Simulating Gross Primary Productivity of Humid-Temperate Pastures

Authors: Skinner, Robert; Corson, Michael; GILMANOV, TAGIR - SOUTH DAKOTA STATE UNIV

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/28/2007
Publication Date: 5/7/2008
Citation: Skinner, R.H., Corson, M.S., Gilmanov, T.G. 2008. Simulating Gross Primary Productivity of Humid-Temperate Pastures. Agronomy Journal. 100:801-807.

Interpretive Summary: Farm systems models been used in the past to study the effects of climate and management on plant growth and competition in pastures. Because of the current interest in carbon sequestration and global climate change, models are now needed to evaluate how climate and management affect the net pasture carbon balance that is a function of carbon uptake through photosynthesis and loss through respiration. This research used daily measurements of gross canopy photosynthesis and total ecosystem respiration to validate the photosynthetic and respiratory components of the pasture growth subroutine of the Integrated Farming System Model. The model did an excellent job of predicting yearly photosynthesis, respiration, and yield, as all were within +/- 11% of observed values. Predictions of yearly photosynthesis were particularly good, differing from observed values by less than 1% in the two years that were simulated. It was more difficult to correctly simulate ecosystem respiration, primarily because plant but not soil respiration was included in the model. At present it is difficult to model belowground respiration because little experimental data is available on the relative contribution of roots and soil microbes to total respiration. Even though total photosynthesis for the year was well represented by the model, photosynthesis tended to be over-estimated in the spring and early-summer then under-estimated in the later half of the year. The model also predicted greater root growth in the spring than what was observed. Further refinement of the model is needed to more correctly balance root and shoot growth, and to correctly simulate the relative contribution of plants and soil to total ecosystem respiration.

Technical Abstract: The Integrated Farm System Model (IFSM) was recently enhanced to represent growth and competition among multiple plant species in pastures. Although this comprehensive model simulates many above- and belowground components of the plant community, calibration and validation were based only on periodic measurements of aboveground forage yield. This research examined the use of daily micrometeorological measurements of gross canopy photosynthesis and total ecosystem respiration to validate the photosynthetic and respiratory components of the pasture growth subroutine of the IFSM. The revised model was able to predict yearly photosynthesis, respiration, and yield all to within +/- 11% of observed values. Predicted yearly photosynthesis differed from observed values by less than 1% in both the calibration and validation years. Simulation of ecosystem respiration was less satisfactory, primarily due to the lack of an explicit representation of soil respiration in the model, and due to the lack of available experimental data to quantify the subcomponents of ecosystem respiration. Despite the excellent agreement between simulated and observed yearly photosynthesis, photosynthesis tended to be over-estimated in the spring and early-summer then under-estimated in the later half of the year. Forage yield, however, was under-estimated during the first growth period then over-estimated later on. The lack of correlation between early-season photosynthesis and yield was due to excessive partitioning of biomass to root growth during that period. Further refinement of the model will require increased partitioning of assimilated carbon to shoots, while avoiding a concomitant increase in photosynthesis. In addition, improved representation of root and soil respiration based on additional experimental data is needed to adequately represent net ecosystem carbon balance.