我国草地面积有多大?

How many areas of grasslands are there in China?

草地是不可或缺的自然资源,但关于我国草地面积到底有多大存在很大争议.本文主要利用植被分布与降水之间的相关关系来探讨我国草地的分布和面积.归一化植被指数(NDVI)数据能够很好地反映植被的覆盖状况,与降水之间也存在良好的相关关系,因此可以通过建立已知草原地区的降水与NDVI的关系来反演草地的分布和面积.利用此方法及与遥感数据相匹配的过去30年(1982~2011年)的平均降水量数据,估算得到我国的草地总面积约为293×104 km2.将研究期间的降水数据每5年求其平均,估算得到我国草地面积的年际变化并不显著,变动于290×104~295×104 km2.

Grassland is the indispensable natural resources which covers a large area of terrestrial China and plays an important role in the ecological construction,ecological security,and food security of the country.The size of the grassland,i.e.,grassland area,is its important natural property,which is the basis to estimate total grassland biomass and productivity.Therefore,an accurate estimate of the grassland area is of great importance in reasonable protection and utilization of the grassland resources.However,the estimated area of China’s grasslands varies greatly among different studies due to different data sources and grassland definitions.Climate determines geographical distribution of natural vegetation,and thus the distribution of a vegetation type can be retrieved based on regional characteristics of climate.In the semi-arid and arid regions of northern China,precipitation is the major limiting factor of vegetation distribution,especially for grassland.On the other hand,normalized difference vegetation index（NDVI）data well reflects the vegetation coverage and is strongly correlated with precipitation in the arid and semi-arid regions.Therefore,the distribution and area of grasslands can be retrieved using the relationship between precipitation and NDVI in the recognized regions of the grassland.For these reasons,we first roughly divide grasslands into two categories according to their concentrative degree of distribution：contiguous temperate grassland（northern grassland）and discrete subtropical grassland（southern grassland）,and then quantify the range and area of the grasslands in these two different regions.Because the grassland distribution in the southern regions cannot be directly determined by climatic variables,we use the vegetation map of China to determine the range and area of the grasslands.For the northern grasslands,we first determine the thresholds of mean precipitation in the ecotones between grasslands and forests and between grasslands and desert vegetation,based on vegetation distribution in the vegetation map of China,and then use the precipitation thresholds to make sure the distributional limits of the grasslands.By conducting a regression analysis between average precipitation and the NDVI average value of vegetation growing season（May–September）over the past 30 years（1982–2011）,we estimate the NDVI threshold for grassland and retrieved its potential distribution.The precipitation and NDVI thresholds for the grassland distribution in our study area are estimated to be 102–437mm and 0.12–0.61 unit,respectively.Finally we document an estimate of the total grassland area of 293×10^4km^2 in China,with a small variation of 290×10^4–295×10^4km^2 over the past 30 years.Uncertainties remain in our estimations because the area might be overestimated or underestimated at regional scale.For instance,a part of deserts in northern Xinjiang was falsely determined as sparse grassland,because the NDVI values of these deserts exceeded the thresholds for grassland;and in the northern Tibetan Plateau,the sparse desert grassland,showing NDVI values lower than 0.05,was falsely determined as desert.Nevertheless,our estimate is generally consistent with that from the vegetation map of China and land cover data of China in 2010.Overall,this study develops a useful methodology which estimates the range and area of the grasslands based on the thresholds of precipitation and NDVI values and generates reliable estimates of the total grassland area in China.