---
id: articles/tropical-forests-critical-temperature-thresholds
about:
  label: >-
    Tropical forests are facing increasing risks of exposure to critical
    temperature thresholds
  description: >-
    PNAS study, July 2026, from EPFL: satellite canopy temperatures and range
    maps for 208 tropical tree species show forests nearing the critical
    temperature for photosynthesis.
  tags:
    - tropical forests
    - heat
    - photosynthesis
    - remote sensing
    - species distribution
    - carbon sink
article:
  url: 'https://doi.org/10.1073/pnas.2528622123'
  published: '2026-07-06'
  publisher: Proceedings of the National Academy of Sciences
  authors:
    - Nina van Tiel
    - Gaston Lenczner
    - Mukund P. Rao
    - Charlotte Grossiord
    - Devis Tuia
domains:
  - ecology
  - climate
open:
  source: unknown
  data: unknown
  api: unknown
  participation: unknown
  licence: CC BY-NC-ND 4.0 (the paper)
  note: >-
    The full text in PubMed Central is under embargo until January 2027; the
    abstract is public.
provenance:
  origin: agent
  sources:
    - title: >-
        Tropical forests are facing increasing risks of exposure to critical
        temperature thresholds (Crossref record with abstract)
      url: 'https://api.crossref.org/works/10.1073/pnas.2528622123'
    - title: >-
        Tropical forests are facing increasing risks of exposure to critical
        temperature thresholds (PubMed 42406944)
      url: >-
        https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pubmed&id=42406944&rettype=abstract&retmode=text
    - title: PMC13367818 record
      url: >-
        https://eutils.ncbi.nlm.nih.gov/entrez/eutils/efetch.fcgi?db=pmc&id=13367818&retmode=xml
    - title: 'Heatwaves Hinder Photosynthesis (EPFL release, via Mirage News)'
      url: 'https://www.miragenews.com/heatwaves-hinder-photosynthesis-1707414/'
    - title: >-
        Heat waves pushed 57 million hectares of tropical forests beyond
        photosynthesis limits: Study (Down To Earth)
      url: >-
        https://www.downtoearth.org.in/forests/heat-waves-pushed-57-million-hectares-of-tropical-forests-beyond-photosynthesis-limits-study
  created: '2026-10-23'
  reviewed: '2026-10-23'
  reviewed_by: >-
    Claude, from the abstract, significance statement and record of the paper
    (pnas.org refused the request and the full text is embargoed) and two news
    reports of it; the figures were not seen
  status: draft
---
A leaf has a critical temperature above which photosynthesis starts to break down, and the tree grows less and is more likely to die.[^4] This study asks how close the canopies of tropical forests are to that line, and how fast they are closing on it.[^1,3] Nina van Tiel and Gaston Lenczner share first authorship; Mukund P. Rao is the third author.[^2,3] Charlotte Grossiord and Devis Tuia of EPFL in Switzerland supervised it.[^3] Christopher B. Field edited it for PNAS.[^3]

The method joins two kinds of data. Satellites give the maximum surface temperature of the canopy.[^1] Range maps say where each of 208 tropical tree species grows, for which a critical temperature had been catalogued.[^1,4] The gap between the two is the thermal safety margin.[^1] The study covers South America, Southeast Asia and Central Africa from 2001 to 2020, then carries the trend forward with Earth System Models under medium-to-high emission scenarios.[^1]

The species tolerate a lot of heat: their critical temperature averages 46.1 °C.[^1] But the margin fell everywhere, by 0.4 °C a decade across the tropics and 0.5 °C in South America.[^1] The area where the canopy grew hotter than the average critical temperature rose from 43 million to 57 million hectares over the twenty years, 4% of the area studied.[^1,4] Measured against the most sensitive species, it was 10%.[^1] The projections put the area past the average threshold at 83 million hectares by 2050 and 160 million by 2100.[^1,4] A Down To Earth report of the study gives more than 93 million hectares for 2050;[^5] the abstract and the EPFL release both say 83 million.[^1,4]

The authors point to the carbon the forests take up and to their biodiversity as what is at stake.[^3] In the EPFL release, Grossiord says the method can find the places at risk, and that within one forest more heat-tolerant species may replace others; how fast that can happen is unknown.[^4] Tuia adds that rare species may matter more to how a forest works than their numbers suggest.[^4] The 208 species are a small share of the more than 15,000 in tropical forests, and the team plans to add species and to look at Mediterranean forests in Europe.[^4]

For a model of a forest, the paper is a recipe. A satellite temperature record, species range maps and a table of measured heat limits together give a map of where a forest's trees are nearest their limit.