PTAEDA-LIGHT: A mechanistic light-competition model for predicting the growth and survival of loblolly pine trees (Pinus taeda)

A model for simulating growth and survival for individual trees in even-aged loblolly pine (Pinus taeda, L.) stands, PTAEDA-LIGHT, was developed by reformulating and linking two existing models: PTAEDA2, an individual tree-based growth and yield model, and tRAYci, an individual tree-based light capture model, which estimates absorbed photo synthetically active radiation (APAR) [as weighted leaf area (WLA)] for individual trees in a stand, given specific locations and crown measurements for each tree and stand-level parameterization of crown characteristics. A full discussion of model development and model structure is presented here, along with a comparison of differences in the predictive power of PTAEDA-LIGHT and its parent model PTAEDA2. It was hypothesized that the inclusion of a spatially-explicit representation of light resource capture would allow for more accurate predictions of tree growth and survival. When the predictions of both models were compared to observed growth and survival in real stands, PTADEA-LIGHT equaled PTAEDA2 in growth prediction and surpassed it in survival prediction, demonstrating for the first time that a mechanistic light competition model could equal and exceed the predictions of a growth model which uses a simpler phytometric competition index. PTADEA-LIGHT, however, had difficulty capturing the full range of variability in height growth, due mainly to the fact that light absorption per tree explained only a small portion of the variability in height growth. The light calculations used in PTEADA-LIGHT also necessitate an increase in simulation time over the simpler model to obtain its results, but this increase is still very small in comparison to the actual time period over which growth and survival are predicted (e.g., minutes of simulation to predict years of stand development).