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.