Submitted to: Global Change Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/29/2007
Publication Date: 5/1/2008
Citation: Phillips, R.L., Beeri, O. 2008. Scaling-up knowledge of growing-season net ecosystem exchange for long-term assessment of North Dakota grasslands under the Conservation Reserve Program. Global Change Biology 14(5):1008-1017. Interpretive Summary: Agricultural management practices influence carbon uptake in the Northern Great Plains (NGP), but effects vary seasonally and annually with environmental conditions. Carbon uptake may be greater for annual crop fields converted to perennial grasses under the Conservation Reserve Program (CRP) because these fields are not mechanically disturbed by tillage or chemically amended with fertilizers that release additional greenhouse gases. However, effects of conversion from annual crops to perennial grasses on plant mediated exchange of C between the land and atmosphere is unknown. Here, we use satellite data tools delineating plant properties to “scale up” current knowledge to address how C uptake varies among perennial grass fields enrolled in CRP. Carbon uptake observed during the growing season from 1997-2006 for CRP fields vacillated with drought and deluge ranging from -366 to 692 g C m-2 growing season-1, with net carbon (C) losses to the atmosphere in 2006. Results indicate assessment of conservation practices on grassland CO2 exchange during the growing season can be remotely estimated at field and landscape scales under variable environmental conditions.
Technical Abstract: Scaling-up knowledge of land-atmosphere net ecosystem exchange (NEE) from a single experimental site to numerous perennial grass fields in the Northern Great Plains (NGP) requires appropriate scaling protocols. We addressed this problem using synoptic data available from the Landsat sensor for ten growing seasons (Apr 15-Sep 30) over a North Dakota field-site, where we continuously measured CO2 exchange using a Bowen Ratio Energy Balance (BREB) system. Seasonal NEE observed at our field-site from 1997-2006 vacillated with drought and deluge, with net carbon (C) losses to the atmosphere in 2006. We used stepwise linear regression with ten years of Landsat and NEE data to construct and validate a model for estimating grassland seasonal NEE from field to landscape scales. Eighty-eight percent of the variability in NEE was explained by year, live biomass, carbon:nitrogen ratio, day of image acquisition, and annual precipitation. We then applied this model on 20,620 ha of North Dakota perennial grass fields enrolled in the Conservation Reserve Program (CRP), including 1272 fields east of the Missouri River and 165 fields west-river. Seasonal NEE for CRP fields was highly variable from 1997-2006, ranging from -366 to 692 g C m-2 growing season-1. Cumulative seasonal NEE was 3170 g C m-2 ten years-1 and 2380 g C m-2 ten years-1 east-river and west-river, respectively. Average cumulative seasonal NEE modeled for fields east- and west-river diverged from one another in 2002-2006, when west-river fields received <70% of the long-term annual average precipitation during these years. Results indicate assessment of conservation practices and climate on grassland CO2 exchange during the growing season can be remotely estimated at field and landscape scales under variable environmental conditions and should be followed up by more work on spatially explicit NEE during the dormant season.