Global Climate Change and Terrestrial Ecosystem Response (GCTE) is an important part of the core research of the International Geosphere Biosphere Programme (IGBP), which has received great attention from the international scientific community and the international community. Vegetation is an active member of the Earth system, and any change in terrestrial ecosystems must respond to vegetation type, quantity, or quality. Vegetation coverage refers to the percentage of the vertical projected area of ​​the aboveground part of the vegetation (including leaves, stems, and branches) on the ground per unit area as a percentage of the total area of ​​the statistical area. Vegetation coverage indicates the degree of vegetation and the area of ​​photosynthesis of plants. It is an important indicator reflecting the growth of surface vegetation communities and an important basic data describing the ecosystem. It has an important indication for regional ecosystem changes. As an important aspect of global change, climate change has an important impact on vegetation coverage. Temperature and precipitation affect plant growth and distribution by affecting plant photosynthesis, respiration and soil organic carbon decomposition by affecting effective accumulated temperature and available water. Vegetation as a comprehensive indicator of the changes in the ecological environment, studying its response to climate change has become one of the main contents of current global change research. Therefore, under the background of global climate change, mastering the inter-annual variation of terrestrial vegetation coverage and exploring the driving role of climate factors have important theoretical and practical significance for evaluating the environmental quality of terrestrial ecosystems and regulating ecological processes.
Fund Project: National Key Basic Research and Development Program (973 Program) Project (2010CB950702); National 863 Program Special Project (2007AA10Z231); Inner Mongolia is located in the northern border of China and is an important ecological barrier in northern China, belonging to arid and semi-arid climate to the southeast. The transition zone of coastal humid and semi-humid monsoon climates varies according to the gradient of rainfall and temperature. The vegetation types are forest, grassland and desert along the northeast-southwest line. Vegetation coverage can reflect the overall situation of the ecological environment in Inner Mongolia. Therefore, the change of vegetation cover in Inner Mongolia and its relationship with climatic factors has always been a hot topic in the scientific research. “12. The existing research is mostly from the same spatial scale (a certain region). Or time scale (interannual level) to explore the relationship between vegetation growth and climatic factors, while ignoring the characteristics of vegetation growth and rainfall, temperature distribution in spatial distribution and time distribution. Therefore, this paper is based on 2001-2010 The MODIS-NDVI remote sensing data obtained the temporal and spatial variation information of surface vegetation coverage in Inner Mongolia. Combined with the rainfall and temperature data of the study area, the forest ecological zone, grassland ecological zone and desert ecological zone were analyzed from different spatial and temporal scales. The interannual and vegetation changes of vegetation coverage respond to climate change, aiming at revealing the impact mechanism of climate change on vegetation growth under different spatial and temporal patterns, exploring the driving role of climate factors on regional vegetation change, and expanding ecosystem services. The radiation effect is of great significance.
2 Data and methods 2.1 Data sources and pre-processing Remote sensing data is derived from NASA EOS/MODIS data from NASA (http://edcims), selected for 2001-2010 MOD13A1 data product, time resolution is 16d, spatial resolution is 500x500m. Use MRT (MODISReproectionTools) to format and re-project the downloaded MODIS-NDVI data, convert HDF format to Tiff format, and cast SIN map to EqualAreaConic projection At the same time, the spatial stitching and resampling of the image is completed. The 16-day MODIS-NDVI data was obtained by using the maximum synthesis method (MVC) to obtain the monthly NDVI data, and the inner Mongolian administrative division map was used to cut the grid image of the monthly NDVI from 2001 to 2010 in Inner Mongolia.
The NDVI16d synthetic data is affected by factors such as aerosol, ice and snow, solar illumination angle and sensor observation angle, and abnormal values ​​or missing data appear. Therefore, in order to make the NDVI time series data correctly reflect the true seasonal variation of vegetation, it is necessary to filter it. In this paper, the NDVI data is smoothed and filtered by Savtzky-Golay (SG) filtering method. The SG filtering method, also known as least squares method or data smoothing polynomial filtering method, uses a least squares convolution fitting method to smooth and calculate the derivative of a set of adjacent values ​​or spectral values. The design idea of ​​SG filtering is to find a suitable filter coefficient to protect the high-order distance, that is, when approximating the basic function, use the high-order polynomial to achieve the least squares fitting in the sliding window. In the research, the cloud layer and the ice and snow pixels are first identified by using the pixel credibility time series data, and the NDVI value is linearly interpolated with the adjacent time period pixel values ​​to improve the data quality of the NDVI. Through repeated iterative processing, the abnormal value of vegetation index caused by cloud and atmospheric conditions is removed, so that the fitted data is closer to the upper envelope of the NDVI time series, and finally the NDVI time series curve tending to true value is obtained.
The meteorological data is the monthly average temperature and monthly precipitation data of 722 standard meteorological stations nationwide from 2001 to 2010 provided by the China Meteorological Science Data Sharing Service Network (http://cdc.cma.gov.cn). According to the latitude and longitude information of each weather station, the Analyst module performs Kriging spatial interpolation on the meteorological data, and obtains a weather data raster image with the same size and projection as the NDVI data pixel. Through the data mask, a raster image of the monthly average temperature and monthly precipitation in Inner Mongolia is cut.
Ecoregions data. Ecological zone refers to a collection of natural communities of land or water that contain certain geographical features. The specific characteristics are: 1) high consistency in species and ecological dynamics; 2) similar habitat conditions; 3) maintenance Ecologically stable ways to interact. TerrestrialEcoregions data divides the world into 825 ecological zones, of which there are 13 ecological zones in Inner Mongolia. According to the climatic characteristics and vegetation distribution characteristics of Inner Mongolia, the existing pixel binary model is not sensitive to the influence of image radiation correction, and the calculation is simple, and it is widely used to calculate vegetation coverage. However, this model also has certain defects. NDVI can not only reflect vegetation coverage information, but also reflect land cover type and leaf area index (LAI) information. Zeng et al. believe that when a pixel NDVI is close to the pure vegetation pixel NDVIv (Ci is close to 1), the increase of NDVI due to the increase of LAI will not affect C, and when a pixel NDVI is smaller than NDVIv (C, < 1) Changes in pixel NDVI (such as interannual variations) are usually related to changes in C and LAI, so the public correlation is used to determine the significance of the interannual variation in coverage, and the negative slope indicates a decrease in vegetation coverage. Conversely, vegetation coverage increases. The significance test of the trend uses the F test, and the significance only represents the level of confidence in the trend change, regardless of the speed of change. The statistic calculation formula is: the actual observation value of the coverage, only its regression value, which is the average value of the coverage of 10 years, and n=10 is the number of years. According to the test results, the trend is divided into the following five levels: extremely significant reduction (0slope) This study uses pixel-based spatial analysis to analyze the partial correlation between vegetation coverage and various climatic factors. First, calculate the simple correlation coefficient, and then obtain the partial correlation. Correlation coefficient. The correlation coefficient between vegetation coverage and temperature or rainfall is calculated as follows: degree or precipitation, x is the average value of vegetation coverage for each month/year, and y is the average temperature or rainfall for each month/year. Value, i is the number of samples.
The partial correlation coefficient between vegetation coverage and temperature based on rainfall, the temperature-based vegetation coverage and the partial correlation coefficient of rainfall are calculated as follows: variable January variable 2, variable 2 and variable 3, variable 1 and variable 3 coefficient.
Based on the existing vegetation coverage, rainfall and temperature data, the partial correlation coefficient between the coverage based on the pixel and the rainfall and temperature is calculated on the annual and monthly scales, and the relationship between coverage and climatic factors is analyzed. When the annual inter-annual vegetation coverage is related to the climate, the annual vegetation coverage is selected by the annual maximum vegetation coverage of the monthly coverage synthesis (MVC), the temperature is the annual average temperature, and the rainfall is Annual total rainfall; when the relationship between vegetation coverage and climate in each month of the year is analyzed by month, the monthly coverage of each year is calculated based on the multi-year average of different months of vegetation coverage and different climate indicators from 2001 to 2010. The partial correlation coefficient between the coverage and the mean temperature and rainfall of each month; in addition, in order to study the time lag of the impact of climatic factors, the coverage of the year from February to December and the climatic factors of the previous month (1-11) were calculated. Partial correlation coefficient between months).
3 Results and discussion 3.1 Spatial pattern of multi-year average vegetation coverage in Inner Mongolia From the spatial distribution of average vegetation coverage in 2001-2010, it can be seen that (a), the vegetation coverage in Inner Mongolia is generally high in the east and low in the west, decreasing from east to west. Characteristics. The 10-year average vegetation coverage of the study area is 0.38, of which the forest ecological area has the highest average coverage, the grassland ecological area is the second, the desert ecological area is the lowest, and the spatial difference is obvious.
The 10-year average vegetation coverage of the forest ecological zone reached 0.57, mainly including the Daxinganling forest area on the Hulunbeier Plateau and the Horqin Sandy Land on the Xiliao River Plain. Located in the Daxinganling forest area in the middle and high latitudes of Eurasia, the vegetation consists mainly of cold-temperate coniferous forests with a coverage of 0.60.7. The Horqin Sandy Land in the transitional zone between the Northeast Plain and the Inner Mongolia Plateau is China. The main agro-pastoral ecotone in the north is farmland, which is the main vegetation type in the region. The annual vegetation covers the grassland ecological zone in the temperate semi-arid climatic zone. The average vegetation coverage in the 10-year period is 0.4. Inner Mongolia grassland is the main body of temperate grassland in China. Water gradient, from the east to the west, there are meadow grasslands with perennial xerophytes and mesophytes, typical grasslands with dry perennial herbaceous plants, and perennial dwarf herbaceous plants with stronger drought. In the desert grassland, the vegetation coverage showed a corresponding decreasing trend, which was 0.40.45, 0.350.4 and 0.30.35 respectively. It is located in the desert ecological zone in western Inner Mongolia, and belongs to the Asian desert plant area in the geographical area. The degree is only 0.16. There are two subtypes of grassland desert and typical desert from the southeast to the northwest in the ecological zone. The vegetation consists of dry and super The Hetao Plain, which is located at the junction of grassland and desert, is artificially irrigated by the Yellow River, and a large number of flowing and semi-mobile dunes are reclaimed into farmland, resulting in higher vegetation coverage, mostly 0.40.5. Average vegetation from the study area. The variation of coverage can be seen as (b), the change rate of vegetation coverage in Inner Mongolia from west to east is 0.2/10°N, and it has obvious longitude zonal (R2=0.93, P<0.001). 97°106°N basically corresponds to the desert ecological zone in space, and the vegetation coverage changes from west to east. The trend of vegetation coverage in the Hetao Plain at 106°107°N is abruptly increased due to the positive disturbance of human activities; 107°N In the east of the region, vegetation coverage increased with the increase of longitude.
3.2 Interannual Variability and Spatial Pattern of Vegetation Coverage in Inner Mongolia From 2001 to 2010, the vegetation coverage of forest, grassland and desert ecological regions in Inner Mongolia showed an overall upward trend, with the most obvious increase trend of desert ecoregions, with an average of about 0.01/10. Year (a); the increase rate of grassland ecological zone is the smallest, only 0.0004/10 years; the average rate of forest ecological zone increase is about 0.002/10 years. The significance test for the rate of change of vegetation coverage in different ecological areas showed that (b) the vegetation coverage in the whole study area increased significantly and significantly increased sleep and significantly increased D. No significant change D significantly reduced significantly significantly 0.7-□ extremely significant Decrease ia" □ significantly reduce 0.6-, □ not significant change ', /. □ significantly increase; > 0.5 - pole Pan-plus one K0.3-0.2-300km forest area grassland area desert area 2001-2010 Inner Mongolia Autonomous Region year Vegetation coverage change rate (a), area percentage (b) and spatial distribution (c) of the three divisions at each significant level and year-to-year variation of vegetation coverage (d) plus area of ​​11.25% and 29.13 respectively of the total area %, the sum of the two is greater than the sum of the area of ​​vegetation coverage that is significantly reduced and significantly reduced (7.65% and 26.61%). The area of ​​vegetation coverage in the forest ecological area changed very significantly, but the area of ​​significant increase and significant reduction was equivalent, accounting for 31.84% and 33.13% of the area, respectively. The vegetation coverage in the grassland area increased significantly and significantly increased. The area ratio is 15.59% and 25.23%, respectively. The sum of the two is slightly larger than the sum of the extremely significant reduction and the significant reduction of the area ratio (13.08% and 27.29%); the area of ​​vegetation coverage in the desert area is the largest, accounting for the area of ​​the area. 30.33%, another 17.13% of the area showed a very significant increase, while the area of ​​extremely significant reduction and significant reduction was only 8.35% and 19.42%. Spatial distribution (c), the region with extremely significant increase in vegetation coverage during the 10-year period It is distributed in the Helan Mountain, the western part of the Mu Us Sandland and the northwestern part of the Ordos Plateau, in the northwest part of the Hunshandake Sandy Land, in the northwest of the Xilin Gol Plateau and in the western part of the Hulunbeier Plateau. The areas with significantly increased vegetation coverage are mainly distributed in the central part of the central Gobi. , the southern margin of the Badain Jaran Desert, the central and eastern parts of the Mu Us Sandy Land, the southern margin of the Kubuqi Desert, and the north of the western part of the Yinshan Mountains. The Mochuan Plain, the north of the Hunshandake Sandy Land and the southwestern part of the Xilin Gol Plateau, the eastern part of the Horqin Sandy Land, the eastern part of the Hulunbeier Plateau and the Daxinganling in the northeastern part of the country; the regional distribution of vegetation coverage is significantly and significantly reduced, except for a few It is scattered around the edge of the Alashan Plateau and part of the Daxinganling Mountains in the northeast. It is mainly distributed along the southern edge of the Wulanchabu Plateau-浑善达克沙地--the southeastern part of the Xilin Gol Plateau.
The annual average vegetation coverage of the three ecological zones in Inner Mongolia from 2001 to 2010 is shown (d). The highest average vegetation coverage in the forest area occurred in 2008, which was 0.59, and the lowest value appeared in 2005, which was 0.55. The average vegetation coverage in the grassland area was the highest in 2008, 0.42, and the lowest in 2001 was 0.36; the average vegetation in the desert area The degree of coverage is not obvious between years, and it fluctuates around 0.16.
3.3 Correlation between vegetation coverage and climatic factors in Inner Mongolia 3.3.1 Spatial pattern of rainfall and temperature distribution in Inner Mongolia Average annual average temperature in Inner Mongolia from 2001 to 2010, annual rainfall (a) and annual average in Inner Mongolia Autonomous Region from 2001 to 2010 The spatial distribution of temperature (b) spatial distribution of annual rainfall is shown. The average annual rainfall in Inner Mongolia in the past 10 years is 294.1mm, and the annual average temperature is 4.4oC. The spatial difference of climatic factors in the study area is obvious: the rainfall shows a trend from southwest to northeast (a), but the temperature decreases from southwest to northeast. Trend (b).
The average annual rainfall in the forest ecological zone is 402.8mm in 10 years, and the annual average temperature is lower, about 2oC. It is located in the Daxinganling forest area in the semi-humid climate zone. The winter is cold, dry and long, and the summer is warm, humid and short. The annual and annual average temperature fluctuation ranges are 350480 oC, respectively. It has obvious cold-temperate continental monsoon climate characteristics; the Horqin sandy land in the transitional area of ​​semi-humid and semi-arid climates has abundant rainfall, and the temperature is higher than the Xing'an Mountains. The area is high, the annual rainfall and annual average temperature fluctuation range are 300450mm and 57oC respectively. The grassland ecological zone is located in the semi-arid climate zone. The average annual rainfall is 301.1mm in 10 years, and the annual average temperature is 2.6oC. Grassland ecological zone The distribution area is wide, and the annual rainfall and annual average temperature in the region also show gradient changes in the southwest-northeast direction. The fluctuation range is 200350 oC. The Inner Mongolia grassland ecological region is a typical temperate continental monsoon climate, and the winter is affected by the high latitude inland bias. The influence of the north wind prevails on the polar continental air mass, which is cold and dry; in the summer, it is affected by the polar ocean air mass or the transgender tropical marine air mass, and the east and southeast winds prevail. Warm and rainy, rainy and hot in the same season. The four seasons are distinct, and about two-thirds of the precipitation is concentrated in the summer. The water and heat are better matched in time, which is conducive to the growth of pasture.
Located in the Inner Mongolia desert eco-region in the hinterland of the Asian continent, the average annual rainfall is 178.3mm in 10 years, and the annual average temperature is higher, about 7.8 oC. The annual rainfall in most areas of the ecological zone is below 150mm. The average temperature is above 7oC. The desert ecoregion is a typical temperate arid monsoon climate. It is controlled by cold air masses in winter, long, cold and snowy. In summer, the subtropical high is rising northward, and the water vapor is increasing gradually in the south, but it is often due to the northerly or cold subtropical high pressure. Improper location, resulting in cold and warm air in the ecological zone, there are not many opportunities, most of the time by the westerly belt system or subtropical high pressure to cause drought and less rain.
3.3.2 The relationship between vegetation coverage and climatic factors is calculated as the time unit in the year. The partial correlation coefficients of vegetation coverage, annual rainfall and annual average temperature of each pixel from 2001 to 2010 are calculated. Statistics show that the average partial correlation coefficient between vegetation coverage and annual rainfall and annual average temperature is 0.2 and 0.04, respectively, and the area with positive correlation with annual rainfall accounts for 74.84/ of the total area, of which 30.52° The area of ​​/ and 11.93 ° / was tested by P < 0.01, and the area positively correlated with the annual average temperature accounted for 54.98 ° / of the total area, of which 23.86 ° / and 7.65 ° / area passed P < 0.05 and P < 0.01 The test indicates that the vegetation coverage in Inner Mongolia is generally more correlated with rainfall. Vegetation growth in different regions has significant spatial differences in response to rainfall (a) and temperature (b).
The average correlation coefficient between vegetation coverage and annual rainfall and annual average temperature in forest ecological region was -0.04 and 0.08, respectively. The area positively correlated with the two areas accounted for 44.68/60.97/, respectively, indicating that the vegetation growth in this area was mainly Affected by temperature. From the perspective of spatial distribution, the correlation between vegetation coverage and rainfall and temperature in Horqin Sandy Land is high, and the correlation coefficient varies between 0.20.5. Due to the relatively cold and relatively abundant rainfall in the Greater Xing'an Mountains, the correlation between annual vegetation coverage and climatic factors in Inner Mongolia Autonomous Region from 2001 to 2010 (a. Correlation with annual rainfall; b. Correlation with annual average temperature) The growth of vegetation is more affected by low temperature, so the correlation between vegetation coverage and annual average temperature is higher, and the correlation coefficient is mostly between 0.20.5. The vegetation coverage and annual rainfall in this area showed a strong negative correlation, and the correlation coefficient was mostly between -0.60, which may be due to the year with higher annual rainfall and lower average annual temperature. According to the annual rainfall and annual average temperature deviation analysis from 2001 to 2010, (a), except for 2008, the average annual rainfall of forest ecological zone is higher than the 10-year average in 2002, 2003, 2004 and 2007. The average temperature is below the 10-year average. In this region, an increase in rainfall means a decrease in temperature, thereby inhibiting vegetation growth. Therefore, the real mechanism of the negative correlation between vegetation coverage and rainfall is the process of temperature change, not the precipitation itself. Zhang Xuezhen et al found that there was a negative correlation between forest vegetation activities and precipitation changes in the humid regions of southern China from 1982 to 2006. The reason was that radiation was considered as a limiting factor for vegetation growth in the region. The increase in precipitation led to a decrease in radiation, which affected vegetation growth. Similar to the results of this study.
The average correlation coefficient between vegetation coverage and annual rainfall and annual average temperature in grassland ecological area was 0.32 and 0.09, respectively. The area positively correlated with the two areas accounted for 89.31% and 65.37% of the area, respectively, indicating that rainfall is affecting the area. The main climatic factors for internal vegetation growth. Vegetation coverage in most areas of grassland ecological area is positively correlated with annual rainfall (a), and the correlation coefficient fluctuates between 0.50.7. The heat conditions in the grassland area are relatively sufficient to meet the needs of vegetation growth, so rainfall has become the main climatic factor controlling the growth of vegetation. Previously, Zhang Geli and others pointed out that precipitation is the main factor driving the interannual variation of Hulunbeier grassland vegetation, which is consistent with the conclusion of this study; Wang Junbang et al found that at the interannual level, the effect of rainfall on vegetation growth in central Inner Mongolia is 2.8 times that of temperature. According to the results of this study, the effect of rainfall on vegetation growth in grassland ecological areas is 3.6 times higher than that of Wang Junbang. The correlation with temperature is not as obvious as rainfall. In general, the correlation coefficient is small, and the area of ​​correlation coefficient between 0.50.7 is obviously reduced, mostly in the southwest and south of the ecological zone. There is a negative correlation between vegetation coverage and annual average temperature in parts of the Hulunbeier Plateau in the north and northeast of the region, which may be due to the unsynchronized water and heat combination in these areas, or it may be due to the fact that the growth of vegetation in this area is mainly affected by rainfall. Its relationship with temperature is not very close. Sun Yanling et al. also found that there is a positive correlation and negative correlation between vegetation growth and climatic factors in Inner Mongolia grassland, and attributed to different vegetation types to obtain soil moisture, so the combination of hydrothermal The response is different.
The average correlation coefficient between vegetation coverage and annual rainfall and annual average temperature in desert ecoregion is 0.31 and -0.04, respectively. The area positively correlated with the two covers 90.53% and 47.59% of the area, respectively, indicating that rainfall is the region. The main controlling factor for vegetation growth. The western margin of the Hunshandake Sandy Land in the eastern part of the region, the northern Wulanchabu Plateau and most of the western Alashan Plateau are warm and dry, with little rainfall, and the vegetation coverage is closely related to rainfall. The correlation coefficient is distributed in 2001- In 2010, the monthly vegetation coverage of Inner Mongolia Autonomous Region was related to the climatic factors of the month (a. correlation with rainfall in the month; b. correlation with average temperature of the month) within 0.20.6. There is a negative correlation between vegetation coverage and rainfall in the northwestern Gobi region and the central Yinshan Mountains. The reason may be: Gobi desert vegetation is sparse, vegetation coverage varies little between years, so it is related to rainfall and temperature. They are weaker; the Yinshan Mountains are higher in altitude, and the low temperature is the limiting factor for vegetation growth in this area, so the correlation with rainfall is weak. Vegetation coverage in most areas of the desert ecoregion has a strong negative correlation with the annual average temperature, which may be due to the low annual average temperature and the low annual rainfall. From the 2001-2010 desert area annual rainfall, annual average temperature deviation analysis (b) can be seen: in addition to 2007, the annual average temperature is higher than the 10-year average of 2001, 2002, 2004, 2006 and 2009 The annual rainfall is below the 10-year average. The negative correlation between vegetation coverage and temperature is due to its closer relationship to rainfall.
3.3.3 The relationship between vegetation coverage and climatic factors of the month is based on the multi-year average of different vegetation months and climatic factors from 2001 to 2010. The rainfall, average temperature and corresponding monthly vegetation cover of each month are calculated. The partial correlation coefficient between degrees. Overall, the correlation between coverage and rainfall was lower than the interannual level in months, with an average of 0.14 (a), and the correlation with temperature was significantly higher than the interannual level, with an average of 0.13 (b). The statistics show that the monthly vegetation coverage of the total area of ​​94.19/ is positively correlated with the monthly rainfall. The area specific gravity of the test by P<<0.01 is 35.71% and 13.19% respectively; the monthly vegetation coverage and the current month of the total area of ​​81.69%. The average temperature was positively correlated. The area specific gravity of P<0.05 and P<0.01 was 31.21% and 9.38%, respectively. It can be seen that the correlation coefficient between monthly vegetation coverage and climatic factors fluctuated at -0.40.4. It is much smaller than the inter-annual level of -0.9 0.9, and the correlation coefficient between the monthly vegetation coverage and the rainfall and temperature in the study area is basically equal. The rainfall and temperature in Inner Mongolia have obvious changes in each month of the year, and the vegetation will only start to grow when the two conditions meet certain conditions, and the growth of vegetation will only be obvious in the appropriate rainfall and temperature range. The linear relationship, therefore, the growth of vegetation during the year is mainly affected by the combination of hydrothermal combination, and the correlation with single climatic factors is reduced.
The average correlation coefficient between vegetation coverage and monthly rainfall and monthly average temperature in the forest ecological region was 0.12 and 0.2, respectively. Unlike the interannual level, there was no strong negative correlation between monthly vegetation coverage and monthly rainfall in the forest area. The correlation coefficient for most regions is in the range of 00.3. The correlation between vegetation coverage and rainfall in Horqin Sandy Land is stronger than that in Daxing'anling Forest Region, while the vegetation growth in Daxing'anling Forest Region at high latitude is affected by temperature, and the correlation coefficient is mostly between 0.10.3.
The average correlation coefficient between vegetation coverage and monthly rainfall and monthly average temperature in the grassland ecological region is 0.15. From the spatial distribution point of view, the correlation between vegetation coverage and rainfall and temperature is obvious in the regional inter-monthly level. Longitude zonality: along the southwest-northeast line, the correlation between vegetation growth and rainfall decreases, while the correlation with temperature increases. With 115°N as the boundary, the west is dry and drier, the evapotranspiration is large, the vegetation growth is more closely related to rainfall, and the east is rich in rainfall, and the temperature becomes the limiting factor for vegetation growth during the year. The sensitivity of different months of vegetation growth to hydrothermal conditions is different, which may be the main reason for the negative correlation between monthly vegetation coverage and climatic factors in some areas of the grassland. Zhang Geli et al found that the vegetation growth of Hulunbeier grassland was more affected by temperature changes in April and May, but it was closely related to the change of precipitation in the previous month from May to August. Vegetation growth had a certain lag to precipitation changes.
The average correlation coefficient between vegetation coverage and monthly rainfall and monthly average temperature in the desert ecoregion is 0.16 and 0.04 respectively. The correlation coefficient between monthly vegetation coverage and monthly rainfall is higher in the eastern region, mostly within the range of 0.250.3. There is a certain degree of negative correlation with monthly mean temperature, probably because the vegetation growth in this area is mainly controlled by rainfall. In the month when the rainfall does not reach the highest value in the year, higher temperature will increase evaporation and reduce the amount of water available for vegetation. Therefore, it has affected the increase of vegetation coverage to some extent. The monthly vegetation coverage in some areas of the Alashan Plateau in the western region is negatively correlated with monthly rainfall and positively correlated with monthly mean temperature. This may be due to the different temperature and rainfall periods in the region, and the seasonal temperature is relatively low, limiting vegetation. Growth, so there is a phenomenon that vegetation growth is negatively correlated with rainfall and is mainly controlled by temperature. This indicates that the vegetation cover in the desert area is sensitive to rainfall and temperature, and it is only beneficial to the growth of vegetation under the appropriate combination of rain and heat.
3.3.4 The relationship between vegetation coverage and climatic factors in the previous month is to study the time-lag effect of vegetation growth on climatic factors. Based on the multi-year average of different vegetation months and climatic factors from 2001 to 2010, calculate within one year. The partial correlation coefficient between the vegetation coverage from February to December and the rainfall and average temperature in the previous month (ie, January-November). In general, the correlation coefficient between monthly vegetation coverage and the previous month's climatic factors (a and b) and its correlation coefficient with the climatic factors of the current month are basically equal, accounting for 88.60% of the total vegetation coverage and the previous month's rainfall. There was a positive correlation, in which the area specific gravity of P<0.05 and P<0.01 was 27.69% and 8.54%, respectively; the monthly vegetation coverage of 79.30% of the total area was positively correlated with the average temperature of the previous month, which was tested by P <0.01. The area proportions were 25.46% and 9.53% respectively. The correlation between different ecological regions and the climatic factors of the previous month was quite different.
The average correlation coefficient between monthly vegetation coverage and monthly monthly rainfall and monthly average temperature in the forest area is 0.12 and 0.2, respectively, which is equal to the correlation coefficient with the climatic factors of the month, and there is no obvious time lag effect as a whole. Whether the vegetation coverage in the forest area has a time-lag effect or cumulative effect on the rainfall response on a longer time scale needs further study.
The average correlation coefficient between the monthly vegetation coverage and the previous month's rainfall in the grassland area is 0.19, which is higher than the correlation coefficient with the monthly rainfall of 0.15, and the average correlation coefficient with the previous month's temperature is 0.13, which is lower than The correlation coefficient of the month's temperature is 0.15. This indicates that there is a time lag effect on the response of vegetation growth to rainfall on the interannual scale, but there is no obvious time lag effect on the response to temperature, which is related to Qu Cuiping and Xu Xu. The results of the study are consistent. The soil types in the steppe area are mainly chestnut soil and brown soil. The lower wind speed and the lower temperature in the relatively desert area enable the rainwater falling to the ground to penetrate into the deeper soil layer and not easily evaporate quickly. The soil water storage and water retention The ability is strong, and the one-pulse precipitation is converted into soil moisture, which can affect the vegetation growth in a certain period of time. This is a possible reason for the time-delay of vegetation growth in the grassland area.
The average correlation coefficient between monthly vegetation coverage and the previous month's rainfall in the desert area is 0.12, which is lower than the correlation coefficient with the monthly rainfall of 0.16, and the average correlation coefficient with the previous month's temperature is 0.12, which is higher than that. The correlation coefficient of the month's temperature is 0.04. The main soil types in the desert area can only maintain the moisture on the soil surface, and the sparse vegetation, frequent high winds and high temperature make the surface soil moisture easy to evaporate. Therefore, the desert area is relatively Vegetation growth is more sensitive to rainfall in the month than it is to the previous month. The higher correlation between the vegetation coverage in this area and the monthly average temperature in the previous month can be considered as caused by different periods of rain and heat.
4 Conclusions and discussion 4.1 Impacts of human activities on vegetation cover change and its relationship with climate factors Previously, many scholars have discussed the relationship between vegetation growth and climate factors in Inner Mongolia. Xu Xu et al. used the NDVI data of NOAA/AVHRR to retrieve the vegetation coverage, and studied the relationship between vegetation coverage and climatic factors in the temperate steppe region of Inner Mongolia from 1982 to 1999. It is believed that the average vegetation coverage of the temperate grassland in Inner Mongolia and the same period of rainfall have There is a strong positive correlation, and the average correlation coefficient reaches 0.646. Sun Yanling et al. used the NDVI data of GIMMS to study the relationship between vegetation cover change and rainfall in Inner Mongolia from 1982 to 2000, and considered that the water condition is the restriction of vegetation growth in Inner Mongolia. Factor, the correlation coefficient between annual vegetation NDVI and annual rainfall is 0.64. The results of this study also show that the vegetation coverage at the interannual level in Inner Mongolia is very sensitive to annual rainfall. However, unlike the above studies, this study The average correlation coefficient between vegetation coverage and rainfall is lower than the above results in the grassland area (0.32) or the entire study area (0.2). Although metadata selection, analytical indicator selection, and experimental error may all cause differences in results, differences in study periods may be the main cause of large differences in correlation coefficients.
Since the beginning of the 21st century, the state has implemented a series of ecological restoration projects such as the treatment of Beijing-Tianjin sand source, returning farmland to forests and grasses, returning grazing to grassland, and enclosing and transferring, and Inner Mongolia is one of the key implementation areas of these projects. In the period of 2001-2010, the increase of human activity intensity has an important impact on the temporal and spatial evolution of vegetation cover, and at the same time, the inter-annual vegetation coverage of the study area is less sensitive to changes in climatic factors, thus making the two The correlation coefficient decreases.
Zhang Baoqing et al. have similar conclusions on the temporal and spatial variation monitoring of vegetation cover in the Loess Plateau from 1982 to 2009. In the 1982-1998 period, the annual average NDVI of the study area showed a good correlation with temperature and precipitation, ie vegetation cover with temperature and precipitation. Ups and downs fluctuated and fluctuated. In the period of 1999-2009, the implementation of large-scale vegetation construction promoted the restoration of vegetation in the area. The annual average value of NDVI in the Loess Plateau increased significantly, but at the same time, the sensitivity of vegetation cover to natural factors such as climate was reduced.
Similar to many research results, this study found that the vegetation of some major dust sources (such as Mu Us Sandland and Horqin Sandy Land) in Inner Mongolia continued to improve during 2001-2010, planting trees, grass grazing, and animal husbandry. The widespread implementation of sand restoration measures played an important role in this process; at the same time, the total number of annual livestock in Inner Mongolia rose sharply after 2000, from 73.05 million in 2000 to 10798.5 in 2010. In the past 10 years, it has increased by 47.9% (Inner Mongolia Statistical Yearbook 1986-2010). The increasing grazing pressure has caused the vegetation coverage in most parts of the central part of the grassland to decline in the past 10 years.
It can be seen that between 2001 and 2010, human activities have an important impact on the spatial and temporal evolution of vegetation coverage in Inner Mongolia, and this influence weakens the dependence of vegetation growth on climatic factors, resulting in a correlation coefficient between the two. The results of the study are low. However, the quantitative assessment of the relative contribution of human activities to the temporal and spatial evolution of vegetation cover in Inner Mongolia remains to be further explored.
4.2 Main conclusions Based on the MODIS-NDVI remote sensing data, this paper inverts the vegetation coverage in Inner Mongolia from 2001 to 2010, and combines the rainfall and temperature data of the same period to analyze the forest ecological zone and grassland ecological zone from different spatial and temporal scales. Interannual and seasonal changes in vegetation in desert ecoregions respond to climate change. The conclusions are as follows: the spatial coverage of vegetation coverage in Inner Mongolia is significantly different, and it is generally characterized by east high and low west and decreasing from east to west. The rate of change from west to east is 0.2/10°N. Research area 2001-2010 The average vegetation coverage is 0.38, of which the average coverage of forest ecological zone is 0.57, the grassland ecological zone is 0.4, and the desert ecological zone is 0.16. From 2001 to 2010, the vegetation coverage of Inner Mongolia showed an upward trend. The rate is desert ecological zone > forest ecological zone > grassland ecological zone.æ€»ä½“è€Œè¨€ï¼Œç ”ç©¶åŒºå†…æ¤è¢«è¦†ç›–度æžæ˜¾è‘—å¢žåŠ å’Œæ˜¾è‘—å¢žåŠ çš„é¢ç§¯åˆ†åˆ«å 总é¢ç§¯çš„11.25%å’Œ29.13%，二者之和大于æ¤è¢«è¦†ç›–度æžæ˜¾è‘—å‡å°‘和显著å‡å°‘çš„é¢ç§¯æ¯”例之和，åŽè€…分别为7.65%å’Œ26.61%.从年际水平æ¥çœ‹ï¼Œç ”究区æ¤è¢«ç”Ÿé•¿æ€»ä½“上与é™é›¨é‡çš„å…³ç³»æ›´åŠ å¯†åˆ‡ï¼Œä½†å˜åœ¨æ˜¾è‘—的空间差异：在森林生æ€åŒºä¸»è¦å—温度影å“，在è’æ¼ ç”Ÿæ€åŒºä¸»è¦å—é™é›¨é‡å½±å“，而è‰åŽŸç”Ÿæ€åŒºä¸ŽäºŒè€…å‡æœ‰å…³ç³»ï¼Œä½†å¯¹é™é›¨é‡çš„ä¾èµ–性更强；从月际水平æ¥çœ‹ï¼Œç ”究区æ¤è¢«ç”Ÿé•¿æ€»ä½“上与é™é›¨é‡å’Œæ¸©åº¦éƒ½å‡æœ‰æ£ç›¸å…³æ€§ï¼Œåœ¨æ£®æž—å’Œè’æ¼ ç”Ÿæ€åŒºæ¤è¢«ç”Ÿé•¿åˆ†åˆ«å—温度和é™é›¨é‡çš„å½±å“较大，而在è‰åŽŸç”Ÿæ€åŒºåˆ™ä¸ŽäºŒè€…的相关性å‡è¾ƒé«˜ï¼›ä¸Žå¹´é™…水平相比，在月际水平上æ¤è¢«ç”Ÿé•¿ä¸Žå•ä¸€æ°”å€™å› å的相关性é™ä½Žï¼Œè€Œæ›´ä¾èµ–于水çƒç»„åˆçš„å…±åŒä½œç”¨ã€‚å…¶ä¸ï¼Œè‰åŽŸåŒºæœˆæ¤è¢«è¦†ç›–度对é™é›¨é‡çš„å“应å˜åœ¨æ—¶æ»žæ•ˆåº”。
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