Trees Less Effective at Sequestering Carbon in a Hotter, Drier World, Study Finds

According to a study pub­lished in the Proceedings of the National Academy of Sciences (PNAS), the high con­cen­tra­tion of car­bon diox­ide in the Earth’s atmos­phere is chok­ing the planet’s trees in warm regions, caus­ing them to reverse their nor­mal func­tion­ing.

“We found that trees in warmer, drier cli­mates are essen­tially cough­ing instead of breath­ing,” said Max Lloyd, an assis­tant pro­fes­sor of geo­sciences at Penn State University in the United States and the study’s lead author. ​“They are send­ing CO2 right back into the atmos­phere far more than trees in cooler, wet­ter con­di­tions.”

During pho­to­syn­the­sis, trees use sun­light, water and car­bon diox­ide from the atmos­phere to build their energy fuels. However, when stressed under high tem­per­a­tures or due to lim­ited water sup­ply, they release the car­bon diox­ide back into the atmos­phere in a process called pho­tores­pi­ra­tion.

The researchers ana­lyzed a global dataset of tree tis­sue sam­ples to assess the amount of car­bon diox­ide trees release when pho­torespir­ing.

They found that pho­tores­pi­ra­tion can be accel­er­ated by up to 100 per­cent in warmer cli­mates, espe­cially when water is scarce. In sub­trop­i­cal cli­mates, includ­ing large swathes of the Mediterranean coun­tries, trees enter pho­tores­pi­ra­tion when aver­age daily tem­per­a­tures exceed approx­i­mately 20 °C.

The find­ing could mean trees in warmer cli­mate zones can no longer oper­ate as car­bon sinks to off­set humanity’s car­bon emis­sions.

By stor­ing and releas­ing car­bon, trees play a cru­cial role in main­tain­ing the ​‘car­bon cycle,’ the con­stant move­ment of car­bon from the land and water through the atmos­phere and liv­ing organ­isms, a process fun­da­men­tal to all life forms on the planet.

Human activ­i­ties, such as mod­i­fy­ing land use and human-induced car­bon emis­sions from burn­ing coal and gas, can dis­rupt the Earth’s car­bon cycle.

The United States Department of Energy has cal­cu­lated that the world’s trees and other plants can absorb around 25 per­cent of the car­bon diox­ide emit­ted in the atmos­phere from human activ­i­ties. However, sci­en­tists expect trees’ abil­ity to sequester CO2 to decrease as the planet con­tin­ues to warm.

“When we think about cli­mate futures, we pre­dict that the CO2 will go up, which in the­ory is good for plants because those are the mol­e­cules they breathe in,” Lloyd said. ​“But we’ve shown there will be a trade­off that some pre­vail­ing mod­els don’t account for. The world will get warmer, which means plants will be less able to draw down that CO2.”

Meanwhile, the sci­en­tist who had urged the world to plant a tril­lion trees to trap human-induced car­bon emis­sions has back­tracked on his rec­om­men­da­tion, argu­ing that mass plan­ta­tions are not the solu­tion they pur­ported to be.

In a 2019 study, Thomas Crowther, a pro­fes­sor of ecol­ogy at ETH Zurich University in Switzerland, sug­gested that 1.2 tril­lion trees could be planted on Earth to absorb as much as two-thirds of human car­bon emis­sions.

Although his study was crit­i­cized by other sci­en­tists who argued that it had over­es­ti­mated the land avail­able for for­est restora­tion, it sparked a race among world lead­ers and orga­ni­za­tions to plant trees, largely over­look­ing their com­mit­ment to cut­ting emis­sions.

At the COP28 cli­mate sum­mit in December, Crowther urged the world’s coun­tries to stop ​‘green­wash­ing’ – the prac­tice of mak­ing mis­lead­ing or false claims about the envi­ron­men­tal ben­e­fits of a prod­uct or ser­vice which his own study had pro­moted.

“Killing green­wash­ing doesn’t mean stop invest­ing in nature,” he said. ​“It means doing it right. It means dis­trib­ut­ing wealth to the indige­nous pop­u­la­tions, farm­ers and com­mu­ni­ties who are liv­ing with bio­di­ver­sity.”

In a recent paper, Crowther wrote that pre­serv­ing exist­ing forests can have a more sig­nif­i­cant impact on reduc­ing car­bon emis­sions than plant­ing new trees to be used as car­bon sequesters.

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• Wired