Stage response of vegetation dynamics to urbanization in megacities: A case study of Changsha City, China

From city to countryside: Unraveling the long-term complex effects of urbanization on vegetation growth in China

The urban environment is the "natural laboratory" of the global ecosystem, and it has complicated effects on vegetation growth, including direct effects (land use transformations) and indirect effects (climatic environment changes). However, the long-term responses of vegetation to urbanization and its associated controlling factors across different spatial scales, from pixels to regions, remains unknown. Here, we unraveled the dual influence of urbanization on vegetation growth and its potential drivers along the urban development gradients in China with satellite observations of leaf area index (LAI) during 2000-2020. The results showed that 65.68 % of pixels in whole China exhibited an increasing trend in vegetation growth, with prominent greening (as indicated by LAI increases) in rural background areas (0.198/10a), significant greening in urban core areas (0.0343/10a), and significant browning in suburban areas (-0.0391/10a). As the process of urbanization intensified, the relationship between urbanization and vegetation growth became increasingly complex, transitioning from linear to non-linear interaction. The overall direct effects of Chinese cities were negative and increased annually. Meanwhile, the positive indirect effects of urban environments on vegetation growth initially declined and then recovered. Cities with high urbanization level (urbanization rate, ULp >70 %) had higher indirect effects (0.24 %) and growth offsets (0.98 %) than that with moderate (ULp = 60 %-70 %) and low urbanization levels (ULp <60 %) (0.18 %, 0.10 %). In economically developed cities, land use changes from construction to vegetation, influenced by urban policies and management strategies, positively impacted urban greening. Overall, more urbanized cities (ULp >70 %) experienced vegetation growth enhancement due to more intense land use changes, whereas less urbanized cities (ULp <70 %) showed the opposite trends. Understanding the direct and indirect effects of urbanization on vegetation growth is crucial for devising effective urban planning and environmental conservation policies. It can help guide future urbanization processes and minimize adverse effects on the natural environment.

Urbanization weakens vegetation resilience in the Pearl River Delta, China

Rapid urbanization has introduced increasingly complex social-ecological processes, intensifying the impacts on vegetation growth. Assessing urban vegetation resilience is critical to understanding urban vegetation growth. However, the current understanding of vegetation resilience in highly urbanized areas, especially regarding the influence of human activities, remains limited, constraining efforts toward sustainable urban vegetation management. In this study, we identified the spatiotemporal heterogeneity of urban vegetation resilience in the Pearl River Delta (PRD) using the lag-one autocorrelation coefficient (AC1) of the enhanced vegetation index (EVI) derived from Landsat imagery. Referring to the general conceptual framework for quantifying the impacts of urbanization on vegetation growth, we assessed the impacts of urbanization on vegetation resilience in the PRD urban agglomeration from 1998 to 2022. Results revealed that 21% of the urban area experienced one to five vegetation loss events, primarily lasting 1-2 years. Although vegetation growth was enhanced along the urbanization intensity gradient, a significant (p < 0.05) downward trend in vegetation resilience was observed, indicating that urbanization restricted the stability and sustainability of urban vegetation. By distinguishing between direct and indirect impacts, we found that the indirect impacts of urbanization on vegetation resilience gradually outweighed the direct impacts over time. Our findings further demonstrate that while intensive management can promote regreening in urban settings, maintaining the prevalent stability of urban vegetation remains challenging. These findings contribute to a better understanding of the human impact on vegetation resilience and offer significant implications for seeking directions to improve urban vegetation resilience.

Facing the challenge of NDVI dataset consistency for improved characterization of vegetation response to climate variability

Non-linear trend detection in Earth observation time series has become a standard method to characterize changes in terrestrial ecosystems. However, results are largely dependent on the quality and consistency of the input data, and only few studies have addressed the impact of data artifacts on the interpretation of detected abrupt changes. Here we study non-linear dynamics and turning points (TPs) of temperate grasslands in East Eurasia using two independent state-of-the-art satellite NDVI datasets (CGLS v3 and MODIS C6) and explore the impact of water availability on observed vegetation changes during 2001-2019. By applying the Break For Additive Season and Trend (BFAST01) method, we conducted a classification typology based on vegetation dynamics which was spatially consistent between the datasets for 40.86 % (459,669 km2) of the study area. When considering also the timing of the TPs, 27.09 % of the pixels showed consistent results between datasets, suggesting that careful interpretation was needed for most of the areas of detected vegetation dynamics when applying BFAST to a single dataset. Notably, for these areas showing identical typology we found that interrupted decreases in vegetation productivity were dominant in the transition zone between desert and steppes. Here, a strong link with changes in water availability was found for >80 % of the area, indicating that increasing drought stress had regulated vegetation productivity in recent years. This study shows the necessity of a cautious interpretation of the results when conducting advanced characterization of vegetation response to climate variability, but at the same time also the opportunities of going beyond the use of single dataset in advanced time-series approaches to better understanding dryland vegetation dynamics for improved anthropogenic interventions to combat vegetation productivity decrease.

Urbanization-driven forest soil greenhouse gas emissions: Insights from the role of soil bacteria in carbon and nitrogen cycling using a metagenomic approach

Increasing population densities and urban sprawl have induced greenhouse gas (GHG) emissions from the soil, and the soil microbiota of urban forests play a critical role in the production and consumption of GHGs, supporting green development. However, the function and potential mechanism of soil bacteria in GHG emissions from forests during urbanization processes need to be better understood. Here, we measured the fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in Cinnamomum camphora forest soils along an urbanization gradient. 16S amplicon and metagenomic sequencing approaches were employed to examine the structure and potential functions of the soil bacterial community involved in carbon (C) and nitrogen (N) cycling. In this study, the CH4 and CO2 emissions from urban forest soils (sites U and G) were significantly greater than those from suburban soils (sites S and M). The N2O emissions in the urban center (site U) were 24.0 % (G), 13.8 % (S), and 13.5 % (M) greater than those at the other three sites. These results were related to the increasing bacterial alpha diversity, interactions, and C and N cycling gene abundances (especially those involved in denitrification) in urban forest soils. Additionally, the soil pH and metal contents (K, Ca, Mg) affected key bacterial populations (such as Methylomirabilota, Acidobacteriota, and Proteobacteria) and indicators (napA, nosZ, nrfA, nifH) involved in reducing N2O emissions. The soil heavy metal contents (Fe, Cr, Pb) were the main contributors to CH4 emissions, possibly by affecting methanogens (Desulfobacterota) and methanotrophic bacteria (Proteobacteria, Actinobacteriota, and Patescibacteria). Our study provides new insights into the benefits of conservation-minded urban planning and close-to-nature urban forest management and construction, which are conducive to mitigating GHG emissions and supporting urban sustainable development by mediating the core bacterial population.

Understanding the indirect impacts of urbanization on vegetation growth using the Continuum of Urbanity framework

Numerous studies investigated the direct impacts of urbanization on the loss and fragmentation of vegetated lands associated with urban expansion. Fewer studies, however, have examined the indirect impacts of urbanization on vegetation related to changes in livelihoods, lifestyles, and connectivity in non-urbanized areas, especially in the context of large-scale urban-rural migration. Here, we employ the Continuum of Urbanity framework to examine how changes in livelihoods, lifestyles, and connectivity in non-urbanized areas associated with urbanization affect vegetation, and thereby to understanding the indirect impacts of urbanization. We found there was a significant trend in human-induced EVI (HEVI) increase in non-urban areas, and such trend was coupled with decreased population density (PD) in forest land and grassland, but increased population density in cropland. The negative correlation between PD and HEVI became increasingly stronger from 2000 to 2011, but weakened since 2011. Livelihood income, lifestyles represented by consumption, and information connectivity to the outside world indirectly impacted HEVI by driving PD changes in non-urban areas. This indirect effect has shifted from positive to negative over the 20 years. These findings suggest that the indirect impacts of urbanization on vegetation growth are complicated and multifaceted, and understanding such impacts would be critically important to help turn urbanization into an opportunity for regional sustainable development.