Tree-cover expansion on former cropland increases connectivity but homogenizes European landscapes

For decades, the idea of planting more trees has been a cornerstone of efforts to fight climate change and protect nature. The logic seems straightforward: trees soak up carbon dioxide, provide homes for wildlife, and cool the planet. In many parts of the world, expanding forests is seen as an unalloyed good, a natural solution to human-made problems. However, ecologists have long recognized that the story is more complicated. Trees are not just passive green backdrops; they are active engineers of the landscape. When trees grow too thickly and too close together, they can shade out the sun-loving plants that thrive in open fields and meadows. They can also change the way fire behaves, potentially making wildfires more intense and destructive. The key to a healthy, diverse ecosystem often lies not in a single type of habitat, but in a mix of open spaces, shrublands, and forests. If the landscape becomes too uniform, dominated by dense, closed-canopy forests, it may lose the variety of homes needed to support the full range of European wildlife.

A new study by researchers at Aarhus University and other institutions across Europe challenges the assumption that more trees always mean better nature. By analyzing satellite data from 2001 to 2021, the team mapped how tree cover, the connectedness of tree patches, and the structural variety of forests have changed across the continent. They found that while tree cover has indeed increased across more than half of Europe, this expansion is often creating a landscape that is denser and more uniform. Specifically, 23 percent of the European land area saw a simultaneous increase in tree cover and the connectivity between tree patches, but a decrease in the structural variety of the vegetation. This means that small, scattered patches of trees are merging into larger, denser blocks of forest, while the complex mix of open and wooded areas that many species rely on is disappearing. The researchers argue that this trend, driven largely by the abandonment of farmland and changes in forest management, could ultimately weaken Europe's ability to support its diverse array of plants and animals.

The study focused on three specific ways to measure the health of the tree landscape. First, they looked at tree cover, which is simply the percentage of the ground covered by tree canopies. Second, they measured connectivity, which describes how well different patches of trees are linked together, allowing animals to move between them. Third, and perhaps most importantly for biodiversity, they assessed heterogeneity, or the structural variety within the vegetation. A highly heterogeneous landscape might have tall trees, short shrubs, open grassy gaps, and varying heights of canopy all in the same area. This variety creates many different "niches" or living spaces for different species. The researchers used high-resolution satellite images to track these three factors across a grid of one-kilometer squares covering the entire continent. They also looked at what was driving these changes, examining factors like climate, natural disturbances like wind and fire, and human activities such as farming and forest management.

The results revealed a clear and widespread pattern. Tree cover increased across 58.3 percent of the European land area, with the most significant growth occurring in southern and central Europe. In many of these areas, the trees were not just growing more densely; they were also becoming more connected, merging into larger, continuous forests. This trend was particularly strong in former agricultural lands. As farmers left rural areas or stopped managing their fields, nature began to reclaim the space. Trees grew back spontaneously, turning open croplands into forests. While this natural recovery is often celebrated, the study found that it frequently came at a cost. In 23 percent of the land area, this expansion led to a situation where trees became denser and more connected, but the structural variety of the landscape declined. The researchers identified this specific pattern—more trees, better connections, but less variety—as the most common outcome across Europe.

The drivers behind these changes are a mix of human decisions and natural processes. The study found that the abandonment of farmland was a major factor. In regions like the former Eastern Bloc, where political and economic shifts led to widespread farm abandonment, trees have rapidly taken over. Similarly, in parts of southern Europe, traditional grazing practices that kept landscapes open have been discontinued, allowing vegetation to grow unchecked. Climate also played a role, with warmer temperatures generally favoring tree growth. However, the study also highlighted that natural disturbances, such as windstorms, can have a negative impact on tree cover, particularly in areas where forests are managed intensively. Interestingly, the researchers found that tree cover tends to increase in areas that are harder for humans to reach, while the structural variety of the vegetation often declines as forests become more accessible to cities and human activity.

The implications of these findings are significant for the future of European biodiversity. The researchers point out that most European wildlife is not adapted to live in dense, closed-canopy forests. Instead, many species, including a vast number of birds, insects, and plants, have evolved to thrive in open or semi-open habitats like grasslands, shrublands, and mosaic woodlands. When these open spaces are swallowed by expanding forests, the habitats for these species disappear. The study suggests that the current trend of spontaneous vegetation succession, where nature is left to grow back without intervention, is leading to a homogenization of the landscape. This means that the rich tapestry of different habitats is being replaced by a more uniform, dense forest cover. While this might benefit species that prefer deep woods, it risks diminishing the capacity of European ecosystems to support the broader diversity of life that depends on open spaces.

To address this, the authors suggest that simply letting nature take its course may not be enough to preserve biodiversity. They argue that restoring natural disturbance processes is essential. In the past, large herbivores and natural fires played a crucial role in keeping landscapes open and diverse, preventing the formation of continuous, dense forests. The researchers propose that reintroducing these processes, such as through the return of large grazing animals or the careful management of fire, could help maintain the structural complexity of the landscape. By doing so, it may be possible to harness the benefits of nature recovery—such as carbon storage and habitat creation—without sacrificing the open habitats that are vital for so many species. The study concludes that without such interventions, Europe may miss a crucial opportunity to build ecosystems that are not only resilient to climate change but also rich in the biodiversity that has defined the continent for millennia.