Urban expansion directly reduces vegetation via impervious surface growth, though indirect urban environmental effects may partially offset these losses. This study examines the Beijing–Tianjin–Hebei (BTH) region from 2000 to 2025 by integrating multi-source remote sensing and socioeconomic data to investigate the effects of urban expansion on vegetation greenness (NDVI) and productivity (NPP). The results show that impervious surfaces expanded from 21,747 km² to 33,688 km². Yet NDVI and NPP did not decline in tandem. Instead, they responded differently. NDVI exhibited broad increases—particularly in the northwest ecological zones. NPP was more sensitive to impervious surface expansion. Further analysis reveals that impervious surface coverage is the primary driver of NPP, while per capita GDP dominates NDVI. Correlation analysis confirms these relationships are consistent in direction. The combined contribution of socio-economic variables (GDP, per capita GDP, and population) substantially exceeds that of impervious surface area. This suggests that economic development, as a whole, is the dominant force shaping vegetation greenness in the region. These findings challenge the conventional trade-off narrative between urban growth and vegetation health. They demonstrate that urban expansion and ecological improvement can advance in parallel. This requires sustained greening investments and coordinated regional planning. The results offer actionable insights for carbon-sensitive land-use regulation in rapidly urbanizing regions.

Net ecosystem productivity (NEP) is an important indicator for quantitatively evaluating carbon sources/sinks in terrestrial ecosystems and plays a crucial role in the global carbon cycle. Based on the improved Carnegie-Ames-Stanford Approach (CASA) model and optimal parameters-based geo-detector (OPGD) model, the spatio-temporal variation characteristics, fluctuation degree and driving factors of NEP changes in the Beijing-Tianjin-Hebei region (BTH region) from 2000 to 2022 were quantitatively analyzed through Theil-Sen trend analysis, Mann-Kendall test and coefficient of variation. The results are as follows: (1) From 2000 to 2022, the NEP in the BTH region showed a fluctuating upward trend. The multi-year average was 136.60 g C·m ^− 2 ·a ^− 1 , and the interannual change rate was 3.76 g C·m ^− 2 ·a ^− 1 . During the study period, the proportion of the area with an NEP value greater than 200 g C·m ^− 2 ·a ^− 1 showed a fluctuating increasing trend. (2) The NEP in the BTH region shows significant spatial heterogeneity, presenting a spatial distribution feature of being high in the northwest and low in the southeast. During the research period, the area of the carbon source area (NEP < 0) accounted for approximately 15.40%, mainly distributed in the main urban areas of various cities and the tidal flats along the Bohai Sea. The high-value areas of NEP were mainly distributed on the Northern Slope of the Yanshan Mountains in the north and the deep mountainous areas of the Taihang Mountains in the west. (3) During the research period, the significantly increased areas in the BTH region accounted for 51.32% of the study area, mainly distributed in the Taihang Mountains and the Yanshan Mountains. The area with a decline in NEP accounted for only 13.64%, which was concentrated in the urban expansion area of the southeast plain of the study area. (4) The proportions of areas with high fluctuation degree and relatively high fluctuation grade were 28.71% and 10.74% respectively, concentrated in the Bashang Plateau in northwest Hebei Province, the southeast plain area and the expansion areas of various cities. The proportions of areas with lower and lower fluctuations were 22.55% and 20.66% respectively, mainly distributed in the Taihang Mountains in the west, the Yanshan Mountains in the north, the main urban areas of various cities and the intermountain basins in the northwest of Hebei Province. (5) NDVI and slope are the main natural driving forces. Among human factors, the q value of population density exceeding 0.2 is the main human driving force. The types of factor interactions were all two-factor enhancement. Among them, the interaction of the combination of NDVI and other influencing factors (altitude, temperature, population density) had a relatively high impact on the change of NEP. The research results can provide valuable references for the study of the carbon sequestration capacity of terrestrial ecosystems in the BTH region, and promote regional green transformation and ecological optimization.