| Forest above ground biomass has great research and application value in forest ecological system.Loess Plateau where soil erosion is serious and the ecology environment is badly interrupted.This study is aimed to provide some references for reconstruction of forest vegetation and improvement of ecological environment in the arid and semi-arid areas of Loess Plateau.In this paper, we estimate the above-ground biomass of the broadleaf forest using remote sensing empirical method, analyze the relationship between above ground biomass and water-heat condition, discuses the factors those influence the spatial distribution of the biomass. The results show that in the Yanhe basin, the average above-ground biomass of the broadleaf forest is about 34.45t/hm2 and the maximum is 209.2t/hm2. Factors such as climate, terrain, species and human activities affect the development of the vegetation. Among these factors, water condition is the key parameter that influences the vegetation status in this area. It is also shown that the area of human activities is related to the location with high biomass. The passive effect of human activities to eco-environment is not quite obvious in this area.Based on light-use efficiency model, a net primary productivity model was proposed. This model considered the difference of ET.The annual sum of NPP in the Loess Plateau was calculated from 2000 to 2008, the result showed that the spatial pattern of NPP in Loess Plateau indicated that NPP high values of increase distributed in the south and east corners, then rapidly decreased from the northwest corners to the southeast center. The trend is closely related with the differences of hydrothermal conditions and the law of zonal differentiation.
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