Submitted to: American Forage and Grassland Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 11/28/2005
Publication Date: 3/13/2006
Citation: Skinner, R.H., Corson, M.S., Gilmanov, T.G. 2006. Simulation of CO2 uptake and partitioning by the integrated farm system model. American Forage and Grassland Conference Proceedings, March 11-13, 2006, San Antonio, Texas. Vol. 15 CDROM. Interpretive Summary: An interpretive summary is not required.
Technical Abstract: The Integrated Farm System Model (IFSM) is a deterministic, process-based model that predicts effects of weather and management on forage yields in temperate regions at a whole-farm scale. The model was recently enhanced to represent the growth and competition of multiple plant species. This enhanced model incorporated plant, water, and soil components of the Simulation of Production and Utilization of Rangelands model (SPUR 2.4). Although this is a comprehensive model that simulates both above- and below-ground components of the plant community, calibration and validation were based only on periodic measurements of aboveground forage yield. This presentation will report on use of daily observations of CO2 uptake and respiration to improve validation of the IFSM. Field data for testing photosynthetic and respiratory outputs of the model were collected in 2003 and 2004 from a pasture at the Penn State University Haller Research Farm. Continuous pasture-scale CO2 fluxes were quantified using a Campbell Scientific eddy covariance CO2 flux system. Temperature and light response curves were used to partition fluxes into photosynthetic and respiratory components. After calibration using 2003 data, simulated forage yield for 2003 was 3553 lbs/acre, or an overestimation by the model of 2% compared with the observed yield of 3480 lbs/acre. At the same time, photosynthesis was overestimated by 0.3% and respiration by 11%. When validated against 2004 data, photosynthesis was overestimated by 0.5%. However, yield was overestimated by 11%, and respiration underestimated by 11%. Even though little difference existed between simulated and observed yearly photosynthesis, modeled uptake in both years was overestimated during the first half of the year and underestimated thereafter. In contrast, simulated respiration rates overestimated observed rates for most of the year in 2003, with the exception of late summer when respiration was underestimated. In 2004 the greatest underestimation of respiration again occurred during late summer. Even though early-season photosynthesis was overestimated by the model, aboveground biomass at the first harvest was underestimated each year while root biomass was overestimated. Future model development should include improved algorithms for root:shoot partitioning. Inclusion of a soil respiration subroutine and additional knowledge of the relative contributions of plant and soil respiration to total ecosystem respiration are also necessary to improve simulations of total ecosystem CO2 flux.