Climate change is making trees grow faster and taller, is it good or bad?

Terrestrial ecosystems undeniably play a vital role in global carbon recycling. It has been already established by various studies that trees of today are growing at a much-accelerated rate in response to elevated carbon dioxide and temperature compared to the past 225 years. However, the significant question that arises here is whether this is good or bad. A recent study published in Nature Communications points out a significant trade-off between the trees’ accelerated growth and their lifespan. This means that increased tree growth rates might lead to shorter lifespans in the long run. Thereby, offsetting the benefits of increased carbon uptake in the present.

Growth-Lifespan Trade-offs 

R. J. W. Brienen and his team analysed more than 210,000 individual tree ring records for 110 diverse tree species from more than 70,000 sites around the world, ranging from the tropics to the Arctic. The researchers in an article titled “Forest carbon sink neutralized by pervasive growth-lifespan trade-offs” point out that, on average, tree lifespan dropped “exponentially” by 23% for a 50% increase in early growth.

The analysis linked the trade-off directly to a shortened life span from accelerated tree growth, ruling out the direct effects of environmental factors like temperature and soil type. 

Researchers speculate that the increases in tree mortality are a result of the trees reaching their maximum potential size. At the same time, rapid growth is also suspected to weaken tree defenses against insects or disease or cause them to grow less densely. Thus, compromising their water transport systems.

The pervasiveness of the findings

The paper observes that the growth-lifespan trade-offs are nearly universal in world terrestrial ecosystems. That is to say, this observation in growth has been made across almost all species and climates. The article also notes that the trade-off is directly linked to faster growth reducing tree lifespan, and not due to covariance with climate or environment. 

“We started a global analysis and were surprised to find that these trade-offs are incredibly common. It occurred in almost all species we looked at, including tropical trees.” Roel Brienen, associate professor of geography at the University of Leeds, and the lead author of the paper, told Gaurdian.

Other studies

Scientists have also dubbed this pace of growth in the world's forests to be a fast food diet of carbon dioxide, which is currently causing them to grow faster. However, researchers provide warning evidence that suggests that forest growth may soon peak as the trees deplete nitrogen in the soil over longer growing seasons. McNeil and others in their paper published in Nature Ecology and Evolution note that trees, like humans, need to have more than one thing in their diets.  The researchers say that the proliferation of carbon dioxide is force-feeding them the one thing they use most. The study underscores the need to restore a balanced diet for forests by severely cutting back or ending altogether the use of fossil fuels.

"There's more carbon dioxide in the atmosphere, and that's the raw material that trees need to convert to sugar, which they use to grow," McNiel told Science Daily. "What is profound is that as all the plants grow faster; they're slowing down climate change." But, McNeil further explains, "the plants of the world can't do that forever. Most of the world is still "greening" in response to climate change, but diminishing nitrogen means future growth will become unhealthier and out of balance,”

Lighter wood

Furthermore, a 2018 study investigated how climate change led to accelerated growth is also leading to lighter wood. Researchers demonstrated that annually growing wood has become lighter by 8-12% since 1900. Lighter wood is less solid and also has a lower calorific value. Researchers noted that less solid wood in living trees increases the risk of damage events such as breakage due to wind and snow in forests. But more importantly, they drew attention to how current climate-relevant carbon sequestration of the forests is being overestimated- taking into account established but outdated wood densities.

Studies have also previously noted how older forests are more resistant to climate change than younger forests. For instance, research in forests of the eastern United States and Canada found that increased forest age reduces the climate sensitivity of forest carbon, and biodiversity to projected increases in temperature and precipitation. In other words, increased age helps to safeguard forests from climate change, which is not the case with younger forests. 

Global Implications 

R. J. W. Brienen and his team highlight the significant effects these findings will have on how we perceive future carbon sequestration and the precision of current Earth system models (ESM) estimates.

Faster growth increases the amount of carbon absorbed, but it also hastens tree mortality, which releases carbon from the tree's reserves back into the atmosphere. The long-term ability of forests to reduce atmospheric CO2 levels might be hampered by this feedback loop of increasing growth leading to higher mortality.  Researchers argue that Earth System Models undertake very limited consideration of this growth-lifespan trade-off, making them overly optimistic and inaccurate in their predictions. 

“Currently, Earth system climate models predict continuation or increases in the size of the carbon sink of mature forests and this study shows the opposite, that increased CO2 compromises forests as a carbon sink.” David Lee, professor of atmospheric science at Manchester Metropolitan University told Guardian. He further says, “the idea that fossil fuel-based emissions can be offset by planting trees or avoiding deforestation really does not stand up to scientific scrutiny.” 

It is important to understand that the research does not negate the importance of growing trees and preventing current deforestation. Rather, it underscores the urgent need to curb present and future carbon emissions by bringing out the consequences inaction would have on the world's ecosystems.

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