Faster tree growth won’t help us hit climate targets

It’s well established that warmer temperatures and more carbon dioxide in the air can make trees grow faster. Scientists have incorporated these relationships into climate models, positing that faster growing trees will result in more carbon stored in forests, helping to mitigate climate change.

But a new study based on tree-ring data spanning the past 2,000 years calls that assumption into question. It shows that trees that grow faster when they are young ultimately don’t live as long as slower-growing trees. The results suggest that future forests will turn over too fast to lock up carbon for an appreciable length of time.

To reach this conclusion, the researchers collected cores and slabs of wood from living and dead trees growing in remote areas. They collected samples from 1,108 Mountain pines in the Spanish Pyrenees and 660 Siberian larches in the Altai-Sayan Mountains of Russia.

Both tree species can live for up to 1,000 years. The samples come from areas that have experienced little to no direct disturbance by humans that could affect their growth rates. Collectively, they represent a 2,000-year record of tree growth spanning both pre-Industrial and Industrial-era climate conditions.

The researchers analyzed the width of the growth rings in each individual tree to determine how fast it grew in the first 25, 50, or 75 years of life. Then they analyzed how early growth rates relate to trees’ eventual longevity.

They found that for the trees, there’s a tradeoff between fast growth and longevity. “Old ages are reached only if juvenile growth is slow,” the researchers write.

The relationship holds in both tree species, in dead as well as living trees, and for analysis of the first 25, 50, or 75 years of growth.

Certain fast-growing tree species, such as those that are the first to grow in open areas after a fire or other disturbance, are known to be relatively short-lived. The new study shows that this relationship between growth rate and lifespan also holds for individual trees of longer-lived species. In turn, that has implications for forest-wide carbon stocks.

“This study suggests that accelerated tree growth (past, present and future) is unlikely to translate into enhanced carbon sequestration, thereby mitigating the global greenhouse effect,” the researchers write. However, it’s not yet certain whether the same relationships would hold for different tree species or in other parts of the world.

Still, the findings call certain climate change strategies into question. Already some countries have started projects to grow secondary forests as a way to achieve negative carbon emissions. But it turns out that the shortcut to cutting greenhouse gases won’t be found in long-lived trees.

Source: Buntgen U. et al.Limited capacity of tree growth to mitigate the global greenhouse effect under predicted warming.” Nature Communications 2019.

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