| The richness and specificity of Xinjiang's grassland have distinct characteristics in China's grassland.At the same time,its economic,ecological and social functions have practical special significance for the development and stability of Xinjiang.However,due to the vast grassland in Xinjiang,seasonal combination management of the plain and upland meadow,the control of natural conditions and the annual changes of climatic conditions,especially the cyclical impact of disastrous climate,the development of animal husbandry is difficult to stabilize.In addition,the grassland ecological environment has recently been degraded due to various reasons such as indiscriminate reclamation and digging,overgrazing,pest damage,etc.Therefore,it is an objective need to carry out grassland monitoring and timely grasp the dynamic changes and development trends of grassland in order to maintain Xinjiang's ecological barrier,develop modern animal husbandry and build a socialist harmonious society in the future.Based on Landsat TM/OLI,MOD13Q and and other relevant measured data,this paper selects Urumqi County,which is rich in grassland resources in the middle part of the northern slope of Tianshan Mountains,as the research area,and uses remote sensing and field measured means to monitor the main ecological parameters of grassland,including the spatial pattern of natural grassland,vegetation coverage,grassland growth,grassland degradation,biomass,grassland ecological service function and other indicators of grassland dynamic change.This paper analyzes the dynamic change characteristics and trend of grassland in Urumqi County in the past 25years,analyzes the influence of natural factors on the grassland vegetation coverage in this region by constructing a geographic weighted regression model(GWR),and discusses the influence of grazing methods and water resources conditions on the grassland vegetation coverage based on the statistical data such as the number of stocks,underground water and surface water quantity.The following conclusions are drawn:(1)Remote Sensing Classification of Natural Grassland TypesThe grassland classification method based on expert decision-making knowledge and MSAVI index and DEM data are used to classify the six main grassland types in Urumqi County.The classification results are accurate enough to meet the utilization requirements.For mountainous plain desert areas,spectral characteristics and spatial distribution characteristics can better improve the classification accuracy.Spectral features focus the research on vegetation as a ground object,ignoring other ground object types in the research area,greatly reducing the classified target ground object types and simplifying the research process.The spatial distribution characteristics mainly enhance the differences among alpine grasses,temperate grasslands,mountain meadows and farmland with obvious elevation characteristics,and reduce the mixing between them.The classification method of expert knowledge depends on expert knowledge in related fields to a large extent.The threshold of MSAVI constructed in this study may also be adjusted according to different seasonal phases,and the altitude threshold may also be changed according to the trend of global warming.(2)Dynamic change of natural grassland and its landscape pattern evolutionBased on the dynamic changes of natural grassland in the study area in the past 25 years,it is concluded that the coverage areas of temperate desert and alpine meadow account for a large proportion of the grassland area in the study area.During this period,alpine meadow,mountain meadow and temperate grassland were in a good recovery state,but temperate meadow grassland,temperate desert grassland and temperate desert showed fewer trends.However,the cultivated land area increased by 9.81×10~3 hm~2,with a large increase.During the whole research period,the change of grassland area was obvious in 2000,and most grassland areas showed a decreasing trend,among which alpine meadow,mountain meadow,temperate meadow grassland and temperate desert grassland showed obvious changes.From the change of spatial landscape pattern,it can be seen that the fragmentation degree of grassland landscape in Urumqi County is continuously increasing,and the grassland landscape is beginning to transform to fine-grained landscape.The overall complexity of patches is increasing.The patch types of various grassland types are concentrated,and each landscape type has greater spatial natural connectivity.(3)Vegetation coverage change and its growth evaluation of natural grasslandConsidering the information distribution characteristics of low vegetation coverage area and bare land in the study area,MSAVI index is selected to calculate the grassland vegetation coverage in the study area,and the correlation coefficient is 0.762.From 1990 to 2015,the area of higher vegetation coverage area first decreased and then increased,with a general trend of decrease.During the study period,the change in 2000 was the most critical among the dynamic changes in the area covered by higher vegetation,because the area covered by higher vegetation was greatly reduced in2000.Although it showed a gradual growth trend in the later period,it still could not reach the level before 2000.From 1990 to 2015,the order of the average vegetation coverage of the six grassland areas is mountain meadow>alpine meadow>temperate meadow grassland>temperate grassland>cultivated land>temperate desert grassland>temperate desert.From the perspective of spatial clustering characteristics,the high-value and low-value areas of vegetation coverage showed basically the same layout characteristics from 1990 to 2015,i.e.the central mountainous area and the southern marginal area of the study area had high vegetation coverage,the northern flat area had low vegetation coverage,and the range of high vegetation coverage areas in the northern plain area expanded significantly from 1990 to 2015.The overall variation of grassland vegetation growth in Urumqi County is characterized by flatness,growth bias and fluctuations in preference zones.The average area of grassland growth areas of each grade(bad,poor,steady,good and better)accounted for 4.92%,12.86%,63.05%,13.65%,and 5.50%of the total monitored area,respectively.Judging from the characteristics of grassland monthly growth,grassland growth in May and June was dominated by steady preference,in July grassland growth in a steady mood,growth good and poor area are synchronously changed.In August,the area in a good growth is increasing,while in September,it shows decreasing trend.(4)Remote sensing monitoring of natural grassland degradationBased on the national standard(GB19377-2003)and the“Grading Standard for Degeneration,Desertification and Salinization of Natural Grassland",this study classifies grassland degradation according to the percentage of grassland vegetation coverage relative to that of non-degraded grassland.It is concluded that slight and moderate grassland degradation are the main types of grassland degradation in the study area from 1990 to 2015,accounting for 29.20%and 12.11%of the total grassland area.The area of seriously degraded and extremely seriously degraded grassland is relatively small,accounting for 1.43%of the total grassland area.The improved grassland area is4.07×104 hm2,accounting for 15.82%of the total grassland area.Mild and moderate degraded grasslands in temperate meadow grassland,mountain meadow and alpine meadow are relatively large,and the extremely seriously degraded parts of temperate desert,mountain meadow and alpine meadow account for about 75%of the extremely seriously degraded grassland area.The improved grassland area is very small and mainly distributed in temperate desert,temperate desert grassland and temperate grassland distribution zone.Mild to moderate degradation exists in all grassland types in Urumqi County,while the area of improved grassland is much smaller than that of degraded grassland.The center of gravity shift of degradation grade shows that the center of gravity in degraded grassland areas of various grades in Urumqi County moves in different directions,mainly because of different degrees of degradation of grassland types in the study area,while the improved grassland mainly extends to the southwest and the north part of study area,and the northern grassland gradually enters a recovery state with the expansion of the northern grazing prohibition area and the promotion of grazing control policies.From the characteristics of cold hot spots of degraded grassland,it can be seen that the aggregation characteristics of“cold spots"and“hot spots"of degraded grassland are obvious during the study period.In space,the distribution areas of degraded grassland and“cold spots"overlap,and their extension directions are consistent with the extension of degraded grassland.The coldest spots are mainly distributed in the grassy areas of the middel mountain belt,i.e.temperate grassland,mountain meadow and alpine meadow.The results showed that there was significant spatial clustering between mild and moderate grassland degradation indexes in middle pert of mountainous area.However,the"hot spots"did not gather in the distribution area of the improved grassland,but overlapped with the distribution area of the unchanged grassland,indicating that there are obvious spatial differences between the degraded grassland and the unchanged grassland pattern in Urumqi County,while the improved area is very small and the concentration is too low,and there is no clear spatial distribution characteristics.(5)Assessment of natural grassland biomass and its ecological service valueBased on the measured grassland yield data from 2011 to 2018 in Urumqi County,the change characteristics of grassland yield in this county in recent 8 years were analyzed.From the average grass yield of grassland in the grassland-livestock balance area from 2011 to 2018,it can be seen that the average grass yield of different grassland types is in the order of mountain meadow>temperate desert>temperate meadow grassland>alpine meadow>temperate grassland,with the largest fresh grass yield compared with other grassland types.According to the biomass changes over the past 8years,the biomass of mountain meadows showed an overall growth trend from 2013 to 2017,while the biomass of temperate grasslands showed little change,showing a slight growth trend on the whole,and the biomass of temperate deserts showed an overall growth trend from 2013 to 2018.According to the change characteristics of grass yield of grassland(temperate desert),although the grass yield showed a relatively large growth trend in 2014,the grass yield in the grazing-forbidden area was not ideal and tended to decrease on the whole.At the same time,the vegetation coverage also showed a decreasing trend,while the bare land area gradually increased.The actual survey records show that although grazing is controlled in the grazing forbidden area,natural factors such as water erosion,desertification and wind erosion continue to affect grassland yield.Based on the biomass data of various grassland types in Urumqi County,this paper revised the ecological service value coefficient of each grassland type and calculated the ecological service value of each grassland type on this basis.It was concluded that the total ecological service value(6 grass and cultivated land)in urumqi county from 1990 to 2015 showed a trend of increasing first and then decreasing,with little change.The order of ecological service value of different grassland types is:mountain meadow>temperate desert grassland>temperate grassland>alpine meadow>temperate meadow grassland>cultivated land>temperate desert grassland.In the past 25 years,the proportion of ecological service value of mountain meadow was more than 55%,reaching 59.92%in 2015.The ecological service value of mountain meadow is much higher than that of other grassland types,which indicates that mountain meadow,as the grassland type with the largest biomass,constitutes the main part of the ecosystem service value of grassland in Urumqi County and contributes the vast majority of the ecosystem service value.The ecological service value per unit area of each elevation zone of grassland in Urumqi County shows a distribution characteristic that it decreases gradually compared with the flat zone and increases with the elevation in the middle-high mountain zone.ESV increases significantly in 1800-2300 m and 2800-3400 m elevation zones,while ESV tends to decrease in 1400-1800m elevation zones.The spatial clustering degree of grassland ecological service value in urumqi county on the scale of 1×1km grid shows that Moran's I value of grassland ecological service value in Urumqi county was0.7926 from 1990 to 2015,showing a trend of decreasing first and then increasing.The ecological service value of the study area is high in the central mountainous area and the southern marginal area,and low in the northern flat area.The range of low ESV concentration areas in the northern plain area was significantly expanded from 1990 to 2015.The high ESV area in Urumqi County is obviously concentrated in the grass areas in the middle,south and west of the study area,including temperate desert grassland,temperate grassland,mountain meadow and alpine grassland.The grassland in this area is in good condition,with high vegetation coverage,low population density and little interference from human activities.ESV low-value accumulation areas are mainly distributed in temperate desert grassland,sparse vegetation distribution zone and the eastern part of Tuoli Village,Yongfeng Village and the northern part of Sardaban Village.According to the calculation results of individual ecological service functions of various grassland types in urumqi county,the contribution rate of mountain meadow to each ecological service function is the largest,and that of temperate desert grassland is the smallest,with the contribution rate ranking as follows:Mountain meadow>cultivated land>temperate desert>temperate grassland>alpine meadow>temperate meadow grassland>temperate desert grassland.Generally speaking,the value of grassland climate regulation,waste treatment,water conservation and soil maintenance services in Urumqi County has continued to grow.Changes in ecological service functions of mountain meadows,temperate grasslands and alpine meadows have a great impact on the value of grassland ecosystem services in the study area.During the study period,these grassland types showed high ecosystem service value and played a key role in the ecosystem balance.(6)Analysis of driving factors of natural grassland changeBased on the data of precipitation,surface temperature,air temperature,slope,elevation,soil organic matter content,pH value and salt content,the spatial correlation between grassland vegetation coverage and natural factors was analyzed by using geographic weighted regression model(GWR).The results show that precipitation,surface temperature and air temperature have great influence on grassland among natural factors,and the simulation result of geographic weighted model reaches 0.76.Slope,elevation,soil organic matter content,pH value and salt have the second effect on grassland vegetation coverage,and the spatial simulation accuracy reaches 0.53.This shows that the main natural factor affecting grassland vegetation coverage in Urumqi County is meteorological factor,and climate change plays a key role in the change of grassland growth,biomass and its ecological service function.The influence of topographic factors and soil factors on vegetation coverage is mainly manifested in the correlation between soil erosion and grassland vegetation coverage,i.e.there is slight and moderate soil erosion in the soil.The spatial distribution of soil erosion is characterized by a negative correlation between vegetation coverage and soil erosion in the study area with the increase of altitude,i.e.with the increase of vegetation coverage,the soil erosion decreases,the low-value covered areas in the north and south have large areas of light,strong and severe soil erosion,and the vegetation coverage and high-vegetation covered areas in the Zhongshan belt have large areas of moderate soil erosion.In addition to spatial data,this paper analyzes the relationship between grassland vegetation coverage and water source and grazing amount based on statistical data such as surface water quantity,underground water quantity and grazing amount.The results show that grassland vegetation coverage and underground water quantity are positively correlated,while grazing amount and vegetation coverage are negatively correlated.The year of groundwater mutation is the same as the year of vegetation coverage mutation,and the vegetation coverage also shows a decreasing trend during overgrazing.There is a negative correlation between grassland vegetation coverage and industrial pollutant emissions,sulfur dioxide emissions and solid waste emissions in social economy.Although the initial inertia is not strong,it shows that industrialization in Urumqi County has brought about one of the potential hazards of negative impacts on grassland vegetation coverage. |
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