Gaps in the canopy: More than 36 million forest areas in Europe no longer have a closed vegetation cover, but are broken through by open spaces. This is the result of a research team that has created the first high-resolution map of canopy openings with the help of over 30,000 satellite images. The causes of the decline in forests include interventions by forestry, but also events such as storms, forest fires or pest infestation.
In Europe, forests cover around 33 percent of the total land area. They play an important role for nature, but also for us humans. The forest ecosystem takes on vital tasks: the trees not only bind carbon dioxide from the atmosphere, but are also part of the water cycle and protect the soil from erosion. The problem: the increasing demand for wood, increased land use and climate change are severely affecting forests.
More and more bald spots in the forest
To get to the bottom of the question of how much forest occurrence in Europe has changed over the past three decades, Cornelius Senf from the Technical University of Munich (TUM) and his colleagues analyzed more than 30,000 satellite images from 1986 to 2016 . In doing so, they looked at the spatial variation, the frequency and the severity of the forest damage, which can be seen from the gaps in the canopy. The analysis enabled the researchers to create a high-resolution map of the canopy of European forests for the first time.
The new map shows that the gaps in the vegetation cover have increased sharply across Europe. Of the total of 210 million hectares of forest areas, over 36 million areas are broken through by open spaces or stands of very young trees. This corresponds to a loss of the canopy of 17 percent within the past 30 years, as the scientists report. However, the size of the forest holes is very different. For example, Sweden has the largest gaps in the canopy with an average of just under two hectares, while the forests in Switzerland have the smallest open spaces on average. At just 0.6 hectares, the gaps in the crown are smaller than a soccer field. In Germany the average area was 0.7 hectares, in Italy 0.75 hectares. The largest opening in the canopy documented by the researchers is in Spain, while the forests in Portugal have the highest number of gaps.
Combination of natural and anthropogenic causes
The newly created canopy map not only shows how much forest area has been lost: it also makes it possible to identify the causes of the changes. As the data show, the frequency of damage in over 70 percent of the forest stands has risen constantly over the years. In contrast, the severity of the respective destruction decreased in many places. “But even if individual gaps are still small, several of them can grow into larger open spaces over time due to further disturbances,” the researchers explain.
Overall, they see the reason for the increase in the canopy gaps in a combination of natural causes and increasing interventions by forest management. The latter has intensified in many countries because the demand for wood has increased steadily in recent years. Natural causes include storm damage, drought and forest fires – for example, a fire in Spain in 2012 burned 17,000 hectares of forest at once. In addition, pests such as bark beetles are spreading further and further and infect large areas of trees that have already been weakened by persistent heat waves.
Prepare for climate change
According to the scientists, the knowledge gained can also open up opportunities for future handling of Europe’s forest landscapes. Because the map shows “where forests run the risk of irreversible damage,” says Senf. This can help to take targeted protective measures against bark beetles, forest fires or storms in the future. The map can also help to adapt forests to climate change.
“The new maps help us to understand how Europe’s forests are changing. Because increasing openings in the canopy are a risk for the forest, but also offer the chance that a new generation of trees, better adapted to climate change, can establish itself, ”explains Seidl. “The maps can help, for example, to identify areas where regeneration needs to be promoted through targeted planting or where the forest can rejuvenate itself.” For recovery, usually only sufficient seed material is needed on the destroyed areas. “In this way, the forest can be made fit for climate change, a task that has become more urgent in the past two years.”
Source: Technical University of Munich; Technical article: Nature Sustainability, doi: 10.1038 / s41893-020-00609-y