It is said to be a plus in the fight against climate change: heat and CO2 accelerate tree growth. But apparently this effect can hardly be counted as an advantage for carbon sequestration, report researchers. For the first time they have comprehensively documented that fast-growing trees all over the world typically die early. Their model simulations show that this effect nullifies the initial gain in biomass in the end.
Normally, an equilibrium would be established: Forests absorb about as much carbon from the atmosphere as they release again when they later decay. However, humans have altered this balance by burning fossil fuels. The atmospheric CO2 levels have risen sharply, which has led to a rise in temperature which, together with the “fertilizing effect” of atmospheric carbon dioxide, has been shown to increase plant growth. So one might expect that forests will buffer some of our carbon emissions – and that is exactly what most Earth system models predict.
But there were already indications that this calculation might not work out in the end. In the Amazon, for example, it was found that the initial increases in growth were followed by an increase in tree mortality. The suspicion arose that this could be due to faster growth that shortens the lifespan of the trees. If this effect is widespread, it could mean that predictions that forests would continue to form a carbon sink may have been overly optimistic. “So far, the connection between growth and longevity has only been proven in a few species and in special locations,” says Roel Brienen from the University of Leeds.
A global principle is emerging
That is why he and his colleagues have now extensively researched the possible connection through a global analysis. The international team of scientists first examined data from more than 80 tree species from many climatically different regions of the world. In addition to information on the age reached, analyzes of the characteristics of the annual rings of trees revealed their growth rates.
In this way, the scientists were able to confirm the principle: trees that grow quickly die young. It has long been known that faster growing species live shorter lives. For example, a balsa tree quickly reaches a height of 20 meters or more, but only lives a few decades, while some species of pine have been growing steadily for almost 5000 years. However, the researchers have now been able to show that the effect can not only be determined when comparing different species, but also within trees of the same species. The study thus confirms that the compromise between growth and lifespan actually occurs almost universally in almost all tree species and climatic zones.
Thwarted the bill?
To study the effects of this effect, the researchers then carried out simulations of the accumulation of carbon in forests. They illustrated how the faster growing trees initially increased the overall level of biomass – but not in the long term. Due to the earlier death, the forest finally loses biomass again and returns to the same level as at the beginning. The models thus show that carbon storage will hardly increase in the long term. It is likely that tree death will now increase globally after the previous growth spurt.
The researchers suspect that one cause of the link between rapid growth and premature death is that the trees reach their maximum size very quickly and then die off. In addition, trees that grow quickly could also invest less in the defense against diseases or pests or develop structures that are more susceptible to drought, the researchers explain. “Our results suggest that the fastest growing trees have characteristics that make them vulnerable, while slower growing trees have characteristics that enable them to survive,” said co-author Steve Voelker of Syracuse University in New York.
Whatever the cause, the mechanism should now be built into scientific models to make realistic predictions about the future development of the forest carbon sink, the scientists say. “Model projections on the persistence of the global forest carbon sink are likely to be imprecise and overly optimistic. Ultimately, our results emphasize once again the urgency of reducing greenhouse gas emissions, ”says co-author Manuel Gloor from the University of Leeds.
Source: University of Leeds, article: Nature Communications, doi: 10.1038 / s41467-020-17966-z