Global tree mortality is getting higher. But we don’t know by how much

Climate change increases tree mortality rates, but figures are incomplete and a global estimate is hard to make. A group of researchers explains how to harmonize data

by Matteo Cavallito

 

Tree mortality rates around the world are increasing, mainly due to climate change and its effects, such as heat waves, droughts, fires, storms, and the spread of plant diseases. These phenomena have a significant impact as they reduce the ability of forests to absorb CO2. That’s why understanding tree mortality is also crucial for climate mitigation management.

However, current monitoring is fragmented and inconsistent at the global level. For this reason, there is a need to systematically integrate data from long-term field surveys with remote sensing to obtain reliable results. This is suggested by a group of scientists who have been working this issue so far.

Incomplete monitoring

“We don’t currently know whether climate change will lead to the death of 10% or 50% of all trees worldwide,” explained Samuli Junttila, professor at the University of Helsinki, in a statement. Junttila is the director of a working group, the Global Ecosystem Health Observatory, which involves over 100 researchers from around the world engaged in monitoring over half a million research projects from 89 countries across all five continents.

Their goal is to generate accurate data on the condition of trees using advanced computer vision methods based on satellite and aerial imagery.

“We need monitoring periods of at least five years to determine whether a particular tree is dead or alive,” said the professor. “In addition to this basic knowledge, it’s essential to understand the characteristics of local habitats and possess data on the age, size and species of trees.” Only then is it possible to begin to hypothesize about overall mortality trends. “The good news is that modern technology already allows much to be done.”

The forests of Asia and Africa are underrepresented

In recent weeks, in particular, the professor drew attention to an article published this year in the journal New Phytologist, which reviews methods, requirements, and gaps in data related to monitoring the phenomenon. Some systems, such as laser scanning already in use in Finland, for example, are making tree monitoring increasingly comprehensive. However, field measurements remain essential. The survey collected information from 316 monitoring programs covering over 40,500 hectares of forest (0.001% of the world total).

Over the years, the authors explain, the area observed has grown steadily but very unevenly, and some regions, such as Africa and Central Asia, remain underrepresented.

The biggest gaps in terrestrial monitoring are found in low-income countries, where resources and infrastructure are lacking and where logistical difficulties and poor security conditions make it difficult to collect data on a continuous basis. Globally, most programs record the status of trees (alive/dead), but only a minority of them also take annual measurements. In 86% of cases, surveys are conducted at least once every 10 years. In two out of three cases, once every five years. Finally, methodological differences and long sampling times limit the ability to identify changes in mortality rates in a timely manner.

Five steps to understanding global trends

To address these issues, an effective global system will therefore need to harmonize heterogeneous data. This is no easy task and, according to the authors, requires at least five steps

1. Promote equitable practices across the community that empower those collecting the data.
2. Invest in ground-based data collection, sustaining long-term efforts and expanding to data-poor regions.
3. Adjust protocols to facilitate comparability and improve quantification of rates and causes of tree mortality.
4. Generate standardised tree mortality metrics from ground-based data that can be widely used by the scientific community and facilitate comparability across studies.
5. Integrate ground-based data with remote sensing data and process-based models to expand current observations temporally and spatially and understand their underlying drivers.

This should lead to achieving a global understanding of tree mortality, based on certain minimum requirements. “Following this roadmap will allow us to create interoperable datasets on tree mortality globally through fair collaboration and ultimately lead to robust predictions of tree mortality trends,” the researchers conclude.