114 such days per year. Temporally, the study identified what the authors call a misalignment-overlap pattern: ozone pollution peaks in June, heatwaves intensify in July and August, and so compound exposure compresses into a narrow midsummer window that coincides exactly with the peak growing season.

When the phenological associations were computed, forests and grasslands parted company in an unexpected way. In forests, holding the total number of heatwave events constant, each additional ozone-coincident heatwave was associated with an earlier end of season, by about 0.293 days per event, with a confidence interval comfortably excluding zero. Grasslands showed the opposite tendency, a positive coefficient of about 0.114 days per event, though it was statistically uncertain. The formal grassland-minus-forest contrast was 0.407 days per event, a clear and significant ecosystem difference. Perhaps most revealing was what happened when the conventional parameterization was compared with the fixed-total one: in grasslands, the positive compound-event association largely evaporated once overall heatwave frequency was accounted for, indicating that most of the apparent grassland signal had been borrowed from the general heatwave regime. In forests, by contrast, the negative association actually strengthened, suggesting that ozone co-occurrence carries genuine information about autumn canopy decline beyond the sheer number of heatwaves.

The physiological logic behind this forest response is plausible, though not yet fully demonstrated. Ozone enters leaves almost exclusively through stomata, the microscopic pores that plants open to absorb carbon dioxide, and once inside it triggers oxidative damage that accelerates senescence. Stomatal behavior, however, is not a passive conduit. It responds strongly to temperature, vapor pressure deficit, and the plant’s water status, and hot, dry conditions can partially close stomata, limiting ozone uptake even as ambient concentrations climb. Forests and grasslands differ markedly in rooting depth, canopy structure, and water-use regulation, which is one reason the same atmospheric exposure might translate into different canopy outcomes. The authors caution that resolving the mechanism will require direct measurements of stomatal conductance, plant water status, and stomatal ozone flux, since event-based counts of threshold exceedances compress away the intensity and duration of exposure that determine the physiological dose plants actually receive.

One of the study’s most vivid findings concerns elevation. In forests, the negative association between ozone co-occurrence and end-of-season timing was strongest at low elevations and progressively weakened upward, at a rate of roughly 0.242 days per event per thousand meters. Across the forest elevation range sampled, from about 155 to 3,258 meters, the conditional coefficient swung from minus 0.518 to plus 0.234 days per event, although the high-elevation estimate carried wide uncertainty. Topography reshapes surface energy balance, water redistribution, and evaporative demand, and previous satellite analyses have shown that elevational patterns of autumn phenology cannot be explained by temperature alone. The pattern suggests that forests are most phenologically sensitive to ozone-coincident heatwaves when those events occur under warmer, more water-demanding lowland conditions, a finding consistent with evidence from the 2022 European heat and drought extremes, where forest carbon uptake and autumn canopy activity depended heavily on the seasonal hydrothermal background.

The study’s authors are candid about its limits. The 2014 to 2023 window is short for characterizing rare compound extremes, and annual event counts inevitably compress differences in event timing, duration, intensity, and antecedent conditions, all of which are known to modulate vegetation responses. Satellite-derived end-of-season dates, moreover, capture canopy greenness rather than the exact cessation of photosynthesis, and different remote-sensing proxies can disagree about the timetable of senescence. Integrating solar-induced chlorophyll fluorescence, eddy-covariance carbon flux measurements, and flux-based ozone metrics such as phytotoxic ozone dose would provide a more physiologically grounded picture of how compound exposure propagates from leaf chemistry to landscape-scale phenology.

Even with those caveats, the implications are substantial. Climate projections indicate that heatwaves will grow more frequent and severe, and that ozone episodes will intensify in many rapidly developing regions, making compound exposure an increasingly routine feature of the growing season rather than a statistical curiosity. The finding that the ecological meaning of a compound-event count depends on whether it reflects more heatwaves or more ozone within those heatwaves offers modelers a sharper instrument: vegetation projections that treat compound events as a single undifferentiated category may misattribute the drivers of phenological change. For China’s forests, particularly the densely exposed lowland forests of the north, the message is that air-quality management is climate-adaptation policy. curbing ozone precursors during heatwave episodes could ease an invisible but measurable pressure that pulls the growing season to an early close, with consequences for carbon storage, autumn land-surface energy exchange, and the ecosystems that depend on the length of the green season.

Subject of Research: Associations between compound heatwave-ozone exposure and autumn vegetation phenology across Chinese forests and grasslands

Article Title: Distinguishing heatwave occurrence from ozone co-occurrence reveals contrasting autumn phenological associations across Chinese ecosystems

Article References: Hou, W., Hu, Z., Wang, L., Wang, M., & Liu, X. (2026). Distinguishing heatwave occurrence from ozone co-occurrence reveals contrasting autumn phenological associations across Chinese ecosystems. Environmental and Sustainability Indicators, 32, Article 101528. https://doi.org/10.1016/j.indic.2026.101528

Image Credits: AI Generated

DOI: 10.1016/j.indic.2026.101528

Keywords: heatwaves, ground-level ozone, autumn phenology, end of season, forests, grasslands, China, compound events, satellite remote sensing, fixed-effects regression, elevation gradient, stomatal ozone uptake