3PG

3PG, short for Physiological Principles Predicting Growth, is a process-based forest growth model used to predict the growth of trees and stands under varying climate and site conditions. Developed in the 1990s by Landsberg and Waring, it provides a link between carbon balance and growth by simulating processes such as photosynthesis, respiration, and biomass allocation.

The model operates at stand or tree level and requires inputs that describe the environment and stand structure. Typical inputs include climate data (temperature, solar radiation, precipitation), soil water-holding capacity, nutrient status, stand density, and canopy properties. Using these inputs, 3PG estimates annual changes in diameter at breast height, height growth, and aboveground biomass, producing growth and yield trajectories over time.

Model mechanics center on carbon supply and allocation. Growth is treated as a balance between carbon gained through photosynthesis and carbon losses from respiration, with allocation to foliage, stems, and roots governed by physiological principles and site productivity. The framework is designed to be adaptable across species and site conditions with relatively simple parameterization, supporting broad application in forest management planning and research.

Applications and impact include climate-change impact assessment, silvicultural optimization, and timber yield forecasting. 3PG has been implemented in multiple software tools and cited widely in forestry literature, contributing to decision support for harvest scheduling, carbon accounting, and ecosystem management.

Other uses of the acronym exist in different disciplines, where 3PG may refer to unrelated concepts such as three-phase electrical systems or statistical distributions. In forestry contexts, 3PG almost always denotes the Physiological Principles Predicting Growth model.