# Coarse-Resolution Satellite Images Overestimate Urbanization Effects on Vegetation Spring Phenology
## 1 Introduction  
Vegetation spring phenology, i.e., the beginning of the seasonal cycle of vegetation growth, is controlled by environmental factors (e.g., temperature, precipitation and light) [1; 2; 3; 4; 5]. Based on this, change in vegetation spring phenology can be regarded as an indicator of climate change such as increased spring temperature [6; 7; 8; 9; 10]. For instance, numerous studies have suggested that vegetation spring phenology occurs earlier along with increasing temperature in middle and high latitude regions of the northern hemisphere [11; 12; 13]. The changes in vegetation spring phenology also have impacts on large-scale carbon or water cycling and may alter the interactions among species [14; 15; 16; 17]. For example, earlier vegetation spring phenology is able to provide higher gross primary production [18], but it can also have a negative influence on some ecosystem services (e.g., carbon loss) [19; 20]. As a result, studying the response and sensitivity of vegetation spring phenology is very important to project future scenarios under the pressure of climate change and other environmental changes caused by human beings. Since the second half of the last century, urbanization, a global process, has accelerated due to the increasing population growth. As we know, urbanization changes the original natural environment (mostly the rural areas surrounding cities) in many aspects, including microclimate, artificial lights, and even ground water. For example, the urbanization process can raise the local air temperature to cause an Urban Heat Island (UHI) effect, introduce more artificial lights to lengthen the photoperiod, and convert more soil to impervious surfaces, which eliminates rainwater infiltration and natural groundwater recharge. As a result, the environmental conditions for vegetation growth between rural and urban areas are significantly different, which causes the rural-urban difference of vegetation spring phenology [21; 22]. In recent years, the difference in vegetation spring phenology between cities and their surrounding rural areas has been used to quantify the urbanization effects on vegetation spring phenology [23; 24; 25]. These studies and the difference of rural-urban vegetation spring phenology are summarized in Table 1. Owing to the capacity for repeated observation and large spatial coverage, satellite images were used in most existing studies to retrieve the rural-urban difference in vegetation spring phenology. From the summary in Table 1, these studies made use of vegetation indices (e.g., NDVI and EVI) from diverse satellite-based remotely sensed images (e.g., Landsat, MODIS, AVHRR and SPOT) with a wide range of spatial resolutions from 30 m to 1 km to extract vegetation spring phenology by various methods. Study areas are mainly in the northern hemisphere, particularly in the cities of China and the United States. From Table 1, it can be seen that all the studies have reached a consistent conclusion that significant spring phenology events occur earlier in urban areas than the surrounding rural areas because of urban-induced rising temperature. Interestingly, however, the magnitudes of rural-urban difference are quite different among these studies listed in Table 1. In other words, the degree of urbanization effects cannot be estimated accurately. Even though previous studies show similar results, the quantitative assessment is inconsistent especially for those studies at the same sites. In Shanghai, China, for example, the average rural-urban difference was approximately 5-10 days using Landsat EVI (30 m) during 2001-2010 [26] compared to 7-15 days using SPOT NDVI (1 km) during 2002-2009 [27]. Similarly, in Salt Lake City, United States, the average rural-urban difference was greater than 15 days using MODIS EVI (500 m) during 2003-2012 [24] while the average rural-urban difference was less than 3.56 days using fused NDVI (30 m) during 2000-2011 [28].