Climate change, deforestation and habitat loss are quietly reshaping the face of global forests, making originally diverse and complex forests increasingly homogeneous, dominated by a group of fast-growing "generalist" tree species, while those slow-growing "specialist" tree species with critical ecological functions are accelerating their demise. A large-scale international study recently published in Nature Plants warns that this "homogenization" trend not only means a decline in biodiversity, but will also weaken the ability of forests to withstand extreme events and weaken their long-term carbon sequestration function.
The research team gathered data from more than 31,000 tree species around the world to construct a functional characteristic picture showing the direction of future forest change, focusing on analyzing how tree species composition evolves, how ecosystem stability changes, and whether forests can continue to undertake key ecological functions such as carbon sequestration, maintaining water and soil, and providing habitat for biological communities. The results show that "fast tree species" with lightweight leaves, low wood density, and the ability to grow rapidly in a short period of time will become increasingly dominant, while "slow tree species" and ecological "specialists" with thick leaves, dense wood, long lifespan, and relatively stable adaptation to the environment will face a higher risk of decline or even extinction.
Jens-Christian Svenning, director of the Department of Biology and the Center for Ecological Dynamics of the New Biosphere (ECONOVO) at Aarhus University in Denmark and one of the corresponding authors of the study, pointed out that the most dangerous tree species are often those with extremely limited geographical distribution ranges. They are highly unique and have irreplaceable functions. Once they disappear, there will be a vacancy in the forest ecosystem that is difficult to fill. He emphasized that a large number of such tree species are concentrated in tropical and subtropical areas, where biodiversity is extremely high and species interact closely. Once the unique local "slow tree species" are squeezed out, the ecological roles they assume cannot easily be taken over by exotic, fast-growing tree species.
The study calls these "slow tree species" the "skeleton" of the forest. These trees, which tend to grow in moist tropical or subtropical forests, are slow-growing but long-lived trees with thick leaves and high-density wood that underpin the forest's long-term stability, carbon storage capacity and resilience to environmental disturbances. However, under the multiple pressures of continued climate warming, frequent extreme weather, and overexploitation by humans, forest structure is tending to be controlled by nature's "sprinters." Fast-growing tree species such as acacia, eucalyptus, poplar, and pine are becoming more and more common. Svenning pointed out that although these trees are easy to afforestation and have quick results, they are more fragile in the face of drought, storms, pests and diseases, and climate shocks, and are not conducive to the formation of a stable carbon pool in the long term.
The study also found that about 41% of the tree species that have been "naturalized" in non-native places have characteristics such as fast growth and small leaves, and are very adaptable to disturbed environments. However, compared with local native tree species, they are often unable to perform the same ecological functions. Svenning said the expansion of these naturalized tree species in increasingly fragmented, frequently disturbed landscapes often increases competition, making it harder for native trees to survive in environments with limited light, water and nutrients, further driving forest homogenization.
Guo Wenyong, the first author of the study and a young professor at the School of Ecology and Environmental Sciences of East China Normal University, pointed out that the impact of the "convergence" of forest structure is particularly severe in the tropics and subtropics. A large number of slow-growing tree species with narrow distribution ranges are concentrated in these areas. Once their habitats are destroyed or occupied by fast-growing tree species, they often have no "escape" and are more likely to disappear completely. At the same time, the model predicts that the number of naturalized tree species and fast tree species that have adapted to disturbed environments will continue to increase. In the cold regions of the northern hemisphere at high latitudes, the invasion of such tree species may even become one of the dominant driving forces of future forest succession.
The research team clearly pointed out that human activities are the primary factor driving changes in forest composition. Human-induced climate change, deforestation for infrastructure, intensive forestry production, commercial logging and the global trade in tree species have all combined to drive the expansion of fast-growing tree species. Guo Wenyong said that in many countries, fast-growing tree species are favored by policies or markets and are used to quickly produce wood or biomass energy. However, from an ecological perspective, they are often more fragile and susceptible to disease, which is not conducive to the long-term health of the forest.
In the study, scientists used a variety of scenarios to simulate the spread and disappearance trends of different tree species in the future. The results showed that naturalized tree species that have already established populations in exotic areas are likely to occupy a larger proportion of forests in the next few decades. In this context, how to prevent the continued loss of “slow tree species” has become particularly urgent. Svenning called for these slow-growing and rare tree species to be given higher priority in forest management and ecological restoration; when planning new afforestation projects, one should not just pursue "fast growth" and "quick results", but should proactively introduce more native tree species with diverse functions to improve the species diversity and overall resilience of the forest.
Researchers suggest that in the construction of protected areas and the restoration of degraded ecosystems, these slow-growing tree species should be consciously restored and cultivated, and combined with the recovery of large animal populations, because the latter is also crucial to processes such as seed dispersal and renewal and succession, and helps to rebuild a more complex and stable ecological network. The article concludes by emphasizing that the current decision-making window is not broad. If the forest structure continues to evolve toward a "rapid and single" evolution, humans will not only lose a large number of unique tree species, but may also weaken the key "buffer" role of forests in addressing the climate crisis.