Heat waves pushed 57 million hectares of tropical forests beyond photosynthesis limits: Study
A new study found that the area of tropical forest experiencing treetop temperatures above the critical thermal threshold for photosynthesis grew from about 43 million hectares two decades ago to 57 million hectares today — an area larger than France.
Researchers compared species-specific critical temperature thresholds, catalogued for 200 tropical tree species, against satellite-derived canopy temperature data collected across tropical forests between 2001 and 2020.
Above the critical threshold, the proteins driving photosynthesis begin to break down, reducing plant growth, cutting carbon uptake, and raising the risk of tree mortality.
The affected area is projected to keep expanding in the coming decades if warming trends continue, adding to concerns about the long-term stability of the tropical forest carbon sink.
The study was published in the Proceedings of the National Academy of Sciences (PNAS) and led by researchers at the École Polytechnique Fédérale de Lausanne (EPFL).
Photosynthetic Thermal Tolerance Threshold (Tcrit)
Plants have a critical leaf temperature (Tcrit) beyond which the enzymes and protein complexes that drive photosynthesis — particularly Rubisco activase and the photosynthetic electron transport chain — begin to denature, causing photosynthetic capacity to collapse rather than decline gradually. A widely cited 2023 study (Doughty et al., published in Nature) established this average critical threshold for tropical canopy leaves at approximately 46.7°C, with sustained exposure beyond it causing measurable leaf damage and reduced transpiration.
Key Details
- Average critical leaf temperature (Tcrit) for tropical forest canopy leaves: approximately 46.7°C (Doughty et al., 2023, Nature).
- Plants normally maintain roughly a 15°C safety margin between typical operating leaf temperature and Tcrit; this margin narrows as ambient warming, drought, and heatwaves push leaf temperatures upward.
- Sustained exposure above Tcrit has been linked to reduced transpiration (leaf cooling capacity) and visible leaf damage in field and experimental studies.
- The new EPFL-led study extends this single-threshold concept by mapping species-specific thresholds (200 species) against two decades of satellite canopy-temperature data, rather than relying on one average value.
The 57-million-hectare figure represents the tropical forest area where canopy temperatures now regularly exceed these species-specific Tcrit values, directly linking the abstract thermal-tolerance concept to an observed, expanding real-world footprint.
Tropical Forests as a Global Carbon Sink
Tropical forests, particularly the Amazon, Congo Basin, and Southeast Asian rainforests, together sequester a substantial share of the carbon dioxide absorbed by land ecosystems annually, making their photosynthetic health directly relevant to the global carbon budget referenced in IPCC assessment reports. Heat-driven photosynthesis decline reduces this carbon uptake capacity and, in extreme cases, can turn forest areas from net carbon sinks into net carbon sources through increased tree mortality and decomposition.
Key Details
- The IPCC's Sixth Assessment Report (AR6) flags terrestrial carbon sinks, including tropical forests, as vulnerable to saturation and potential reversal under continued warming and land-use change.
- Reduced canopy transpiration also lowers the volume of water vapour tropical forests release into the atmosphere, a process linked to regional and even distant rainfall patterns (the "biotic pump" effect), meaning forest heat stress can compound drought risk beyond the forest itself.
- Large-scale Amazon dieback is frequently cited as a possible climate "tipping point," where reduced rainfall recycling and increased fire risk could trigger a shift from rainforest to a drier savanna-like state.
- India's own tropical/moist forests (Western Ghats, North-East India, Andaman & Nicobar) are ecologically comparable systems, relevant to India's Nationally Determined Contribution (NDC) target of creating an additional carbon sink of 2.5-3 billion tonnes of CO2-equivalent through forest and tree cover by 2030.
A shrinking thermal safety margin across a widening area of tropical forest directly threatens the carbon-sink function that global climate mitigation pathways (including India's own NDC forestry targets) depend upon.
Heatwave-Induced Forest Stress and Mortality Risk
Heat stress operates alongside drought as a compounding driver of tree mortality; even without soil moisture deficit, elevated air and leaf temperatures alone can push canopy leaves past their thermal operating range, especially during heatwave events when temperatures spike well above seasonal norms. Studies from Switzerland and other temperate regions cited alongside this research indicate that such heat-driven physiological stress is not confined to the tropics, though tropical species — adapted to a narrower historical temperature range — are considered more vulnerable to relatively small increases in peak temperature.
Key Details
- Tropical species typically evolved under a narrower range of temperature variability than temperate species, giving them comparatively less physiological headroom to acclimate to sudden heat spikes.
- Leaf-level cooling mechanisms (transpiration, leaf angle adjustment, boundary layer effects) can partially offset short heat exposures, but these mechanisms are themselves impaired under drought, when stomata close to conserve water and transpirational cooling is reduced.
- The study's 2001-2020 satellite dataset allows detection of a rising trend (43 to 57 million hectares) rather than a single-year anomaly, indicating a structural climate-driven shift rather than a one-off heatwave event.
The consistent upward trend documented over two decades — rather than an isolated spike — is what elevates this finding from a localized heatwave story to a structural signal about tropical forest resilience under sustained global warming.
- Tropical forest area exceeding critical canopy temperature thresholds: grew from ~43 million hectares (early 2000s) to 57 million hectares (present), an area larger than France.
- Average critical leaf temperature threshold (Tcrit) for tropical canopy photosynthesis: ~46.7°C (Doughty et al., 2023, Nature).
- Typical thermal safety margin for tropical leaf photosynthesis: roughly 15°C above ambient operating temperature.
- Study period: satellite temperature data spanning 2001-2020, compared against critical thresholds for 200 tropical tree species.
- Published in the Proceedings of the National Academy of Sciences (PNAS), led by Charlotte Grossiord's Plant Ecology Research Laboratory, EPFL.
- India's NDC forestry target: an additional carbon sink of 2.5-3 billion tonnes of CO2-equivalent through forest and tree cover by 2030. [Unverified — figure drawn from general NDC context, not confirmed within the source article itself]