**3. Results**

#### *3.1. The Green-up Date of Mongolian Oak*

ExG values obtained from digital cameras clearly indicate the phenological changes of Mongolian oak (Figure 3). The green-up date of Mongolian oak, derived from the curvature K value of the digital camera ExG, was the earliest in the urban area and gradually delayed moving through the suburban and rural areas toward the natural area with 94, 95, 95, 95, 97, 102, and 117 days in Mts. Nam, Mido, Umyeon, Buram, Cheonggye, Sori (Gwangneung), and Jeombong, respectively, based on the day of year (DoY) (Table 4).

**Figure 3.** (**<sup>a</sup>**,**b**) show the seasonal courses of ExG in the Mongolian oak stands of each site. (**c**) shows the logistic models of green-up based on ExG, and (**d**) shows the rate of change of curvature K in study sites. The time at which the rate of change in curvature exhibits local maxima indicates the green-up date.


**Table 4.** Green-up dates estimated by two different criteria, ExG and EVI, and the difference between observed and expected green-up dates from ExG and EVI. *Obs*: observed, *Exp*: expected, *Diff*: difference.

EVI values obtained from MODIS images also clearly indicate the phenological changes of Mongolian oak (Figure 4). The green-up date of Mongolian oak, derived from the curvature K value of the MODIS image EVI, showed a similar trend to the result from the digital camera, with 94, 96, 97, 95, 96, 104, and 114 days in Mts. Nam, Mido, Umyeon, Buram, Cheonggye, Sori (Gwangneung), and Jeombong, respectively, based on the DoY (Table 4).

**Figure 4.** (**<sup>a</sup>**,**b**) show the seasonal courses of EVI in the Mongolian oak stands of each site. (**c**) shows the logistic models of green-up based on EVI, and (**d**) shows the rate of change of curvature K in study sites. The time at which the rate of change in curvature exhibits local maxima indicates the green-up date.

In addition, to clarify the differences among sites due to artificial interference, the expected dates of green-up were obtained through latitude, altitude, and elevation correction based on the natural area, Mt. Jeombong. This was compared with the actual observed

dates of the study site (Table 4). The green-up date observed from the ExG of Mt. Nam was DoY 94, which is 16 days earlier than the expected DoY 110. The green-up date observed from the ExG of Mt. Mido was DoY 95, which is 18 days earlier than the expected DoY 113. The green-up date observed from the ExG of Mt. Umyeon was DoY 95, which is 15 days earlier than the expected DoY 110. The green-up date observed from the ExG of Mt. Cheonggye was DoY 97, which is 14 days earlier than the expected DoY 111. The green-up date observed from the ExG of Mt. Buram was DoY 95, which is 15 days earlier than the expected DoY 110. The green-up date observed from the ExG of Gwangneung (Mt. Sori) was DoY 103, which is 11 days earlier than the expected DoY 114 (Table 4).

The green-up date observed from the EVI of Mt. Nam was DoY 94, which is 16 days earlier than the expected DoY 110. The green-up date observed from the EVI of Mt. Mido was DoY 96, which is 18 days earlier than the expected DoY 114. The green-up date observed from the EVI of Mt. Umyeon was DoY 97, which is 15 days earlier than the expected DoY 112. The green-up date observed from the EVI of Mt. Cheonggye was DoY 96, which is 14 days earlier than the expected DoY 110. The green-up date observed from the EVI of Mt. Buram was DoY 96, which is 15 days earlier than the expected DoY 111. The green-up date observed from the EVI of Gwangneung (Mt. Sori) was DoY 104, which is 11 days earlier than the expected DoY 115 (Table 4).

As a result of analysis, the correlation between green-up dates derived from ExG and EVI values and land use intensity showed a significant negative correlation (Figure 5).

**Figure 5.** Relationship between urbanization ratio and green-up dates derived from ExG (**upper**) and EVI (**lower**) (*p* < 0.05).

#### *3.2. Accumulated Growing Degree Days (AGDD)*

Green-up dates estimated by AGDD in each study site are shown in Table 5. Green-up dates expressed as DoY were 94, 95, 95, 97, 96, 103, and 114 in Mts. Nam, Mido, Woomyeon, Cheonggye, Buram, Sori (Gwangneung), and Jeombong, respectively (Table 5). AGDD for green-up dates derived from the ExG of the digital camera were 160.3 ◦C, 158.2 ◦C, 156.3 ◦C, 162.2 ◦C, 162.3 ◦C, 156.6 ◦C, and 160.5 ◦C, and AGDD for green-up dates derived from the EVI of MODIS images were 160.3 ◦C, 166.9 ◦C, 162.6 ◦C, 153.2 ◦C, 168.3 ◦C, 163.3 ◦C, and 154.4 ◦C in the aforementioned site order (Table 5). As a result of analyzing the correlation between the land use intensity of the study sites and the date when the AGDD value reached 159 ◦C, they showed a significant negative correlation (Figure 6).

**Table 5.** Green-up dates estimated by AGDD and AGDD values for green-up dates derived from ExG and EVI.


**Figure 6.** Relationship between dates when AGDD reached 159 ◦C and urbanization ratio (*p* < 0.05).

#### *3.3. Seasonal Trajectory of the Sap Flow*

The seasonal change in sap flow is expressed in Figure 7. Green-up dates expressed in DoY were 94, 96, 97, and 104 in Mts. Nam, Woomyeon, Cheonggye, and Sori (Gwangneung), respectively (Figure 8). The difference between the green-up dates derived from sap flow and the ExG of the digital camera was within one day, and the date was the same in Mts. Nam and Cheonggye. The difference between the green-up dates derived from the sap flow and the EVI of MODIS images was within one day, and the date was the same in Mt. Nam and Gwangneung (Mt. Sori) (Table 6). The trajectory change in the curvature K value

derived from the sap flow of the plant showed a very similar trend to that of the curvature K value derived from the digital camera and MODIS satellite images (Figure 8).

**Figure 7.** Changes in sap flow velocity during the study period in each site.

**Figure 8.** SFM logistic models of green-up (**a**) and curvature K (**b**) in study.


**Table 6.** Comparisons of green-up dates derived from vegetation phenology (ExG and EVI) and sap flow (SFM).
