2000—2015年宁夏草地净初级生产力时空特征及其气候响应

Spatiotemporal characteristics of grassland net primary production in Ningxia Province from 2000 to 2015 and its response to climate change

草地是宁夏陆地生态系统的重要组成部分,估算其净初级生产力(NPP)对宁夏草地可持续利用与管理至关重要。采用MODIS数据和CASA模型对2000—2015年间宁夏草地生态系统NPP进行了估算,通过一元线性回归趋势分析、Hurst指数等方法研究草地NPP的时空变化规律及未来演变趋势,并分析草地NPP与气象因子的相关性。结果表明:(1)基于CASA模型的宁夏草地NPP模拟精度高,其估算值与实测多年草地NPP均值具有良好的线性关系(R=0.93,P<0.01),与MOD17产品的草地NPP空间分布基本一致。(2)近16 a宁夏草地年均NPP为148.28 g C m^(-2)a^(-1),且存在波动上升的趋势,其线性增长率为3.84 g C m^(-2)a^(-1)(P<0.01)。(3)宁夏草地NPP整体处于上升趋势,草地NPP增长的草地面积达98%,且其增率自南向北递减;宁夏草地NPP的Hurst指数在0.27—0.81之间,均值为0.53,大部分草地的NPP变化趋势具有较强同向持续性。(4)在年时间尺度上,宁夏草地NPP主要受降水量的影响,与气温的相关性较弱;在月时间尺度上,生长季草地NPP与月总降水量的相关性高,且不存在时间滞后响应现象,而与月均温的响应则存在1个月的时间滞后性,宁夏大面积分布的干草原与荒漠草原NPP对气温响应滞后是导致这一现象发生的主要原因。

Net primary productivity(NPP) is an important indicator of terrestrial ecosystems and accurately estimating NPP is fundamental in terrestrial carbon cycle research. Ningxia Province is dominated by grassland and the estimation of grassland NPP can provide a scientific basis for the sustainable use of grassland resources and the improvement of grasslandmanagement. In this study,the grassland ecosystem NPP in Ningxia Province was estimated using a Carnegie-Ames-Stanford Approach(CASA) model and Moderate Resolution Imaging Spectroradiometer(MODIS) data from 2000 to 2015. To study the spatiotemporal characteristics and future evolution trend of grassland NPP,the correlation between the grassland NPP and meteorological factors was analyzed using a simple linear regression model and the Hurst index. The results showed that the CASA model accurately simulated the grassland NPP in Ningxia Province. The correlation coefficient between estimated NPP by the CASA model and measured NPP was 0. 93(P < 0. 01),and the spatial characteristics of the estimated NPP image were similar to that of the NPP image of MOD17. In the whole province,the average annual grassland NPP estimated by the CASA model was 148.28 g C m^-2a^-1during 2000-2015. Meanwhile,the average annual grassland NPP increased with fluctuations during the past 16 years and the linear growth rate was 3.84 g C m^-2a^-1(P<0.01). Regarding the spatial variation of grassland NPP in Ningxia Province,the NPP in 98% of the grassland increased during the past 16 years and this increasing rate decreased from South to North. The Hurst index of grassland NPP in the past 16 years ranged from 0.27 to 0.81 and the mean value was 0.53,which indicated the grassland NPP will maintain the same trend of change in the future. At an annual scale,the average grassland NPP in Ningxia Province was mainly affected by annual precipitation,and had a weak correlation with the average annual temperature. At a monthly scale,there was also a weak relationship between the grassland NPP and temperature in the growing season;however,the monthly total precipitation exerted an important influence on the grassland NPP accumulation. Furthermore,there was a one-month time lag in the monthly NPP response to mean temperature,but this phenomenon did not exist between the monthly NPP and total precipitation. The main reason for this phenomenon was possibly the hysteresis response to temperature in the steppe and desert grasslands,which were the main types of grassland in Ningxia Province.