(ORDO NEWS) — Warm climate savannas are responsible for about 30% of the bioproductivity of the earth’s land, they also account for 70% of the earth’s fires.
Therefore, it was believed that the fight against fires there and the overgrowth of trees in the savanna zone should bind a lot of carbon dioxide, slowing down warming. New data shows that everything is noticeably more complicated – and more interesting.
Unlike most zones on the planet, the main part of the tropical savannas is subject to systematic fires. About 62% of CO2 emissions from fires in the world fall on the savannah, whose inhabitants arrange fires due to their cultural traditions.
Northern forests contribute much less to these emissions. Based on this, scientists proposed to fight fires in such regions, as well as plant trees there. It was believed that trees were able to bind carbon dioxide in their tissues for decades, and in a fire-free environment, bound carbon might never leave local soils.
By themselves, such appeals did not give anything. But the growth of population in tropical countries has led to the plowing of a significant part of the savannah, as a result of which the locals began to burn it less often (savannah pastoralists are most often burned). In addition, the ongoing global greening on Earth has dramatically increased the number of trees in the savannas.
The authors of the new work – an international team of scientists – published in Nature , tried to test how much such processes helped to sequester carbon dioxide in this biome. And, therefore, how much they fight global warming.
It turned out that a serious decrease in the frequency of fires observed in savannas in the 21st century led to an increase in above-ground woody biomass in savannas by 157%, or 2.57 times (savannas, which burned out on average once every three years, were taken as a reference point). The canopy area in the savannah increased by 79% and the average tree height by 32%.
However, the total amount of carbon stored in the soil in savannahs with fewer fires has not changed much. As it turned out, during combustion, carbon is released mainly from the top centimeters of the soil. And its main part is located at depths of tens of centimeters, and fires do not affect it in any way.
The researchers compared the total amount of carbon fixed by vegetation in the savannah, where fires were suppressed for 60 years, with areas where fires were once every three years.
It turned out that in the first ecosystem, the total amount of fixed carbon over 60 years was only 35.4% higher than the amount of carbon bound in the regularly burned savannah.
The figure of 35.4% looks impressive, only if you do not consider that per year it means an increase in carbon sequestration of about 0.6%. In addition, almost all of this increase came from aboveground tree biomass.
Trees in the savannah live for decades, and soon their dead trunks will fall to the ground, where the carbon from them will inevitably return to the ecosystem, reducing the long-term carbon sequestration advantage for unburned savannahs to about zero.
One of the reasons for this unexpected result, scientists note, is that savannahs, where fires are frequent, retain most of the woody biomass in the soil.
This is what happens to trees so that they start new shoots after fires. When there are no fires, trees reduce the underground part of their biomass, thereby slowing down the carbon dioxide sequestration in the soil.
The final conclusion of the authors of the new study is simple: the possibilities of planting trees in the fight against global warming are exaggerated. Here it is worth adding one more point that is not touched upon in the work.
The reflectivity of typical woody vegetation is normally somewhat lower than that of grass. Therefore, the increase in the number of trees in unburned savannahs means an increase in the thermal budget of the planet due to the absorption of more sunlight.
The tropics themselves heat up slightly from this: additional energy enhances the convective rise of air from the surface upwards, and such a rise is the main mechanism for the formation of precipitation in this zone.
However, the “heating” of the overgrown savanna with trees leads to the movement of warm air to the poles, raising the temperature in temperate and high latitudes. All this further calls into question the idea that overgrowing the planet with trees can somehow slow down global warming.
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