Task Force probes ‘whys’ behind increased tree mortality

Several trees of different species died after the strong hot and dry period during the 2015-16 El Niño drought in Central Amazon (photo). Now scientists are trying to understand the impacts of this drought event, and subsequent tree mortality, on the carbon stocks in the Amazon basin. Photo by Adriane Esquivel Muelbert

Tree mortality appears to be increasing at unprecedented rates.

One may be tempted to think: So what? Trees regenerate. They’ll grow back.

But, for a lot of reasons, it’s not quite that simple.

“Forests are incredibly complex ecosystems that have taken centuries – even millennia – to establish,” says Dr. Henrik Hartmann, of the Max-Planck Institute for Biogeochemistry in Jena, Germany. He and four deputies, Adriane Esquivel Muelbert, Aster Gebrekirstos, Nadine Ruehr and Bernhard Schuldt, are coordinating IUFRO’s Monitoring Global Tree Mortality Patterns and Trends Task Force.

“Increased tree mortality means that forests can hold less carbon for a shorter period of time. Therefore, there will be more carbon in the atmosphere with known feedback dynamics on climate warming.

“The accelerating tree mortality rates may indicate a climate change risk for forest survival. And forests are important for the Earth system and to human welfare.

“We need to know how forests will cope with the changes we are imposing on the Earth system.” (The term “Earth system” refers to Earth’s interacting physical, chemical, and biological processes.) That,
Dr. Hartmann says, is one of the main drivers behind his Task Force (TF).

“Trees and forests that are hundreds of years old cannot be replaced as quickly as they disappear and the migration of trees into regions that become suitable for establishment is much slower than the rate of disappearance,” he says.

“These ecosystems,” says Dr. Adriane Muelbert, lecturer in Global Forest Ecology at the School of Geography, Earth and Environmental Sciences of the University of Birmingham, England, “once they are gone, or when they’re permanently disturbed, may not be able to recover, or they may recover in a very different form.

“In some cases, losing a forest may change the local conditions that allowed the forest to establish there in the first place and the forest will be unable to grow back.”

As an example, Dr. Bernhard Schuldt points to the island of Haiti/Dominican Republic. There, it was human actions that made forests disappear. “Deforestation caused intense soil erosion that still hinders forest re-establishment. There is just not enough soil to have trees regenerate and grow to maturity.

“Examples of climate-driven forest disappearance are not as well documented, as this is a rather recent scientific branch. There are indications of climate-induced desertification in the southwestern U.S., but data are still sparse,” says Dr. Schuldt, who is with the Julius-von-Sachs-Institute of Biological Sciences, Ecophysiology and Vegetation Ecology at the University of Würzburg, Germany.

Dr. Hartmann notes “this TF is tackling a timely and urgent topic – potential changes in trends of global tree mortality. Our mission is to provide an empirical basis on global tree mortality for sustainable policy making.

“This is important because currently we have no integrated large-scale assessment on the general state of our forests, despite many independent studies indicating that tree mortality is increasing.

“The TF aims to provide the information needed for today’s policy makers and forest managers to ensure forest persistence for generations to come,” Dr. Hartmann says.

Mortality of Scots pine (Pinus sylvestris) near the city of Jena, Germany. Large tracks of pine stands have died following the 2018 drought in Central Europe despite the species’ high drought tolerance. Photo by Henrik Hartmann

Recognizing the many challenges ahead for the TF, Dr. Nadine Ruehr of the Institute of Meteorology and Climate Research at Germany’s Karlsruhe Institute of Technology in Garmisch-Partenkirchen says: “First and foremost is to be able to reach all those who can contribute to our initiative. Engaging with a global community is not straightforward. The real challenge is to be truly inclusive, going beyond the academic circle and across languages and cultures.

“Then there are huge technical and scientific challenges to harmonize different data sets that have been collected based on a variety of protocols.”

One other challenge, noted by Dr. Aster Gebrekirstos of the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, is a lack of data from some areas. She says: “There are regions, such as Siberia and the Congo Basin, that are very understudied and where we have virtually no understanding of forest dynamics. By integrating enough data from the field and satellite observations we can understand how these forests are changing and potentially bring attention to the community to steer more research efforts into those areas.”

“But, on the other hand,” she says, “our goals would have been much more difficult to achieve a few decades ago. Living in an era where we can easily engage with the international community online, access satellite data and have the necessary tools to process and store large amounts of data, is definitely a fundamental factor to achieve our goals.”

Dr. Hartmann adds: “This TF also reflects a truly global effort. We include people from many different countries with different cultural and political backgrounds. Their contributions will provide the input we need to make our initiative successful in the different biomes of the world.”

He says that ultimately the entire global population will benefit from the persistence and continued existence of forests and that his TF envisions generating a “coherent picture of global tree mortality and the causes, which can be broadly used in research and policy making.

“Ideally, by the end of the TF’s five-year ‘lifespan’, we will have accessed and integrated most existing data sources on forest conditions,” Dr. Hartmann says, “and can assess whether and where forest health is threatened at the global, regional and local scale. That can then allow policy makers and forest managers to make decisions that can alleviate those threats.”

IUFRO Task Force Monitoring Global Tree Mortality Patterns and Trends:
https://www.iufro.org/science/task-forces/tree-mortality-patterns/

The IUFRO Task Forces are established on a temporary basis during each 5-year IUFRO Board term and focus on emerging key forest-related issues. The nine current TFs will run till 2024 at which time their relevance will be assessed in relation to the forest issues of the day.