Hotter-Drought Fingerprint – Can Earth’s Forests Survive Global Warming?

Taken in 1993, this photo shows the mortality of historic Atlas Cedar forests in Morocco. Credit: Csaba Mátyás, Emeritus Professor, University of Sopron, Hungary

A global team of scientists determines the ‘footprint’ of how much heat the drought is too much for the forests.

The authors conclude that limiting the Earth’s warming will determine the survivability of many of Earth’s forests.

How hot is too hot, and how dry is too dry, for Earth’s forests? A new study from an international team of researchers has found the answers – by examining decades of dying trees.

Just published in the journal Nature Communication, the study compiles the world’s first database of precisely georeferenced forest mortality events, at 675 locations dating back to 1970. The study, which encompasses all forested continents, then compares this information to existing climate data to determine conditions climatic conditions of heat and drought that caused these documented episodes of tree mortality.

“In this study, we let the Earth’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, then analyzed the climate during the mortality events, compared to long-term conditions.”

After performing the climate analysis on the observed forest mortality data, Hammond noted, a pattern emerged.

“What we found was that globally there was this consistently hotter and drier pattern – what we call a ‘hotter drought footprint’ – which can show us how it has to be unusually hot or dry for the forests to be at risk of death,” said Hammond, an assistant professor in the UF/IFAS Department of Agronomy.

The fingerprint, he says, shows that forest mortality events occurred consistently as the typically hottest and driest months of the year became even hotter and drier.

“Our warmer drought footprint revealed that global forest mortality is linked to intensified climate extremes,” Hammond said. “Using climate model data, we estimated the frequency of these previously deadly climatic conditions under additional warming, relative to pre-industrial climate – 22% more frequent at plus 2 degrees.” Celsius (plus 3.6 degrees Fahrenheit), 140% more frequently at plus 4 degrees Celsius (plus 7.2 degrees Fahrenheit). »

These higher temperatures would more than double the frequency with which forests around the world experience tree-killing droughts, he adds.

“Plants do a phenomenal job of capturing and sequestering carbon,” Hammond said. “But the death of plants not only prevents them from fulfilling this essential role of carbon sequestration, but the plants also begin to release carbon as they decompose.”

Hammond says that relying, in part, on trees and other plants to capture and sequester carbon, as some proposed climate solutions suggest, makes it essential to understand how hot is “too hot” and how dry is “too dry”. “Otherwise, mortality events, like those included in our database, could wipe out predicted carbon gains.”

One of the study’s co-authors, Cuauhtémoc Sáenz-Romero of the Michoacana University of San Nicolás de Hidalgo in Mexico, gave an example of how recent climate patterns have affected a Mexican temperate forest.

“In recent years, the hot dry season from March to May is even drier than usual, but also hotter than ever,” he said. “This combination puts a lot of stress on the trees before the arrival of the next rainy season from June to October. For example, in 2021, more than 8,000 mature trees were killed by bark beetles in the Monarch Butterfly Biosphere Reserve in central Mexico. The effect of the La Niña Current from the Pacific Ocean resulted in drier and warmer conditions; a deadly combination that has favored pest outbreaks.

Hammond has also developed an interactive application on the website of the International Tree Mortality Network to host the database online and allow others to submit additional observations of forest mortality to the database.

The organization, founded and coordinated by co-author Henrik Hartmann of the Max Planck Institute in Germany, among others, is a collaborative effort between scientists from all forest continents and aims to coordinate international research efforts on forest dieback events. Hammond is the leader of the network’s data management group.



Using maps or aerial images, scientists assign them real coordinates.

“verified in the field”

Information confirmed or validated by direct observation and measurement. In the case of machine learning, it involves verifying the results to precision.

“We hope this paper will create some urgency around the need to understand the role of warming on forest mortality,” Hammond said. “Furthermore, we hope that our open-access database will enable additional studies, including other climate footprints at local and regional scales. Today’s climate modeling and remote sensing research communities need sets of field-verified data to validate their predictions of important processes like forest mortality. One of the really important parts of this study was bringing all of this data together for the first time, so that we could ask a question like this- here on a planetary scale.

Reference: “Global field observations of tree mortality reveal warmer drought footprint for Earth’s forests” April 5, 2022, Nature Communication.
DOI: 10.1038/s41467-022-29289-2

In addition to Hammond, Sáenz-Romero, and Hartmann, it is also co-authored by A. Park Williams, University of California, Los Angeles; John Abatzoglou, University of California, Merced; Henry D. Adams, Washington State University; Tamir Klein, Weizmann Institute of Science; Rosana López, Universidad Politécnica de Madrid, Spain; David D. Breshears, University of Arizona; and Craig D. Allen, University of New Mexico.

Teresa H. Sadler