2010–2017年藏北高寒退化草地禁牧恢复效果评价
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摘要
针对退化草地进行禁牧封育是现行草原生态保护奖励补助政策的重要举措之一,科学地评估禁牧成效并探究其对气候的响应机制对于未来禁牧工程布局优化具有重要意义。本研究以西藏那曲和阿里两地市2 413块禁牧地为对象,计算草原生态保护补助奖励政策实施前(2000–2009年)和政策实施后(2010–2017年)禁牧草地归一化植被指数(normalized difference vegetation index, NDVI)均值差(ΔMean_(NDVI))和趋势差(ΔTrend_(NDVI))来评价禁牧有效性,并结合两个时段内生长季温度和降水的均值差和趋势差探讨气候变化对高寒草地禁牧效果的影响。研究发现:(1)前后两个时段禁牧有效ΔMean_(NDVI)> 0和ΔTrend_(NDVI)> 0地块比例分别为67.3%和40.5%,禁牧出现负面影响,ΔMean_(NDVI) <0和ΔTrend_(NDVI) <0的地块比例分别为22.5%和31.0%;禁牧无显著影响,ΔMean_(NDVI)≈0和ΔTrend_(NDVI)≈0的地块比例分别为10.2%和28.4%;(2) NDVI与生长季温度和降水相关性系数平均值分别0.27和0.37;前后两个时段NDVI与生长季温度相关性没有显著改变;草原和荒漠区禁牧地NDVI与生长季降水相关系数从0.236提升到0.370。(3)线性回归和方差分析均显示生长及温度和降水对NDVI影响显著但解释度很低(约2%)。本研究表明,藏北高原围栏禁牧对草地植被恢复有效性及其效益规模具有明显的空间异质性。因此,建议禁牧已出现负面影响的地块应及时开放为放牧地,有效的地块未来应探明其最佳禁牧年限,无效的地块应探讨除气候变化以外人文因素的影响。
Allowancing and awarding local herdsmen families for degraded grassland restoration, via grazing exclusion, is an important eco-compensation policy in China. A scientific assessment on how grazing exclusion and climate change affects ecological restoration is essential for improving the layout of enclosures at a broad spatial scale in the future. In this study,we focused on 2 413 enclosures that are distributed in Nagchu and Nagri, Tibetan Autonomous Region, China, and calculated the differences in the means and trends of the Normalized Difference Vegetation Index(DMean_(NDVI) and DTrend_(NDVI)) in these exclosures between two subperiods, prior to(2000–2009) and after(2010–2017) the current policy of payment for grassland restoration by fencing. The differences in the means and trends of the growing season temperature(GST) and precipitation(GSP) between corresponding subperiods were also calculated, for better understanding of how climatic variables affect NDVI change in fenced grasslands over time and across space. Our results show that exclosures with aΔMean_(NDVI) > 0 and ΔTrend_(NDVI) > 0, account for 67.3% and 40.5% of all exclosures on the Northern Tibetan Plateau,respectively, indicating that grazing exclusion has effectively restored these degraded grasslands. The proportion of exclosures with a ΔMean_(NDVI) < 0 and ΔTrend_(NDVI) < 0 are 22.5% and 31.0%, respectively, indicating that grazing exclusion has negative influences on those fenced grasslands. A ΔMean NDVI ≈ 0 and a ΔTrend_(NDVI) ≈ 0 can be found in 10.2% and 28.4% of exclosures, respectively, indicating that grazing exclusion by fencing has no evident influence there. In addition, we found that the NDVI of fenced grasslands were correlated with the GST(r = 0.27) and GSP(r = 0.37), and the correlation coefficients of NDVI with GST did not change between the two subperiods. In alpine and desert steppe zones, the correlation coefficients of NDVI with GSP increased from 0.236 in 2000 –2009 to 0.370 in 2010 –2017. The results of the linear regressions and analyses of variance further indicate that the GST and GSP have significant influences on the NDVI but can explain only a small fraction of its variance(approximately 2%). In summary, the effectiveness of grazing exclusion on the restoration of degraded grasslands showed a heterogeneous pattern over space across the Northern Tibetan Plateau.Therefore, we suggest opening the exclosures where fencing has negative influences, clarifying the optimal time for grazingexclusion where fencing has positive influences, and exploring the potential effects of other anthropogenic factors where fencing has no evident influences on grassland restoration.
Allowancing and awarding local herdsmen families for degraded grassland restoration, via grazing exclusion, is an important eco-compensation policy in China. A scientific assessment on how grazing exclusion and climate change affects ecological restoration is essential for improving the layout of enclosures at a broad spatial scale in the future. In this study,we focused on 2 413 enclosures that are distributed in Nagchu and Nagri, Tibetan Autonomous Region, China, and calculated the differences in the means and trends of the Normalized Difference Vegetation Index(DMean_(NDVI) and DTrend_(NDVI)) in these exclosures between two subperiods, prior to(2000–2009) and after(2010–2017) the current policy of payment for grassland restoration by fencing. The differences in the means and trends of the growing season temperature(GST) and precipitation(GSP) between corresponding subperiods were also calculated, for better understanding of how climatic variables affect NDVI change in fenced grasslands over time and across space. Our results show that exclosures with aΔMean_(NDVI) > 0 and ΔTrend_(NDVI) > 0, account for 67.3% and 40.5% of all exclosures on the Northern Tibetan Plateau,respectively, indicating that grazing exclusion has effectively restored these degraded grasslands. The proportion of exclosures with a ΔMean_(NDVI) < 0 and ΔTrend_(NDVI) < 0 are 22.5% and 31.0%, respectively, indicating that grazing exclusion has negative influences on those fenced grasslands. A ΔMean NDVI ≈ 0 and a ΔTrend_(NDVI) ≈ 0 can be found in 10.2% and 28.4% of exclosures, respectively, indicating that grazing exclusion by fencing has no evident influence there. In addition, we found that the NDVI of fenced grasslands were correlated with the GST(r = 0.27) and GSP(r = 0.37), and the correlation coefficients of NDVI with GST did not change between the two subperiods. In alpine and desert steppe zones, the correlation coefficients of NDVI with GSP increased from 0.236 in 2000 –2009 to 0.370 in 2010 –2017. The results of the linear regressions and analyses of variance further indicate that the GST and GSP have significant influences on the NDVI but can explain only a small fraction of its variance(approximately 2%). In summary, the effectiveness of grazing exclusion on the restoration of degraded grasslands showed a heterogeneous pattern over space across the Northern Tibetan Plateau.Therefore, we suggest opening the exclosures where fencing has negative influences, clarifying the optimal time for grazingexclusion where fencing has positive influences, and exploring the potential effects of other anthropogenic factors where fencing has no evident influences on grassland restoration.
引文
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