## 1 Introduction  
The world's urban population has been growing quickly from only 30% of the global population residing in urban areas in the 1950s to as much as 54%, or 3.9 billion urban residents, in 2014. It is projected to increase toward 66% by 2050 [1], so it is important to understand the effects of urban areas on the environment. One such effect is the elevated temperature in urban areas, which in turn affects the growth cycle of plants known as plant phenology. Urban heat island (UHI) is the term used to describe higher temperatures in an urban environment compared to the surrounding rural areas [2]. UHI is mainly a result of changing a natural landscape into a man-made urban texture [3] and arises due to the modified surface affecting the storage and transfer of heat, water, and airflow [4]. The UHI has a measurable effect, as previous research has shown a small UHI effect of 0.17 \({}^{\circ}\)C for cities as small as 2-3 km\({}^{2}\) and a larger effect of 2.9 \({}^{\circ}\)C during the day and 2.3 \({}^{\circ}\)C during the night for larger cities of \(>\)100 km\({}^{2}\)[5]. Since the UHI is comparable to projected global temperature increases [6], urban environments have been used as a surrogate for future climate change (i.e., [7]). The relationship between urban intensity and the course of annual developmental events in plants, known as plant phenology, has been documented [8]. From a biological perspective, phenological research predominantly addresses the timing of switches between recurrent phases of organisms [9]. In cool and temperate regions, to maximize growth, trees need to extend the growth period as long as possible while avoiding frost damage [10]. It has been confirmed that bud burst is under strong temperature control, with leaves appearing earlier and growing faster during a warmer spring [11]. In the fall, growth cessation and dormancy are in response to the shortening of the photoperiod [12], however, low temperature also causes growth cessation [13]. Plants, in general, will develop earlier in cities by a factor of days to weeks when compared to their rural settings. However, there is no clear association with autumn phenophases. Comparing the urban to rural trends of phenology allows for the evaluation of phenological trends in differing conditions and assessment of impacts of climate change [6]. Earlier greening of land cover and later autumnal leaf fall may alter seasonal climate through biogeochemical processes and physical properties [14]. Phenological changes will have varying impacts on albedo [15] and surface temperature [16]. In addition, the length of the growing season plays a key role in the amount of CO\({}_{2}\) in the atmosphere [17]. Earlier spring onset leads to an increase in Gross Primary Production (GPP) and ecosystem respiration, with GPP having the larger increase resulting in a higher Net Ecosystem Productivity (NEP) [18]. From a biological perspective, due to species responding differently to warming temperatures, mismatches in the phenology across trophic levels may already be occurring due to non-uniform shifting of co-occurring organisms, changing the biological communities [19]. Satellite derived measurements of land surface reflectance have been used to study land surface phenology (LSP) over large areas (for example, [20]). When applied to examine the length of growing season for urban and rural areas, it was found that the growth season was about 15 days longer in urban areas [21]. However, this type of analysis has historically been restricted due to clouds, atmospheric conditions, and the infrequent satellite coverage [22]. Removing these atmospheric effects has a relatively long history [23]. However, even after this correction, the level of detail that urban maps provide is dependent on the spatial resolution of the sensor [24] and the error in phenology detection increases with a reduction in temporal resolution [25]. Therefore, both high temporal and high spatial satellite sensors each have complementary advantages when observing urban phenology.