Giant trees in the Amazon are getting thicker—and likely taller—as rising atmospheric carbon dioxide helps fuel growth across intact rainforest plots. A new 30-year analysis spanning 188 sites reports a 3.3% per-decade increase in mean basal area, a standard measure of trunk girth, suggesting that the forest’s largest stems are adding wood faster than previously recognized, even amid climate stressors and warming extremes. Researchers caution, though, that the gains apply to intact forests; fragmentation, fires, and deforestation could erase the advantage.

Key Takeaways

– shows Amazon giant trees increased trunk basal area by 3.3% per decade across 188 plots over 30 years, 95% CI 2.4–4.1. – reveals large-tree prevalence expanded about 3.3% per decade, with no die-off trend detected among the biggest stems monitored. – demonstrates gains are strongest in intact forests; authors warn deforestation, fires, and fragmentation could erase CO2-driven size increases. – indicates compounding at 3.3% per decade yields roughly 10.2% thicker trunks over three decades, boosting biomass and carbon storage capacity. – suggests climate resilience: a 30-year study found “we’re not seeing signs of them dying off,” yet dubbed gains “qualified good news” for intact forests.

How giant trees are bulking up on CO2

Published September 25, 2025 in Nature Plants, the study finds that mean tree basal area—a proxy for trunk thickness—rose by 3.3% per decade across Amazonia over three decades (95% confidence interval: 2.4–4.1), consistent with a CO2 fertilization effect enhancing growth where forests remain intact [1]. A companion university release reported a comparable increase of 3.2% per decade, underscoring the consistency of the signal across the network of monitored plots [2].

The mechanism is straightforward: with more CO2 in the air, photosynthesis can accelerate, enabling trees to fix more carbon and build more wood, provided water and nutrients are not limiting. Researchers emphasize that both small and large stems gained, though the largest trees—the main carbon vaults—showed particularly strong thickening, reinforcing their disproportionate role in biomass storage [5]. The result is an Amazon that, in places undisturbed by chainsaws and flames, is adding woody tissue faster than it is losing it [1].

Tracking giant trees across 188 Amazon plots

The analysis draws on 188 permanent forest plots monitored by an international network over 30 years, led by Adriane Esquivel-Muelbert and involving nearly 100 researchers, providing one of the most comprehensive longitudinal datasets of Amazon tree dynamics to date [1]. The team remeasured trees repeatedly to calculate basal area trends, enabling a continent-scale view of how stems are responding to atmospheric and climatic change across gradients of rainfall, soil, and disturbance [2].

Media summaries emphasize two core findings: the basal area of large trees increased at about 3.3% per decade, and the population share of big stems expanded at roughly the same rate across the network, pointing to a fattening forest rather than a thinning one [3]. Crucially, field teams reported no widespread die-off among giant trees within intact plots—a signal of resilience even through recent drought years [3].

What 3.3% per decade means for carbon and climate

A 3.3% per-decade rise compounds to roughly 10.2% thicker trunks over 30 years, calculated as (1.033)^3 – 1. While basal area is not the same as biomass, it is a robust proxy for wood volume; all else equal, fatter stems imply more stored carbon. Because the largest trees hold a dominant share of biomass and long-term carbon, incremental thickening of these giants delivers an outsized carbon dividend relative to gains in smaller stems [2].

That dividend matters for climate because intact tropical forests absorb and store a significant fraction of anthropogenic CO2 emissions. The observed thickening indicates the Amazon’s undisturbed stands have, for now, retained or even strengthened their carbon sink function at the plot scale measured here. Researchers caution that this benefit is conditional: it depends on keeping forests intact and on climatic stresses not intensifying beyond physiological thresholds [1]. If droughts, heatwaves, and fire frequency continue to escalate, the sink could saturate or reverse, nullifying the structural gains seen so far [5].

Giant trees, intact forests, and deforestation risks

The authors are explicit that the “good news” story applies to intact forest; once fragmentation and deforestation intrude, the growth edge can vanish. The University of Birmingham team highlighted the exceptional carbon value of large trees while warning that clearing and breaking up forests could negate the CO2-driven size increase by raising mortality and fire exposure along edges [2]. News coverage similarly emphasized that roads, fires, and land clearance remain major threats—even as the biggest trees are thickening where they remain protected [3].

Environmental reporters pointed to active risk zones, including Brazil’s BR-319 highway corridor and aggressive land-clearance fronts in Bolivia, as examples where opening up intact blocks invites ignition, logging, and degradation that target the very giants now buffering the climate [4]. Oliver Phillips, a co-author, called the result “qualified good news,” underscoring that protecting intact forest is the precondition for maintaining the observed benefit [4].

What scientists say about giant trees’ resilience

Lead author Adriane Esquivel-Muelbert summarized the resilience signal plainly: “We’re not seeing signs of them dying off,” referring to giant trees within undisturbed plots, a key observation after recent record-breaking droughts [3]. Beatriz Marimon called the finding “a good news story,” while stressing that policy must focus on preventing fragmentation and deforestation that would convert a carbon sink into a carbon source [2]. Phillips’ “qualified good news” framing reiterates the same caveat: without intactness, size gains can quickly be overwhelmed by elevated mortality, fire, and heat stress [4].

Journalistic explainers added important nuance: while CO2 fertilization can temporarily boost growth, it is not a free pass against climate change. Drought severity, VPD (atmospheric dryness), and heat extremes have been rising in the basin; if they keep intensifying, they could curtail growth or tip large trees toward higher mortality, erasing the gains detected in the monitored plots [5].

Methodological notes and what to watch next

The Nature Plants paper’s central statistic—mean basal area increasing 3.3% per decade with a 95% CI of 2.4–4.1—reflects a clear, statistically supported directional change over three decades across diverse Amazon subregions [1]. The independent university release’s 3.2% estimate aligns closely, increasing confidence that the pattern is not an artifact of site selection or measurement [2]. Media outlets consistently reported the dual signal of thickening stems and expanding prevalence of large trees at roughly 3.3% per decade across the 188-plot network [3].

Looking ahead, three tests will determine durability. First, will future droughts and heatwaves amplify beyond trees’ hydraulic safety margins, pushing mortality high enough to offset CO2-fueled thickening? Second, will policy curb deforestation and road-building—such as the contentious BR-319 project—that elevate ignition risk and fragment habitats? Third, will nutrient limitations (notably phosphorus) and soil-moisture constraints cap the fertilization effect despite ample CO2? Researchers and explainers stress that accelerating climate extremes and land-use pressures could soon reverse today’s structural gains if protections fail [4][5].

Why giant trees matter for climate math

In tropical forests, a small fraction of stems—often less than 5%—can hold a third or more of aboveground biomass, making giant trees pivotal in carbon accounting. Thickening these already dominant stems is akin to adding capacity to the forest’s biggest vaults. That is why a seemingly modest 3.3% per-decade structural gain, sustained over multiple decades, can translate into a meaningful, compounding boost to the biome’s carbon stock, provided mortality does not climb and disturbance remains low [2]. The present findings suggest that intact Amazon stands are inching in the right direction—even as policy and protection determine whether that trend can last [1].

Bottom line for policy and markets

For policymakers, the message is both optimistic and urgent. Protecting intact Amazon forest may allow giant trees to keep translating atmospheric CO2 into durable wood, reinforcing the biome’s role as a natural climate buffer. But the same data set that delivers the 3.3% per-decade good news also implies fragility: open the canopy with chainsaws or roads, and the thickening advantage can quickly unravel, turning a sink into a source through fire, heat, and edge effects [3][4]. In carbon markets and national inventories, that makes high-integrity conservation—and prevention of fragmentation—as consequential as any engineered removal strategy [2][5].

Sources:

[1] Nature Plants (via PubMed) – Increasing tree size across Amazonia: https://pubmed.ncbi.nlm.nih.gov/40999006/

[2] University of Birmingham – The fattening forest: trees of the Amazon are getting bigger: https://www.birmingham.ac.uk/news/2025/the-fattening-forest-trees-of-the-amazon-are-getting-bigger [3] NBC News (NBC10 Philadelphia) – Giant trees of the Amazon get taller as forests fatten up on carbon dioxide: www.nbcphiladelphia.com/news/national-international/giant-trees-amazon-taller-forests-carbon-dioxide/4276404/” target=”_blank” rel=”nofollow noopener noreferrer”>https://www.nbcphiladelphia.com/news/national-international/giant-trees-amazon-taller-forests-carbon-dioxide/4276404/

[4] The Guardian – Big trees in Amazon more climate-resistant than previously believed: www.theguardian.com/environment/2025/sep/25/study-shows-big-trees-in-amazon-more-climate-resistant-than-previously-believed” target=”_blank” rel=”nofollow noopener noreferrer”>https://www.theguardian.com/environment/2025/sep/25/study-shows-big-trees-in-amazon-more-climate-resistant-than-previously-believed [5] LiveScience – Amazon rainforest trees are resisting climate change by getting fatter from CO2 in the atmosphere: www.livescience.com/planet-earth/climate-change/amazon-rainforest-trees-are-resisting-climate-change-by-getting-fatter-from-co2-in-the-atmosphere” target=”_blank” rel=”nofollow noopener noreferrer”>https://www.livescience.com/planet-earth/climate-change/amazon-rainforest-trees-are-resisting-climate-change-by-getting-fatter-from-co2-in-the-atmosphere

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