Heat, Drought Causing Forest Die-Offs Globally

New research indi­cates that ris­ing global tem­per­a­tures and pro­longed peri­ods of drought are caus­ing for­est die-off events glob­ally.

A team of inter­na­tional sci­en­tists has exam­ined for­est die-offs in 675 loca­tions since 1970.

By ana­lyz­ing these episodes and com­par­ing them with exist­ing cli­mate data, the sci­en­tists were able to iden­tify the local tem­per­a­ture and drought con­di­tions that deter­mined the tree mor­tal­ity events. A global geo­ref­er­enced data­base was then cre­ated.

In the study pub­lished in Nature Communications, the researchers said there is a lack of data about how for­est ecosys­tems will sur­vive ris­ing aver­age tem­per­a­tures asso­ci­ated with anthro­pogenic cli­mate change.

“Lacking is the quan­ti­ta­tive deter­mi­na­tion of com­mon­al­ity in cli­mate anom­alies asso­ci­ated with pulses of tree mor­tal­ity… required for under­stand­ing the role of extreme cli­mate events in over­all global tree die-off pat­terns,” the sci­en­tists wrote.

Widespread tree die-offs may prove to be excep­tion­ally dam­ag­ing to global efforts to mit­i­gate the impacts of cli­mate change.

The lat­est State of the World’s Forests report by the United Nations Food and Agriculture Organization and the U.N. Environment Program said these unique ecosys­tems cover only 30 per­cent of the global land area but are home to the vast major­ity of the ter­res­trial plants and ani­mal species.

Forests play a vital role in sta­bi­liz­ing the cli­mate by reg­u­lat­ing ecosys­tems and pro­tect­ing bio­di­ver­sity. Apart from sup­port­ing the liveli­hood of large pop­u­la­tions, the researchers empha­sized how forests are com­posed of more than 60,000 tree species, store nearly half of the ter­res­trial car­bon and sequester up to a third of annual anthro­pogenic car­bon emis­sions.

According to the International Olive Council, ​“a hectare of olive trees can­cels out one person’s annual car­bon foot­print.”

“Our analy­sis quan­ti­fies a global ​‘hot­ter-drought fin­ger­print’ from these tree-mor­tal­ity sites, effec­tively a hot­ter and drier cli­mate sig­nal for tree mor­tal­ity, across 675 loca­tions encom­pass­ing 1,303 plots,” the researchers wrote.

“The fre­quency of these observed mor­tal­ity-year cli­mate con­di­tions strongly increases non­lin­early under pro­jected warm­ing,” they added. ​“Our data­base also pro­vides the ini­tial foot­ing for fur­ther com­mu­nity-devel­oped, quan­ti­ta­tive, ground-based mon­i­tor­ing of global tree mor­tal­ity.”

“What we found was that at the global scale, there was this con­sis­tently hot­ter, drier pat­tern… that can show us how unusu­ally hot or dry it has to get for forests to be at risk of death,” William Hammond, co-author and plant eco­phys­i­ol­o­gist at the University of Florida, added in a note.

The sci­en­tists empha­sized how ris­ing tem­per­a­tures threaten forests in sev­eral ways, includ­ing ampli­fy­ing atmos­pheric drought, inten­si­fy­ing soil drought and caus­ing heat stress to the plants. As the tem­per­a­ture rises, soils and trees lose water more quickly.

A warmer envi­ron­ment can also trig­ger lethal events in unex­pected ways, accord­ing to Cuauhtémoc Sáenz-Romero, co-author and researcher at the Universidad Michoacana de San Nicolás de Hidalgo in Mexico.

“In recent years, the dry and warm March to May sea­son is even more dry than usual, but also warmer than ever,” he said. ​“This com­bi­na­tion is induc­ing a lot of stress on the trees before the arrival of the next June-to-October rainy sea­son.”

“For exam­ple, in 2021, more than 8,000 mature trees were killed by bark bee­tles in the Monarch Butterfly Biosphere Reserve in cen­tral Mexico,” Sáenz-Romero added. ​“The effect of the La Niña Pacific Ocean stream resulted in drier, warmer con­di­tions; a deadly com­bi­na­tion that favored pest out­breaks.”

According to the researchers, the study shows a gen­eral accel­er­a­tion of destruc­tive cli­mate con­di­tions des­tined to worsen over time as tem­per­a­tures con­tinue to rise.

“Earth’s his­tor­i­cal forests in par­tic­u­lar face a chal­leng­ing future, includ­ing dra­matic changes in the extent, com­po­si­tion, age and struc­ture of these unique and irre­place­able forests, with plan­e­tary-scale con­se­quences for bio­di­ver­sity and the cycling of water and car­bon,” the authors wrote.

In their con­clu­sions, sci­en­tists empha­sized how forests are essen­tial in efforts to mit­i­gate the impacts of the cli­mate cri­sis. However, their abil­ity to stand as car­bon sinks is strictly related to their sur­vival abil­ity.

“Plants do a phe­nom­e­nal job cap­tur­ing and seques­ter­ing car­bon,” Hammond said. ​“But the death of the plants not only pre­vents their per­form­ing this crit­i­cal car­bon-cap­tur­ing role, plants also start releas­ing car­bon as they decay.”

“Our find­ings show that lim­it­ing warm­ing to 2 ºC above pre-indus­trial lev­els could reduce the fre­quency of these cli­mate con­di­tions asso­ci­ated with observed tree mor­tal­ity events to less than half that pre­dicted at 4 °C,” the researchers wrote.

“Efforts to pro­tect the world’s cli­mate from exces­sive warm­ing likely will be deci­sive in deter­min­ing the future per­sis­tence of many of Earth’s forests,” they added.

The data­base com­posed by the researchers is now avail­able on the International Tree Mortality Network web­site, where col­lab­o­rat­ing sci­en­tists can also add data to pro­vide a larger pic­ture and a broader under­stand­ing of for­est die-off events.

• Nature Communications
• United Nations Food and Agriculture Organization