How hot is too hot and how dry is too dry for the world’s forests? A new study by an international team of researchers has found the answers – by looking at trees that have been dying for decades.
Just appeared in the magazine nature communication, The study creates the first global database of precisely georeferenced forest diebacks at 675 locations dating back to 1970. The study, which includes all forested continents, then compares this information to existing climate data to determine the climatic conditions of heat and drought that caused these documented episodes of tree mortality.
“In this study, we’re letting the world’s forests do the talking,” said William Hammond, a University of Florida plant ecophysiologist who led the study. “We collected data from previous studies documenting where and when trees died, and then analyzed how the climate was during the mortality events compared to long-term conditions.”
After performing climate analysis on observed forest mortality data, Hammond noted a pattern emerged.
“What we found was that on a global scale, there is this consistently hotter, drier pattern — what we call a ‘hot drought fingerprint’ — that can tell us how unusually hot or dry it needs to get for forests to be at risk are deadly,” said Hammond, an assistant professor in the UF/IFAS Department of Agronomy.
The fingerprint, he says, shows that forest mortality events occurred whenever the normally hottest and driest months of the year became even warmer and drier.
“Our fingerprint of hotter droughts has shown that global forest mortality is associated with increased climate extremes,” Hammond said. “Using climate model data, we estimated how common these previously deadly climate conditions would be with further warming compared to pre-industrial era climates — 22% more common at plus 2 degrees Celsius (plus 3.6 degrees Fahrenheit) to 140% more common at plus 4 degrees Celsius (plus 7.2 degrees Fahrenheit).”
Those higher temperatures would more than double how often forests around the world experience tree-killing droughts, he adds.
“Plants do a phenomenal job of capturing and sequestering carbon,” Hammond said. “But not only does plant death prevent them from fulfilling this critical role as carbon scavengers, plants also begin to release carbon as they decay.”
Hammond says understanding how hot is “too hot” and how dry is “too dry” is crucial when partially relying on trees and other plants to capture and sequester carbon, as some proposed climate solutions suggest. “Otherwise, mortality events such as those contained in our database may nullify projected carbon gains.”
One of the study’s co-authors, Cuauhtémoc Sáenz-Romero of the Universidad Michoacana de San Nicolás de Hidalgo in Mexico, provided an example of how recent climate patterns were affecting a Mexican temperate forest.
“In recent years, the dry and warm season from March to May is even drier than usual, but also warmer than ever,” he said. “This combination causes a lot of stress on the trees ahead of the arrival of the next rainy season from June to October. In 2021, more than 8,000 mature trees were killed by bark beetles in the Monarch Butterfly Biosphere Reserve in central Mexico. The action of the La Niña Pacific Current resulted in drier and warmer conditions. a deadly combination that fueled pest outbreaks.”
Hammond has also developed an interactive application on the International Tree Mortality Network website to host the database online and allow others to submit additional observations of forest mortality to the database.