Climate Change Kills Trees — Yet Some Forests Are Growing Thicker
In recent years, news reports and scientific assessments have increasingly described “forest damage caused by extreme weather” and “tree mortality from drought.” Indeed, it is true that many forests have suffered from extreme heat waves, desiccation, and wildfires. For that reason, we often assume that “global warming = tree dieback.”
However, two recently published peer-reviewed papers challenge this common assumption.According to these studies, trees in both North and South America are getting thicker. In other words, even amid climate change, trees are quietly but steadily increasing in size. Behind this unexpected trend lies a surprising mechanism: the combined effects of rising atmospheric CO₂ and global warming themselves are promoting growth.
Based on these two papers, let’s explore what is actually happening across the American continents.
The Northern Forests: Growth of Large-Diameter Trees in the U.S. — The Blessing of Warm and Humid Seasons
A study conducted in U.S. forests (Chisholm & Gray, 2025, PNAS) analyzed long-term national forest inventory data and reported that the number of large-diameter trees (≥ 100 cm DBH) has generally increased over the past several decades.
In particular, in the eastern United States, the average annual increase reached around 3 %, a surprisingly high growth rate. This trend appears to be linked to a longer growing season caused by warming and improved moisture conditions due to increased precipitation. In other words, rather than severe droughts or cold spells, it is the “warmer and wetter climate” that has favored tree growth in some regions. The authors also point out that reduced logging pressure, forest recovery, and recent climatic stabilization together have supported this phenomenon of “thickening trees.”
Reference: Chisholm, R. A., & Gray, A. N. (2025). Populations of large-diameter trees are increasing across the United States. Proceedings of the National Academy of Sciences (PNAS).
The Southern Forests: Expanding Basal Area in the Amazon — The Shadow of the CO₂ Fertilization Effect
Meanwhile, a 30-year long-term observation of the Amazon rainforest (Esquivel-Muelbert et al., 2025, Nature Plants) revealed that the total forest basal area (the cross-sectional area of tree trunks) has increased by about 3 % per decade.Remarkably, this upward trend has persisted even after extreme events such as major droughts and El Niño years.
The authors attribute this mainly to the CO₂ fertilization effect. When atmospheric CO₂ increases, plants can perform photosynthesis more efficiently under the same light conditions. Moreover, since smaller stomatal openings are sufficient for CO₂ uptake, water loss decreases and water-use efficiency (WUE) improves.
As a result, trees become less affected by warming and drying, and overall, “thickening forests” are emerging.
CO₂ and Warming Are Broadening the “Physiological Optimum” in Some Regions
Normally, plant growth has a “physiological optimum”—if it becomes too hot or too cold, growth slows. However, the recent rise in CO₂ appears to be quietly broadening that optimum range.
With more CO₂ available, photosynthesis is stimulated, enabling trees to “tolerate” moderately higher temperatures and drier conditions. This mitigates part of the stress caused by climate change and, as a result, shifts the average trend toward “thicker growth.” Of course, nature is not consciously balancing itself. Yet this can be seen as an ecosystem-level buffering response to atmospheric change—an intriguing phenomenon.
It is almost as if the Earth itself were inhaling the excess CO₂, and the trees were swelling their bodies in response.
Beyond the Limit: When Drought, Fire, and Heat Stress Cross the Threshold
That said, this trend of “thickening forests” will not last forever. Even if rising CO₂ provides a temporary fertilization effect, extreme warming and frequent droughts could push mortality beyond growth.
In 2025, large-scale wildfires again swept across North America and Canada, releasing massive amounts of stored carbon into the atmosphere. Heat waves, desiccation, and hurricane damage were also widely reported.
When the pace of thickening falls behind the pace of destruction, what we may witness is not just “a halt in growth,” but the collapse of accumulated forests. As long as the CO₂ fertilization effect persists, forests will continue to grow— but once that threshold is crossed, the same CO₂ could become fuel that amplifies the fire.
Are Thickening Forests the Earth’s Stress Response—or a Temporary Expansion?
From the temperate forests of the north to the tropical jungles of the south, these two studies together reveal a hopeful sign: even in an era of climate extremes, forests continue to grow. Yet this may not represent lasting stability, but rather a temporary ecological response to environmental stress. Forests thickening may be the planet quietly steadying its breath under strain. Whether this “silent giantification” signals recovery or the prelude to collapse, we may now be standing at that very crossroads.
To understand and continue observing this phenomenon may hold the key to deciphering the next move of our living planet. How this transition unfolds from here remains deeply fascinating.
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