Modeling repeated fertilizer response and one-time midrotation fertilizer response in loblolly pine plantations using FR in the 3-PG process model

We also developed a dynamic function to model the FR to accurately predict growth in loblolly pine stands fertilized one time with N and P during the middle of the rotation. We used data from a series of midrotation loblolly pine plantations across the southeastern United States to model change in FR (ΔFR). This was done using an optimization process where the temporal distribution of the ΔFR was modeled using the Weibull function. Baseline FR, intensity of N fertilization, and year since treatment were used as input regressors to model dynamics of FR following midrotation fertilization. The model accurately described temporal changes in FR that enabled 3-PG to accurately predict response to midrotation fertilization with N and P. When FR values generated using this function were input into the 3-PG model, 80% of the variation in the observed aboveground biomass in fertilized loblolly pine plantations was described by the 3-PG simulated aboveground biomass.