**3. Results**

#### *3.1. Spatial Distribution and Trend of NTL for Global PAs*

We superimposed the global PAs and the global NTL, except for Antarctica (Figure 3). The result revealed that the DN values for most of the PAs were lower than that of the surrounding areas. Further assessment ranked the average NTL DN values of all PAs on each continent as: Europe > Asia > North America > South America > Africa > Oceania. The DN values in Europe was much higher than those in the other continents (Figure 4). The ranking of the average change trend on all continents was consistent with the average DN value, i.e., the highest was in Europe and the lowest was in Oceania (Figure 5).

**Figure 3.** Spatial distribution of NTL in association with global terrestrial protected areas (PAs). The orange-colored areas represent the PAs; the dark blue-colored area (including oceans) were in darkness; the yellow-colored areas had a higher NTL digital number (DN) value than the green-colored areas; the white-colored area had no data.

**Figure 4.** (**a**) Average NTL DN value for every PA and (**b**) average NTL DN value for every continent. For (**a**), we used the natural breaks (Jens) method to divide the PAs into three levels according to the average DN value. The green-colored PAs were those with the average value of 0, the yellow-colored PAs had a lower than average DN value, and the red-colored PAs had a higher than average DN value. For (**b**), the ranking of the average NTL DN value of PAs in each continent gave the order as Europe > Asia > North America > South America > Africa > Oceania.

**Figure 5.** (**a**) Average trend of NTL of the global terrestrial PAs and (**b**) average trend of every continent. For (**a**), we used the natural breaks (Jens) method to divide the PAs into three levels according to the average trend of NTL. The blue-colored PAs had an average trend of 0, the yellow-colored PAs had a lower than average trend, and the red-colored PAs had a higher than average trend. For (**b**), the ranking of the average NTL trend of PAs in each continent gave the order as Europe > Asia > North America > South America > Africa > Oceania.

#### *3.2. Average NTL Level in Di*ff*erent PAs and Bu*ff*ers*

Figure 6 illustrates that NTL DN values in types III and V PAs were significantly higher than that of the other five types of PAs in the interior and buffer zones. With the buffer radius increased, the average NTL DN values of all PAs increased first and then decreased (Figure 7). The highest DN values of different types of PAs appeared in the range of 1–10 km.

**Figure 6.** NTL level on different buffers for each type of PA. Columns with different colors represent the interior of PAs and different buffer zones.

**Figure 7.** NTL level for each type of PA in buffer zones. Columns with different colors represent different types of PAs. The light blue shadowed areas represents the mean DN values of PAs and buffers.

From the time series, the NTL level within and outside PAs increased without exception, but the NTL distributions among seven types of PAs had different characteristics (Figure 8).

**Figure 8.** *Cont.*

**Figure 8.** Changes within PAs and in different buffer zones in the NTL level of different types of PAs in the time series. The numbers in the grid represent the average NTL DN values. The color of the grids is from blue to red, and the corresponding values are from small to large. (**a**) strict nature reserve (Ia), (**b**) wilderness area (Ib), (**c**) national park (II), (**d**) natural monument or feature (III), (**e**) habitat/species managemen<sup>t</sup> area (IV), (**f**) protected landscape/seascape (V), and (**g**) protected area with sustainable use of natural resources (VI).

Type Ia PAs represent strict nature reserves. As shown in Figure 8a, the average NTL DN value in buffer zones of type Ia PAs increased in the 0–10 km buffer zone and then decreased with over the 10–100 km buffer zone, reaching the maximum DN value in the 5–10 km buffer zone. The average DN value in the 0–100 km buffer zone of type Ia was much higher than the average DN value within the boundaries of the PAs.

Type Ib PAs represent wilderness area with large unmodified or slightly modified areas (Figure 8b). The average DN value within boundaries of type Ib was lower than that of type Ia PAs with the lowest NTL value among all types of PAs. The average DN value of the type Ib buffer increased from the 0–1 km to the 50–100 km buffer zones.

Type II PAs represent national parks (Figure 8c). The change trend of the DN value of the type II PAs buffer was similar to that of Ia (Figure 8a), suggesting a trend of increasing in the close buffer zones (0–10 km) and decreasing in distant buffer zones (10–100 km), with the maximum DN value in the 5–10 km buffer areas. The fluctuation of the NTL DN value of type II was the lowest among all types of PAs.

Type III PAs represent natural monuments or features (Figure 8d). The difference between DN values within PAs and 0–100 km buffer zones were the highest among all types of PAs. The NTL outside of type III PAs increased in the 1–5 km buffer zone and then decreased, indicating more human development in this range.

Type IV PAs represent habitat/species managemen<sup>t</sup> areas (Figure 8e). The nearest (0–1 km) buffer zone had the lowest DN value. The area with the highest NTL level was concentrated in the 1–10 km buffer zone.

Type V PAs represent protected landscapes/seascapes (Figure 8f). The DN value of type V PAs was much higher than that of the other types of PAs (Figure 9). The average DN value and the range of the buffer zones was also the highest among all PA types (Figure 9). The brighter areas around the PAs were concentrated within 1–25 km, and the 0–1 km and 25–100 km areas were darker.

**Figure 9.** The NTL level of different types of PAs in their interior and surrounding buffer zones. The blue shadowed area represents the fact that the 1–10 km buffer zone had the highest NTL level.

The average DN values in type VI PAs buffers were the lowest among all PAs (Figure 8g). The DN values of the 0–1 km buffer zone were the lowest among all types of PAs. As distance from the boundaries of the PAs increased, the DN value in the buffer zones increased, and reached a maximum in the 25–50 km buffer zone.

The results showed that the NTL levels within PAs were lower than that of surrounding areas, indicating that human development was limited and controlled by the boundary of the PAs. The NTL level within and outside the PAs increased from 1992 to 2013 (Figure 8). The areas with the highest NTL level in the buffer zone constantly approached the boundary of the type Ia PAs (Figure 8a).

In general, the ranking of the NTL level among the seven types of PAs was V > IV > III > VI > II > Ia > Ib (Table 2). The ranking of NTL level in the 0–100 km buffer zones outside the boundaries of types of PAs was V > III > IV > Ia > II > Ib > VI (Figure 7). The ranking of the NTL level of buffer zones and the interior was 5–10 km > 1–5 km > 10–25 km > 25–50 km > 50–100 km > 0–1 km. The NTL level within the PAs was significantly lower than that observed outside the PAs (Table 2). For most types of PAs, e.g., Ia, II, III, IV, and V, the brightest areas around the PAs were concentrated in the 1–25 km buffer zone. However, for types Ib and VI, the brightest areas around PAs were concentrated at further distances (Figure 9).


**Table 2.** NTL level of interior and buffer zones for the seven types of PAs.

#### *3.3. NTL Growth Rate in Di*ff*erent PAs and Bu*ff*ers*

The 1992–2013 NTL growth rates of every type of PAs and their bu ffer zones were doubled, except within the 0–1 km for type III PAs (Figure 10), indicating enhanced human development within and around PAs. The NTL growth rate for type Ia PAs was the highest both within and the surrounding outside among all types. Especially, NTL growth rates within the PAs and in the 0–10 km bu ffer zones were much higher than that of other types. The DN value of the interior Ia increased from 0.04 in 1992 to 0.19 in 2013, representing an increase of 378%. The 0–1 km bu ffer zone increased by 441% from 0.54 in 1992 to 2.93 in 2013. The growth rates of the 1–5 km and 5–10 km bu ffer zones were 372% and 274%, respectively. The NTL growth rate from 1992 to 2013 for type Ib PAs decreased from 217% for the internal area to 135% for the furthest bu ffer zone. The growth rate in the 0–1 km bu ffer zone was 215%, which was much greater than that of other bu ffer zones. NTL growth rates for type Ib PAs were much lower than that of Ia. The NTL growth rate for type II PAs ranged from 160% in the 5–10 km bu ffer zone to 226% in the outermost 50–100 km bu ffer zone. The growth rate within the type II PAs was 222%, ranking second. For type III PAs, the NTL growth rate was 114% within the protected boundary area. The average growth rate of the bu ffer zones was the lowest among all types of PAs. The growth rate increased with distance, from 90% in the 0–1 km bu ffer zone to 170% in the 50–100 km bu ffer zone. The NTL growth rate within type IV PAs (178%) was slightly higher than that of its surrounding areas (157–175%). The di fference in growth rates between bu ffer zones was small, with a minimum of 157% in the 1–5 km bu ffer zone and a maximum of 175% in the 5–25 km bu ffer zone. The NTL growth rate within the type V PAs was 157%. The growth rate of the 0–1 km bu ffer zone was 164%, which was the highest among all type V PA bu ffers. The growth rate of the remaining bu ffer zones decreased with distance from the boundary, i.e., from 133% in the 1–5 km bu ffer zone to 151% in the 50–100 km bu ffer zone. From 1992 to 2013, the NTL growth rate of type VI PAs was 245%. The bu ffers near the boundary had a higher growth rate than that of the bu ffers farther away. The highest growth rate (263%) occurred in the 0–1 km bu ffer zone, and the minimum growth rate was 142% in the 50–100 km bu ffer zone.


**Figure 10.** Growth rate of NTL within every type of PA and the bu ffer zones.

#### *3.4. Trends in Di*ff*erent PAs and Bu*ff*er Zones*

NTL DN values of all PAs and their bu ffers showed a significant increasing trend from 1992 to 2013 (Figure 11). Except for the type V PAs, the NTL trends within the other six types of PAs were lower than that in any other bu ffer zone at the 0–100 km. The change trend within type Ib PAs was the lowest of all the PAs, with a value close to 0. The change trend of type V (0.13 DN/year) was the highest, which was greater than the trend in 50–100 km bu ffer zone. The average trend of type V PAs bu ffers was the lowest among all types of PAs.


**Figure 11.** Trends within every type of PAs and of buffers.

The trend for type Ia PAs was significantly higher than that of type Ib, indicating that Ia was affected more by human development. The change trends of different types of PAs showed characteristics. The rank of mean values of change trends in buffer zones were V > III > Ia > IV > II > Ib > VI. The change trend of the type V PAs was the highest, e.g., 0.26/year in 1–5 km and 0.24/year in 5–10 km buffers.

The trends in type V, III, Ia, and IV PAs were concentrated in the 1–10 km buffer zone, indicating active urban development. The type VI, Ib, and II PAs had the lower change trends inside PAs, but higher change trends in the peripheral areas far from the PA boundary. The change trends in the area near the PA boundary between 0–10 km were relatively low.
