2.2.5. Analyzing Spatial–Temporal Evolvement of the Forest Cover

The dissolve tool was used to integrate spatial distribution data of forest cover for six periods into ArcGIS, which were classified into two codes: forest land and non-forest land. The intersect tool was used to calculate data intersection between two periods as a group. The area field was added to the attribute table and calculated. Next, the attribute table was converted into shapefile attribute format (.dbf) and opened in Microsoft Excel. The commands "Pivot Table" and "Pivot Diagram" in the data menu were implemented, which generated a transfer matrix of the forest cover with two periods after appropriate changes were made. A transfer matrix with different periods was generated when the preceding process was repeated.

Data on soil type, elevation, soil, and lithology were generated from the distribution layer of soil type, sea level elevation, slope gradient, and lithology, respectively. Overlay analysis of the layers of spatial distribution information on forest cover under different historical periods was performed with the aforementioned layers in OVERLAY EVENTS. Under different classification conditions, the spatial distribution information of the forest cover was extracted under various periods concerning different soil types, elevation, gradient, and lithology. The corresponding distribution diagrams were drawn, and the area calculation function of ARC/INFO was used to establish area statistics. The transfer matrix method was used to calculate the area and direction of spatial transfer for the forest cover under different periods and natural backgrounds.

#### **3. Results**

#### *3.1. Overall Changed Process and Characteristics in Time*

From Figures 4 and 5, it can be seen that the change in forest area and scale showed a trend of first decreasing and then increasing in the study area from 1944 to 2013, and an obvious turning point appeared in 1986. These changes can be roughly divided into two stages: forest degradation before 1986 and forest restoration after 1986. It can be seen intuitively from Figure 4 that the forest coverage of the six years studied is quite different. The forest area was at its maximum in 1944 (up to 24.28% of the study area) and its minimum in 1986 (only 8.5% of the study area), with a difference of 2.84 times. The forest area decreased annually before 1986 and gradually increased after 1986, reaching 19.97% of the study area in 2013. However, even though ecological restoration and improvement were carried out from 2000 to 2013 (Figure 5), the forest coverage rate still did not reach the scale at the beginning of the research period.

**Figure 4.** The spatial distribution information maps of forest landscape in the different historical times. (We constructed this map using ArcGIS9.3 (http://www.esri.com/arcgis/about-arcgis) (accessed on 25 December 2021)).

**Figure 5.** The evolution process of forest landscape and important historical events related to it. (We constructed this figure using WPS office (https://platform.wps.cn/) (accessed on 30 December 2021)).

#### 3.1.1. Rate of Change in the Forest Cover under Different Historical Periods

In the analysis of the previous part, it has been found that the evolution trend of forest cover area in the whole study period is first decreasing and then increasing. However, it remains to be analyzed which period changes more quickly. It can be seen from Table 1 that before 1986, the evolution frequency was negative (referring to the decrease in forest area), and after 1986, it was positive (the increase in forest area).

**Table 1.** The annual changing rates of forest landscape in different historical periods. (We constructed this table using WPS office (https://platform.wps.cn/) (accessed on 19 December 2021)).


Specifically, from 1944 to 1977, although the overall change rate was −10.43%, the average annual change frequency was only −0.32% because the two years were separated by 33 years. The minimum frequency of change was from 2000 to 2007, with a change rate of only 1.82%. The difference between the highest and lowest frequency of change is nearly 5.73 times. However, a big change does not necessarily mean a quick change rate. This variable is also related to the number of years. From 1986 to 2000, the annual average rate of change was the largest, about 0.52%. Although the overall frequency of change in this period is 7.61%, which is lower than 10.43% in 1944–1977, the average annual frequency of change was higher. It shows that in this period from 1986 to 2000, a series of national ecological protection projects and policies have obvious effects on the restoration of forest cover, such as the Yangtze River Shelter-belt Project (1989), Pearl river Shelter-belt Project (1996), and Reforestation Project (1999) (Figure 5).

## 3.1.2. Transfer Direction of the Forest Cover under Different Time Series

It can be seen from Figure 6 that there are obvious differences in changes between forest and other land types in various historical periods. Specifically, from 1944 to 1977, the conversion of forest to other land types was 1165 km 2, which was significantly larger than the conversion area of other land types to forest in this period (Figure 6a). From 1977 to 1986, although the changes from forest to other land types and the changes from other land types to forested land were scattered in space, it was also obvious that the changes from the forest to other land types were more (Figure 6b). From 1986 to 2000, the change in other land types to the forests was the most obvious, which was the fastest period of ecological restoration, mainly in the northwest of the study area (Figure 6c). From 2000 to 2007, other land types still changed to forest (467 km2), which mainly happened in the southeast of the study area (Figure 6d). From 2007 to 2013, the forest was converted into other land types (255 km2), mainly occurring in the middle of the study area, which may be related to the increase in temporary forest in construction land caused by urbanization during this period [59]. At the same time, during this period, other land types of 366 km2 were also converted into forest (Figure 6e). From the whole research period from 1944 to 2013, the conversion of forests to other land types is higher than that of other land types (Figure 6f), which is also consistent with the overall analysis results of the previous part of forest coverage.

#### *3.2. Changes in Forest Cover under Different Influencing Factors*

The changes in forest cover under the different elements of factors such as elevation, slope, soil types and lithology in different time periods are shown in Table 2 and Figure 7. Table 2 shows the change area of forest under various factor levels in different time periods. Figure 7 shows the percentage of forest change area in different factor levels in the total forest area in different historical periods. In Figure 7, the positive and negative values before the abscissa percentage indicate the changing direction of forest increase and decrease, respectively. The negative percentage indicates the proportion of decreased area in the total forest area at the beginning of this period, and the positive percentage indicates the proportion of increased forest area in the total forest area.

**Table 2.** The distribution and change information of forest landscape under various natural background conditions during different historical stages (unit: km2). (We constructed this table using WPS office (https://platform.wps.cn/) (accessed on 19 December 2021)).


**Figure 6.** The space transfer matrix maps of forest landscape in different historical periods. (We constructed this map using ArcGIS9.3 (http://www.esri.com/arcgis/about-arcgis) (accessed on 27 December 2021)).

**Figure 7.** The percentage of forests changing area with different elements grades in the total forest area in different historical periods. (We constructed this figure using WPS office (https://platform. wps.cn/) (accessed on 27 December 2021)).
