**1. Introduction**

Urbanization is one of the major social and scientific changes spreading around the world [1,2]. It significantly alters land surface conditions and has profound impacts on terrestrial ecosystem processes and services [3–7]. Changes in land use release greenhouse

**Citation:** Kim, AR.; Lim, C.H.; Lim, B.S.; Seol, J.; Lee, C.S. Phenological Changes of Mongolian Oak Depending on the Micro-Climate Changes Due to Urbanization. *Remote Sens.* **2021**, *13*, 1890. https://doi.org/ 10.3390/rs13101890

Academic Editors: Xuanlong Ma, Xiaolin Zhu, Jiaxin Jin, Yuke Zhou and Qiaoyun Xie

Received: 5 April 2021 Accepted: 10 May 2021 Published: 12 May 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

gases into the atmosphere by changing the patterns of carbon storage and accelerate climate change by breaking the balance of the carbon budget [2]. An increase in atmospheric CO2 due to intensive use of land and fossil fuel destroys the balance of the global carbon cycle maintained in an equilibrium fashion [2,8–11]. In addition, increased development areas and populations cause changes in weather factors and affect terrestrial ecosystems [3–6,12]. Inadvertent weather factor changes in urban areas form an important effect on regional climate change [13–16].

Urbanization is an important anthropogenic influence on climate and has significantly affected terrestrial ecosystems [12,17]. It can modify local climate on daily, seasonal, and annual scales [18] and increase extreme climate events [19–22]. Changes in climate resulting from urbanization, therefore, can be considered a type of climate change on a local scale [23]. Such a change in local climate in an urban area is referred to as the "urban heat island (UHI) effect". The UHI effect is characterized by elevated temperatures in urban areas, compared to the surrounding rural areas [7,24,25]. It can affect regional climate change, increase environmental pollution, elevate energy and water consumption, affect the development of meteorological events such as increased precipitation, and have a significant impact on human health [16,26,27].

In phenological research, urban areas are important areas for study because they enable an assessment of the potential future effects of climate change on plant development [17,28]. Increased temperature by UHI can affect vegetation phenology such as the start of the season (SOS) and the end of the growing season (EOS) [25,29–31]. It is very important to understand the impact of UHI on phenology because the intensity of the UHI effect is similar to the expected temperature change in the near future [7].

Current climate change has a strong impact on vegetation phenology [6,32–37], and it causes changes in the timing of plant developmental phases, affecting the carbon budget of the terrestrial ecosystem [38–41]. Phenological characteristics are closely related to variation in weather factors [6,42–45], many of which can affect vegetation phenology such as green-up, budburst, and leaf senescence [17].

Among the numerous techniques to observe phenology, using digital camera and MODIS satellite images requires less time and cost to collect data [46,47]. Time-lapse photography provides very exact temporal sampling at daily intervals for assessing phenology. Satellite remote sensing provides decades of records of vegetation phenology across larger spatial scales than other technologies [48]. Remote sensing also has the advantage of providing large temporal records of vegetation phenology over larger spatial scales than other techniques [17,48]. In particular, the method using both digital camera and MODIS satellite images has sufficient spatial resolution to obtain detailed information about vegetation and land cover types and can collect data with more flexible spatial resolution, thereby resolving the problems pointed out in the existing data collection [17,26,46,49].

Recently, beyond the level of checking phenological phases by observing the external forms of plants, a study method to confirm the plant phenological phases through physiological responses such as sap flow time series estimates was also proposed [17,50]. Water availability is regulated by the timing and periodicity of leaf production and is very important for plant growth [51,52]. According to many studies investigating the relationship between phenology and sap flow, sap flow is closely related to the change in leaf area [50,53,54], since plants draw water from the xylem by tension from the leaves during the transpiration period [55], and the amount of transpiration increases depending on the formation of leaves. Therefore, sap flow usually has a linear relationship with leaf area development [17,50,54]. Phenological events emerging through appearance may be difficult to observe accurately and precisely due to various influences [56,57], and the method of color wavelength analysis of the leaves applies well for deciduous plants, but there will be limitations to evergreen plants. In this respect, a method of monitoring physiological changes according to seasonal changes may be more versatile [58]. In addition, since the phenophase using instrumental techniques can be better specified than pure observations and qualified guesses, the onset of spring phenological stages such as leaf

area development can be easily identified from sap flow measurements [50]. Ecophysiological measurements such as sap flow measurement can provide additional value in the objectification of phenological studies [50]. Therefore, sap flow has been widely utilized as a tool to diagnose the development of the phenological phases of plants [30,51,54,59].

The objectives of this study are to (1) find the phenological trajectory of vegetation through analysis of MODIS satellite and digital camera images, (2) investigate how microclimate change caused by urbanization affects vegetation phenology, (3) diagnose the phenological trajectory of vegetation through the physiological response of plants, and (4) prepare an ecosystem managemen<sup>t</sup> strategy to adapt to climate change.

#### **2. Materials and Methods**
