Determination of Fertility Rating (FR) in the 3-PG Model for Loblolly Pine (Pinus taeda L.) Plantations in the Southeastern United States

A method was developed to predict the soil fertility rating (FR) used in the model 3-PG for loblolly pine plantations based on the relationship between stand productivity and site index. Then FR was used in 3-PG to predict loblolly pine yield and mortality on 21 sites across the southeastern United States. When observed yield and stem number were compared against the simulated values, 89% of the variation in yield and 89% of the variation in stand density were explained by simulated values. The model also performed well using FR derived a priori from site index when tested against an independent data set containing the control plots from a mid-rotation fertil13 ization study. Although there was a slight positive bias in the predicted volume, 73% of the variations in observed volume was explained by 3-PG. 3-PG performed poorly on sites where stochastic events affected mortality and soil nutrient supply varied substantially through time. The USDA NRCS SSURGO dataset contains site index values for loblolly pine for the major soil series in most of the counties in the southeastern US. The potential of using site index from SSURGO data to predict regional productivity of loblolly pine was assessed by comparing site index values from SSURGO with field inventory data in the study sites. Good correlation was observed between site index reported in SSURGO database and site index observed in field inventory across the major soil series in the southeastern US. Site index values from SSURGO dataset were used to derive FR values to predict loblolly pine productivity at a regional scale. When the 3-PG model was used with FR values derived using site index values from SSURGO database to predict loblolly pine productivity across the broader regions, the model provided realistic outputs of loblolly pine productivity.