RESPONSES OF GRASS PRODUCTION TO PRECIPITATION IN A MID-LATITUDE TYPICAL STEPPE WATERSHED

Net primary production (NPP) is commonly used to measure productivity of steppe grasses and has been extensively studied. However, NPP variations within an enclosed watershed as influenced by precipitation and environmental factors (e.g., soils) have been poorly documented in existing literature. The objectives of this study were to: ( 1) examine spatiotemporal variations of NPP across a Eurasian typical steppe watershed using remote sensing and field data, and ( 2) determine effects of precipitation on the spatiotemporal variations. In addition to the Moderate Resolution Imaging Spectroradiation (MODIS) data from 2000 to 2010, field data on NPP, soils, and groundwater were measured by the authors in 2012 and used in this study. Advanced statistical (e.g., Mann-Kendall and correlation) and visualization techniques were employed to analyze the data. The results indicate that the NPP for a downstream area within the study watershed tended to be smaller than the NPP for an upstream area. The NPP varied almost synchronically with precipitation and was especially dependent on the May to July precipitation for a given year. The May to July precipitation could explain 12% to 65% of the NPP variances and played more roles in the growth of steppe grasses in a lower area than in an upper area. The NPP spatial pattern and the spatially differentiated effects of precipitation might be controlled by environmental factors of topography, soil texture, soil permeability, soil salinity, and grazing intensity.