## 4 Discussion
### Impacts of Urbanization on Vegetation Growth  
Vegetation plays an important role in the urban environment, however, the patterns and mechanisms of vegetation dynamics caused by urbanization are still poorly understood due to the limited availability of traditional ground-based observations [51]. Here, the FVC derived from remote sensing data with 30 m resolution, in combination with the LUCC dataset, were used to quantify vegetation activity and the impact of urbanization on vegetation growth. From 2004 to 2020, the vegetation coverage in Harbin city exhibited an overall insignificant decreasing and stable trend, but fluctuated on the local scale, which was consistent with the findings of other studies [52; 53]. The FVC, as with other vegetation indicators (EVI, GPP, NDVI, LAI), was reduced significantly with increasing urban intensity [53; 54]. By separating the direct and indirect impacts of urbanization on vegetation growth along with urban intensity, we further confirmed that urbanization could exert both positive and negative impacts on vegetation growth. The direct impact, which generally refers to the replacement of natural land cover by impervious surfaces and the reduction in vegetation cover and growth, is usually negative [55]. By contrast, the indirect impact is usually caused by the unique natural environments of urban areas (e.g., urban \"heat island\" warming, lower air humidity, higher CO\({}_{2}\) concentration, water shortage) and human management of urban vegetation, which may enhance or inhibit vegetation growth [56]. It has long and widely been believed that there is more impervious surface, dense buildings, and less open space in areas of high urbanization, which led to plants getting limited light, thereby increasing the relative allocation of plant biomass to shoots [57]. Yet, a previous study revealed that despite poor solar conditions and high air pollutant concentrations in central urban areas, aerosol particles can increase diffuse radiation, which can enhance canopy light use efficiency [58]. More surface sealing, soil compaction, and pollution caused by urbanization may affect root health and litterfall decomposition, and further affect the urban carbon cycle [59; 60]. Additionally, urban vegetation growth also benefits from human management practices (e.g., irrigation, fertilization). The higher temperature caused by \"urban heat island\" warming and atmospheric CO\({}_{2}\) concentrations can effectively lengthen the growing season in cold regions [61] and may enhance the autotrophic respiration and photosynthetic processes of vegetation. This may be one of the drivers of the enhanced indirect impact (\(w_{i}\)) that accompanies increasing urbanization intensity. It is because of the unique conditions of urban spaces that they are often seen as \"harbingers\" of future global change [62]. It can be said that the impact of urbanization on vegetation growth can provide valuable information on how other non-urban ecosystems may react to future climate and environmental changes. The driving factors of an FVC change study [52] found that the vegetation change in Harbin is influenced by various factors (e.g., temperature, precipitation, radiation, and CO\({}_{2}\) concentrations). Other environmental factors, such as plant species, O\({}_{3}\), and nitrogen deposition, may also have an impact on the growth of vegetation in urban areas [62], which may be an important reason for the complex changes of indirect impact, that merit further investigation. Moreover, we found, as in previous studies [23; 25; 63], enhanced growths (FVC) caused by urbanization can offset to some extent the direct loss of vegetation caused by land conversion, especially in medium urban intensity areas, which might be explained by more resources of ecological governance invested in peri-urban areas.