Location: Hydrology and Remote Sensing Laboratory
Title: Spatial distributions ofC3 and C4 grass functional types in the U.S. great plains and their despendency on inter-annual climate variability Authors
Submitted to: Remote Sensing of Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 11, 2013
Publication Date: August 15, 2013
Repository URL: http://handle.nal.usda.gov/10113/59827
Citation: Wang, C., Hunt, E.R., Zhang, L., Guo, H. 2013. Spatial distributions ofC3 and C4 grass functional types in the U.S. great plains and their despendency on inter-annual climate variability. Remote Sensing of Environment. 138:90-101. Interpretive Summary: There are two types of grasses, cool season and warm season, which have maximum growth in the spring and summer, respectively. The initial product of photosynthesis for cool-season grasses is 2 three-carbon sugars; hence the grasses have C3 photosynthesis. However, the initial product of photosynthesis for warm-season grasses is a four-carbon acid; thus these grasses have C4 photosynthesis. Grasslands in areas with relatively high and low amounts of precipitation are termed tall and short, respectively. The native American tallgrass prairie and shortgrass steppe are both dominated by warm-season grasses, whereas the native northern mixed-grass prairie are dominated by short cool-season grasses. Cool-season grasses generally have higher forage quality, so these grasses are planted in pastures, and may be considered to be C3 tallgrass functional type. The objective of this study was to use NASA Moderate Resolution Imaging Spectroradiometer (MODIS) normalized difference vegetation index (NDVI) data to classify the Great Plains in the United States as warm or cool season based on the growing season and tall or short based on maximum growth. Ten years of data were used because of the high amount of variability in climate. The three types of native grasslands were distributed as expected with east-west gradients of tall-short grasses, respectively, and north-south gradients of C3-C4 grasses, respectively. The south-eastern Great Plains was expected to be warm-season grasses based on potential vegetation maps, but the MODIS classification suggested these areas were tall cool-season grasses, most likely resulting from extensive modification of the native grasslands to pastures. Using maps of actual vegetation composition instead of potential vegetation may provide important information about plant responses to climatic change.
Technical Abstract: Grassland ecosystems in North America are primarily composed of C3 and C4 plant functional types (PFTs) with their relative cover varying spatially and temporally. This study used 500-m MODIS surface reflectance products (MOD09A1) from 2000 to 2009 to extract an NDVI time series of C3 and C4 PFTs in different floristic regions (tallgrass prairie, shortgrass steppe, and southern and northern mixed-grass prairies). A set of phenology metrics, including end of season, growing length, peak NDVI value, peak date, and summer dry-down, were found optimal in delineating these grass types and major annual crops. Taking climate variations into consideration, a robust decision tree classifier was developed from the phenology metrics to map C3 and C4 PFTs in the Great Plains. The results generally showed the expected latitudinal and longitudinal distributions of C3 and C4 PFTs. C4 grasses mostly located in the south while C3 grasses were in the north. A large amount of tallgrass C3 PFT was mapped in the southern mixed-grass prairie, which may have been introduced species to southern-state pasturelands. Grass PFTs were also distributed in accordance with floristic regions. Tallgrass PFTs dominated the tallgrass prairie while shortgrass PFTs were more common in the shortgrass steppe and the northern mixed prairie. The inter-annual spatial variations of PFTs were statistically correlated with climate factors over the past 10-years. The strength and directions of correlations varied in different floristic regions. Increased spring precipitation favored the growth of tallgrass C4 in the tallgrass prairie but suppressed shortgrass C4 in the shortgrass Steppe. Warmer spring temperatures in the mixed prairie resulted in higher cover of shortgrass C4 in replacement of tallgrass C4 in this land. This spatio-temporally explicit information about C3/C4 abundance may provide quantitative inputs in ecological forecasting for various climate change scenarios.