Fertilizing effect of CO2 on tropical vegetation revised down

The new data has led to a downward revision to the fertilizing effect of carbon dioxide (CO2) on tropical vegetation. These results are consistent with previous work and show that water availability and forest fire activity are more critical factors in the evolution of forest cover. The study was published May 5 in the journal The science.

Increasing the concentration of carbon dioxide in the atmosphere tends to fertilize vegetation, like in an agricultural greenhouse, and stimulate the growth and expansion of forest cover. This effect, which is particularly pronounced in the tropics, thus accelerates carbon sequestration through photosynthesis and partially offsets anthropogenic emissions.

CO2 fertilizer, outdated concept?

At least that’s what we thought ten or twenty years ago. Since then, numerous field studies have shown that the response of tropical vegetation to a CO2-enriched atmosphere is more complex than one might imagine rather simplistically. As a result, various processes tend to oppose fertilization and the losses from these competing influences are far from trivial.

New study published in the journal The science bring the point home a little more. Analyzing the evolution of tropical vegetation in West Africa over the past 500,000 years, a team of researchers found that changes in CO2 have surprisingly little effect on the progressive increase and decrease in forest cover.

Credits: SRBR / iStock.

We compared past environmental change data from sediments recovered from Bosumtwi Lake in Ghana with climate modeling and global atmospheric CO2 changes from ice cores over the past 500,000 years. “explains William Gosling, lead author of the study.

The dominant role of humidity and forest fires

In their article, the scientists assessed the contribution of carbon dioxide to observed changes compared to five other factors: temperature and its seasonality, water availability, wildfire activity, and the number of herbivores present in the ecosystem. However, it turned out that the most determining factors were the availability of water and the activity of forest fires. Even the density of herbivorous mammals is ahead of carbon dioxide.

The benefits of increased CO2 are much smaller than originally thought. Gold, many climate and vegetation models still overestimate the impact of CO2 and therefore underestimate the impact of climate change on vegetation. “, says Jonathan Overpeck, one of the co-authors of the study. ” We have shown that the amount of CO2 in the atmosphere does not matter if there is not enough water, if there are fires every year, or if all the seedlings are eaten by animals. summarizes William Gosling.

a tropical forest
Credits: flickr.

If the study’s apparent limitation is that it concerns only a geographic entity, it nonetheless reinforces previous work and insists somewhat more on the needimprove the representation of interactions between climate and vegetation in models. In addition, tree planting projects are still too often based on the outdated concept of an almost mechanical relationship between CO2 and vegetation.

The idea that you can just plant trees to sequester carbon, and that those trees will be protected from hotter, drier climates by high levels of atmospheric CO2, is not a safe bet. warns Jonathan Overpeck. “ Efforts to promote carbon sequestration in tropical vegetation must carefully consider the roles of moisture, fire, and herbivores if they are to be successful. adds William Gosling.

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