Forests across the world are undergoing a subtle but profound transformation, one that scientists warn could permanently weaken their ability to support life on Earth. A large international study has found that many forests are losing their most ecologically valuable trees and becoming increasingly dominated by fast-growing, generalist species. While these changes may appear gradual, researchers say the long-term consequences for biodiversity, climate regulation and ecosystem stability could be severe.
Trees play a foundational role in sustaining life. They absorb and store carbon dioxide, stabilize soils, regulate water cycles and provide habitat for countless species of animals, fungi and insects. Forests also support human societies by supplying timber, food, shade and recreational spaces. But mounting pressures from climate change, deforestation and land-use change are now reshaping forest composition on a global scale.
The findings come from a major study published in Nature Plants, which analysed data from more than 31,000 tree species worldwide. By examining changes in species traits, growth strategies and geographic distribution, researchers were able to assess how forests are likely to evolve under increasing environmental stress.
The study reveals a clear trend: forests are becoming more uniform. Fast-growing tree species that thrive in disturbed environments are expanding, while slower-growing, highly specialized native species are declining. This shift reduces overall biodiversity and makes forests less resilient to droughts, pests and extreme weather events.
According to Jens-Christian Svenning, professor at Aarhus University and one of the study’s lead authors, this transformation threatens the very structure of forest ecosystems. He explains that many of the trees now disappearing play unique roles that cannot easily be replaced by faster-growing species.
The trees most at risk are what scientists call “slow specialists.” These species typically have dense wood, thick leaves and long lifespans. They grow slowly but are exceptionally good at storing carbon and stabilizing ecosystems over long periods. Such trees are especially common in tropical and subtropical forests, where biodiversity is highest and ecological relationships are tightly interconnected.
“These species form the backbone of forest ecosystems,” Svenning says. “They contribute to long-term stability, resilience and carbon storage. When they disappear, forests lose much of their functional strength.”
In contrast, fast-growing trees sometimes described as nature’s “sprinters” grow quickly but invest less in structural strength. Species such as eucalyptus, acacia, poplar and pine can dominate disturbed landscapes, but they are more vulnerable to drought, storms and disease. Over time, forests dominated by these species become less reliable carbon sinks and more prone to collapse under environmental stress.
The study also highlights the growing influence of naturalized tree species those introduced by humans that now grow wild outside their native range. Nearly 41 percent of these species share traits such as rapid growth and small leaves, making them well suited to degraded or frequently disturbed environments.
While these trees can spread quickly, researchers caution that they rarely fulfill the same ecological roles as native species. Instead, they often intensify competition for light, water and nutrients, making it even harder for native trees to recover. Over time, this accelerates forest homogenization and further erodes ecosystem diversity.
Tropical forests face the greatest losses
The impacts of these changes are expected to be most severe in tropical and subtropical regions. According to the study’s first author, Wen-Yong Guo of East China Normal University, many vulnerable tree species already have extremely limited geographic ranges. If their habitats are cleared or overtaken by fast-growing competitors, they risk disappearing entirely.
At the same time, colder regions in the Northern Hemisphere are likely to see increased invasion by fast-growing, disturbance-tolerant species. This means that while the mechanisms may differ, forest homogenization is expected to intensify almost everywhere.
Human activity is the primary driver behind these shifts. Climate change, large-scale logging, infrastructure development, intensive forestry and global trade in tree species all contribute to reshaping forest composition. Fast-growing trees are often favored for commercial reasons, but scientists warn that ecological health is being sacrificed for short-term gains.
Researchers stress that the trend is not inevitable, but reversing it will require deliberate action. The study calls for forest management strategies that prioritize slow-growing and rare tree species, particularly in restoration and reforestation projects.
“When new forests are established, we should focus far more on ecological quality, not just speed of growth,” Svenning says. Promoting diverse native species can strengthen forests against future climate shocks and support richer animal communities that are also essential for ecosystem functioning.
As forests continue to change quietly across the globe, scientists warn that losing these foundational tree species could undermine decades of conservation and climate efforts. Protecting the slow, resilient trees that hold ecosystems together may be one of the most urgent and overlooked tasks in safeguarding the planet’s future.