3.4.1. Features of Inter-Annual Change of Climatic Factors

According to the inter-annual trends of the mean air temperature, mean relative humidity, and cumulative precipitation in the growing season of the study area from 2000 to 2015 (Figure 7), the mean air temperature reached a maximum of 11.47 ◦C in 2008, then gradually decreased, and reached its minimum of 9.69 ◦C in 2010. The multi-year mean air temperature was 10.56 ◦C at a decline rate of 0.37 ◦C/10a and the linear trend was not obvious. The mean relative humidity reached a minimum of 46.77% in 2006 and then gradually increased, reaching a maximum of 53.8% in 2013, and the multi-year mean relative humidity was 49.94%. It generally showed an upward trend with a rate of 0.91%/10a, and the linear trend was not obvious. The minimum of cumulative precipitation (210.69 mm) occurred in 2007 after which there is a rising trend, reaching a maximum of 378.87 mm in 2013, and it showed a downward trend after 2013, which generally showed an upward trend. The multi-year mean cumulative precipitation was 267.43 mm, at a growth rate of 5.833 mm/a.

**Figure 7.** Inter-annual change of air temperature (**a**); relative humidity (**b**); and precipitation (**c**) in IMAR from 2000 to 2015.

#### 3.4.2. GRAs between the Growing Season EVI and the Climatic Factors

The GRA results of the EVI with climatic factors (Figure 8a) show that the GRG between EVI and air temperature was 0.388–0.880 with a mean value of 0.609. The areas with high GRG were mainly located in Hulunbuir, western Xilingol League, and eastern Bayannur League, the elevation in the areas below 2000 m. However, the EVI value of the high GRG area was low, and the vegetation types were mainly swamps, meadow steppe, typical steppe, and desert steppe. While the low GRG areas were mainly distributed in the south-central part of Chifeng and southeastern Erdos, the elevation below 2000 m, the EVI value of the area was high, and the vegetation types were mainly evergreen broadleaf forests, deciduous broadleaf forests, swamps, and typical steppe. It indicated that air temperature had a greater influence on the vegetation in the low-EVI areas (steppe and meadow steppe, etc.) than that of the high EVI areas (coniferous forest, broadleaf forest, etc.).

The GRA results between EVI and relative humidity in the study area (Figure 8b) display that the GRG between EVI and relative humidity was 0.385–0.902, with a mean value of 0.623. In northern Hulunbuir and western Xilingol League with the elevation of 200–1000 m, the GRG was relatively high and the vegetation types were mainly evergreen coniferous forests, deciduous coniferous forests, deciduous broadleaf forests, coniferous and broadleaf mixed forests. In the southern Chifeng, eastern Tongliao and southern Hohhot, the elevation from 100 to 2500 m, the GRG was relatively low and the vegetation types were mainly typical steppe, desert steppe, and shrubs. It can be seen that relative humidity had a much greater impact on forest ecological areas than on steppe ecological areas.

According to Figure 8c, the GRA results between EVI and cumulative precipitation in the study area can be found that the GRG between EVI and cumulative precipitation was 0.398–0.893, and the mean value was 0.649. High GRG areas were mainly located in the northwestern Hulunbuir and

southeastern Hinggan League, the elevation below 2000 m. Evergreen coniferous forests, deciduous coniferous forests, deciduous broadleaf forests, meadow steppe and swamps were the main vegetation types in the high GRG areas. Low GRG areas were mainly distributed in the southeastern Hulunbuir, northern Hinggan League and southern Chifeng with elevation of 300–1500 m. The vegetation types of the low GRG area were mainly swamps, typical steppe, desert steppe, and coniferous and broadleaf mixed forests. The results show that although the impact of precipitation on the forest ecological areas was slightly higher than that on the steppe ecological areas, precipitation and EVI showed a high correlation in the two major areas.

According to the GRA results, there was a distinct spatial difference in the degree of vegetation response to climatic factors in the study area. In forest ecological areas, the response of vegetation to precipitation and relative humidity was generally higher than that of air temperature, the relational correlations between vegetation and precipitation and between vegetation and relative humidity showed an obvious difference. In the steppe ecological areas, the response of vegetation to precipitation and air temperature was generally higher than that of relative humidity, but the GRGs of EVI and precipitation and EVI and air temperature were very close. Figure 8 shows that at certain air temperatures, the water from precipitation directly affected the surface of the vegetation, but the water from relative humidity existed in the air. Therefore, in the typical steppe and forest areas with moderate air humidity in the northern part of the study area, precipitation had a closer relationship with vegetation growth. On the contrary in the desert steppe areas with less precipitation in the western and central parts of the study area, relative humidity played a more dominant role in vegetation growth than precipitation.

**Figure 8.** GRG between EVI and air temperature (**a**); relative humidity (**b**); and precipitation (**c**).
