Temporal-Spatial Characteristics and Factors Influencing Crop NPP across Northeastern China
东北地区农田净初级生产力时空特征及其影响因素分析

The farm productivity in northeastern China was conceptually divided into five classes in terms of Moderate Resolution Imaging Spectroradiometer (MODIS) net primary productivity (NPP) data products for the period 2000-2006, meteorological data, and land use data with the support of GIS technology, i.e., the highest production, higher production, middle production, lower production, and the lowest production. On the basis of this, the authors examined in detail the characteristics of spatial-temporal distribution of the NPP of the farmland across northeastern China during the period 2000-2006, and elaborated the effects of climate and topography on agricultural productivity. Results showed that the dry land over the study region accounted for roughly 89.8% of the total farmland area, and the area of paddy field took up around 20.2%. As far as the spatial distribution of NPP is concerned, the NPP of crop increased gradually from the west to the east, from the north to the south. The mean annual NPP of dry land ranged approximately from 309.8 gC/(m2·a) to 329.5 gC/(m2·a). The mean annual NPP of paddy field changed from 323.5 (m2·a) to 341.7 (m2·a). Interannual trends in mean annual NPP of both dry land and paddy field were the same, i.e., the NPP was gradually increasing but as demonstrated in this work such trends varied to certain degree in some years. In general, from 2000 to 2006, the mean NPP of farmland in northeastern China increased from 309.1gC/(m2·a) to 331.4gC/(m2·a). The correlation coefficients between NPP of dry land and paddy field, and climatic and topographical factors showed that the mean annual NPP of dry land and paddy field in the growing season suggested a marked positive correlation with precipitation, with a correlation coefficient of 0.53 and 0.37 for dry land and paddy field, respectively. The temperature of the growth season and the mean annual NPP of dry land and paddy field showed statistically significant negative correlation. As the farmlands are generally located in the Northeast China Plain with relatively low altitudes, topographic factors (e.g., elevation, slope, and aspect) had little effect on the mean annual NPP of dry land or paddy field. It can be drawn that precipitation is the most critical factor determining the NPP of farmland in northeastern China.