西北地区植被净初级生产力估算模型对比与其生态价值评价

净初级生产力(NPP)是绿色植物在单位时间和单位面积上所能产生的有机干物质总量,是地球碳氧循环研究的关键问题之一。通过使用不同模型进行NPP物质量测算并换算为价值量,分析2000—2009年西北地区NPP价值的时空分异特征,了解西北地区近年植被生态效应的变化格局与趋势。结果表明:(1)CASA模型估算出耕地、草地、常绿针叶林的平均NPP分别为4.15 t.hm 2、4.87 t.hm 2和7.35 t.hm 2,统计模型中综合模型与北京模型和CASA模型估算结果最为相近;(2)研究区NPP价值随时间序列递增,耕地与建设用地的增幅集中在2000—2003年,而草地与未利用地增幅集中于2006—2009年,草地增长最快,2009年达7.91 107万元;(3)以县域为单元分析NPP价值空间差异,空间分布的不对称性、趋向均值的偏离程度有所增加,但高值集聚特征也有所加强;(4)NPP价值的空间集聚变化有低值快速升高、高值继续升高、较高值出现下降3种趋势,青藏高原与黄土高原NPP价值增速最快。NPP价值升高趋势比下降趋势更明显,反映了西北地区生态环境呈现整体好转、局部恶化的态势。

Contrast in vegetation net primary productivity estimation models and ecological effect value evaluation in Northwest China

Net primary productivity (NPP) is the total dry organic matter produced by green plants in unit time and area. It is one of the key parameters used in carbon and oxygen cycle research. Not much NPP research has covered the whole of Northwest China and model estimates of NPP have also varied considerably. Photosynthesis is the main mode vegetation generates dry mater and is the core link between natural carbon and oxygen cycles. The synthesis of NPP is accompanied by energy fixing, carbon sinking and oxygen production; which are three indispensable ecological processes for human survival and development. By estimation of NPP based on photosynthetic equation, ecological effects via the synthesis of NPP can be effectively and quantitatively evaluated. This study compared different NPP evaluation models and analyzed (at both temporal and spatial scales) different characteristics of NPP for the period from 2000 to 2009 in Norwest China. The study showed that the Comprehensive model and the Beijing model most closely with the CASA model estimates. Calculated mean NPP for croplands, grasslands and evergreen needle leaf forests were 4.15 t·hm^-2, 4.87 t·hm^-2 and 7.35 t·hm^-2, respectively. Human activity had enhanced vegetation in dry areas not to completely depend on rain. This implied that some differences existed in the estimation of the NPP by statistical models. Estimation results of NPP values by different methods through evaluating dry matter production, carbon fixing and oxygen release functions showed that NPP values increased with time. Also NPP values for grass increased at the fastest rate in the study area, which was 7.91×1011 Yuan in 2009. NPP value recalculated based on county unit showed three spatial-temporal patterns: in the high value area, it first decreased then increased; in low value area, it first increased then decreased; NPP value gradually diversely distributed. The spatial agglomeration tendency of change in NPP showed that the lowest value units rose rapidly, the highest value units rose gradually and the medium units dropped gradually. The asymmetry and deviation from the mean intensified and the agglomeration of high values also strengthened. The rise in NPP was more apparent than the decline, which lead to smaller NPP values difference from the east to the west and higher NPP values difference from the south to the north. The spatial decentralization development of counties favored a better ecological environment, although with local deterioration. For the weak ecological environment in Northwest China, it was necessary to partition NPP into ecological services value estimation and ecological safety evaluation at regional scale. The combination of direct agricultural income and potential ecological assets was more effective for sustainable regional development planning and control.