A research team led by Dr. LI Chunlin from the Landscape Ecology Group of the Institute of Applied Ecology (IAE, Shenyang), Chinese Academy of Sciences, has made significant progress in revealing how global urban expansion affects the carbon sequestration capacity of vegetation. Their findings clarified the pathways and mechanisms of carbon storage changes at different stages and scales of urban development.

Urban expansion has profoundly reshaped the structure and function of terrestrial ecosystems. It not only alters land cover but also reshapes biogeochemical cycles, fragments landscapes, and modifies local climates. Strengthening the net ecosystem productivity (NEP) of vegetation has become a shared global goal to increase carbon sinks and mitigate climate warming. For the past two decades, however, limited observational data and modeling techniques have left uncertainties in understanding how NEP responds to urbanization on a global scale. Dr. LI’s team addressed this challenge by systematically investigating the spatial and temporal impacts of urban expansion on vegetation’s carbon sequestration capacity between 2000 and 2020.

The researchers found that global urban areas expanded from 515,000 square kilometers to 736,100 square kilometers over the period, with an average annual growth rate of 1.8 percent. Asia contributed most to this increase, accounting for 53.84 percent of the global total, while China alone contributed 36.41 percent. During the same period, vegetation NEP rose overall, with an annual increase of about 1.05 gC m⁻². Europe recorded the fastest NEP growth, followed by Asia and North America, while South America, Africa, and Oceania experienced slower gains. Major tropical rainforest regions, such as the Amazon, Congo, and Southeast Asia, remained the world’s most important carbon sinks.

By analyzing data from 2,904 cities, the team uncovered how expansion alters NEP spatial heterogeneity. Urban areas in developed countries generally experienced NEP increases, partly due to ecological restoration and greening policies, while many cities in developing countries, especially newly built areas and suburbs, faced declines. The extent of urban influence also varied with city size: megacities affected vegetation NEP up to 70–80 kilometers beyond their boundaries, compared with 30–40 kilometers for small and medium-sized cities. Globally, expansion resulted in 320,400 square kilometers of negatively impacted areas, compared with 86,700 square kilometers where NEP improved.

The researchers further examined the direct and indirect effects of expansion across urban–rural gradients in 508 typical cities. They found that direct impacts on NEP were largely negative, averaging –32.6%. In contrast, indirect effects, mediated through changes in land management, microclimates, and socio-economic activities, were positive, averaging 19.6 percent. These indirect effects helped offset nearly 15% of direct losses. The researchers found that climate played a critical role in mediating indirect effects: arid and cold regions showed stronger positive indirect effects, while tropical cities had the weakest. Human activities, measured by factors such as the urban heat island effect (UHI) and human footprint (HF), often outweighed climatic influences. Notably, developed countries tended to exhibit higher indirect benefits, but in areas of rapid expansion, developing countries demonstrated stronger ecological responsiveness.

This body of work systematically clarifies the mechanisms through which global urbanization influences the carbon sink function of vegetation. The results provide a scientific foundation for improving urban ecosystem carbon sequestration, optimizing expansion patterns, and advancing global carbon neutrality goals.

The findings have been published under the titles “Global urbanization indirectly ‘enhances’ the carbon sequestration capacity of urban vegetation” in Geography and Sustainability and “Dual impacts of urban expansion on the carbon sequestration capacity of urban vegetation: A multi-scale global study” in Ecological Indicators.