Quantification of Urban Heat Island-Induced Contribution to Advance in Spring Phenology: A Case Study in Hangzhou, China

Response of vegetation phenology to urbanization in urban agglomeration areas: A dynamic urban-rural gradient perspective

Being an important theme in global warming, the response of vegetation phenology to urbanization has become an increasing concern at both the local and global levels. Previous studies have focused on spatial or temporal responses across urban-rural gradients; thus, the influence of urbanization on vegetation phenology along the dynamic urbanization gradient has not been well quantified. In this study, we comprehensively analyzed the response of vegetation phenology to urbanization in the Guangdong-Hong Kong-Macao Greater Bay Area (GHM-GBA) from a dynamic urban-rural gradient perspective. The results show that the response of vegetation phenology to urbanization level has a distinct spatiotemporal difference across the urban-rural gradient. Compared to rural areas, the change rate of advancements in the start-of-season (SOS) in urban domains was 1.16 DOY/year and that of the end-of-season (EOS) was 0.63 days/year from 2001 to 2020. In the GHM-GBA region, 61.03 % of the remote sensing pixels showed an advancing trend for SOS and 55.75 % for EOS. Urbanization advanced the SOS and EOS but did not extend the growing season length, and the SOS and EOS were advanced by 7 and 6 days along the urban-to-rural gradient, respectively. For every 10 % increase in urbanization levels, the SOS and EOS advanced by 1.085 and 1.091 days across the urban-rural gradient, respectively; the spring land surface temperature (LST) advanced the SOS at a rate of 1.71 days/°C, while the autumn LST advanced the EOS at a rate of 1.88 days/°C. The phenological shift in the urban-rural gradient was more significant than that over time, which was mainly because of land surface warming under different urbanization levels. These quantitative findings are of great importance for understanding the complicated impacts of urbanization on vegetation phenology and for developing models to predict vegetation phenological changes under future urbanization.

Spatial heterogeneity of first flowering date in Beijing's main urban area and its response to urban thermal environment

Phenology - the rhythm of periodic plant life cycle events - was significantly shaped by urban climate, with flowering as one most sensitive phenophase. Apart from the widely noticed urban-rural phenological discrepancy caused by heat island effect, driven by the aggravating spatial unevenness of urban thermal environment, the spatial heterogeneity of flowering time was also found within the urbanized area of some metropolitans, bringing multiple impacts on urban ecology, landscape and public health. This research aimed to reveal the intraurban spatial variation and response characteristics of Beijing's trees flowering phenology that remained largely unclear before. We analyzed the spatial heterogeneity pattern of the first flowering date (FFD) for 42 deciduous woody species in Beijing's main urban area (MUA), and explored the species-specific phenological response to local thermal environment. The sample plots were set in 9 green spaces distributing from urban center to northwest suburb in Beijing's MUA, the FFD data was collected by ground-based phenological observation, and local thermal environment was measured with land surface temperature (LST) retrieved from MOD11A1 products. The main results are as follows: (1) A significant spatial variation for FFD existed among 9 sample plots and the maximum spatial difference of FFD reached 6.76 ± 1.77 days in average, FFD showed an overall delay trend from urban center in 2^nd Ring to outskirts beyond 5^th Ring with 3^rd Ring as a critical line for significant phenological difference. (2) The FFD of 35 species was found to be negatively correlated with [Formula: see text] (average of daily mean LST above 0 °C before mean FFD) in the sample plot (p < 0.05) with a response sensitivity of 2.99 ± 0.87 days/°C, which reflected the significant impact of LST variation during flower development period. Furthermore, the spatial difference and response sensitivity of FFD for a specific species were found to be negatively associated with its mean FFD value (p < 0.05), i.e., the flowering time of early-blooming species tended to be more sensitive to thermal environment variation compared with late-blooming ones. This research illustrated how flowering phenology responded to the heterogeneous intraurban thermal environment in Beijing's MUA, which can improve our understanding of the vegetation dynamics in a constantly changing urban environment. And as a critical indicator of trees' climate vulnerability assessment, the species-specific phenological response sensitivity could also guide species selection in urban forest construction.

Spatiotemporal Characteristics and Heterogeneity of Vegetation Phenology in the Yangtze River Delta

Vegetation phenology and its spatiotemporal driving factors are essential to reflect global climate change, the surface carbon cycle and regional ecology, and further quantitative studies on spatiotemporal heterogeneity and its two-way driving are needed. Based on MODIS phenology, meteorology, land cover and other data from 2001 to 2019, this paper analyzes the phenology change characteristics of the Yangtze River Delta from three dimensions: time, plane space and elevation. Then, the spatiotemporal heterogeneity of phenology and its driving factors are explored with random forest and geographic detector methods. The results show that (1) the advance of start of season (SOS) is insignificant—with 0.17 days per year; the end of season (EOS) shows a significant delay—0.48 days per year. The preseason temperature has a greater contribution to SOS, while preseason precipitation is main factor in determining EOS. (2) Spatial differences of the phenological index do not strictly obey the change rules of latitude at a provincial scale. The SOS of Jiangsu and Anhui is earlier than that of Zhejiang and Shanghai, and EOS shows an obvious double-clustering phenomenon. In addition, a divergent response of EOS with elevation grades is found; the most significant changes are observed at grades below 100 m. (3) Land cover (LC) type is a major factor of the spatial heterogeneity of phenology, and its change may also be one of the insignificant factors driving the interannual change of phenology. Furthermore, nighttime land surface temperature (NLST) has a relatively larger contribution to the spatial heterogeneity in non-core urban areas, but population density (PD) contributes little. These findings could provide a new perspective on phenology and its complex interactions between natural or anthropogenic factors.

Characterizing Spatial Patterns of the Response Rate of Vegetation Green-Up Dates to Land Surface Temperature in Beijing, China (2001–2019)

The phenology indicator of vegetation green-up dates (GUD) is prone to being affected by changes in temperature. However, the influencing degree of urbanization-induced temperature warming on vegetation GUDs among different vegetation species along the urban-rural gradient remains inadequately described. In this study, based on the long-term (2001–2019) satellite-derived vegetation GUDs and nighttime land surface temperature (LST) of forests, grasslands, and croplands along the urban-rural gradient with Beijing (China) as a case study area, the responses of vegetation GUDs to temperature changes were quantitatively analyzed, taking into account the vegetation types and distances away from the urban domain. The results show that (1) long-term GUDs and LST are significantly negatively correlated, characterized by a weaker significant correlation near the urban area when compared with its surrounding areas, with the greatest absolute linear correlation coefficients (r) happening at rings 32 km (rmax = −0.93, forests), 20 km and 48 km (rmax = −0.83, grasslands), and 34 km (rmax = −0.82, croplands), respectively; (2) the magnitude of change in GUDs over the past 19 year (2001–2019) are significantly positively correlated with these in LST near the urban area, demonstrating a distance-decay trend, with the greatest advance in GUDs occurring at the ring nearest the urban area, by about 20 days (forests), 24.5 days (grasslands), and 15.6 days (croplands), respectively; (3) the spatial pattern of the response rate of GUDs change to LST change (days K−1) also showed a declining trend with distance, with GUD advanced by 6.8 days K−1 (forests), 7.5 days K−1 (grasslands), and 4.9 days K−1 (croplands) at the closest ring to the urban, decreasing to about 2.3 days K−1 (48 km), 4.1 days K−1 (18 km), and 1 day K−1 (18 km), respectively, indicating a notable influence of temperature warming on vegetation GUDs near the urban domains.