The study, published in Forest Ecosystems, presents a refined update to the 3-PG (Physiological Processes Predicting Growth) model. Its major innovation is adding a carbon storage pool specifically for stem growth, making it possible for the model to account for the "carry-over effect" in which trees use carbohydrates stored from previous years to form new wood, particularly early in the growing season when photosynthesis is low. Including such physiological processes improved simulations in both ring widths and isotopic compositions.
"Tree rings record decades of growth and environmental change, so improving how models reproduce them helps us understand forests' long-term responses to climate and stress," the authors explained. "By introducing carbon storage into 3-PG, we made the model more consistent with how trees actually function."
The researchers also incorporated a height-based growth modifier that replaces the traditional age-based modifier. This adjustment more accurately reflects how increasing tree height, rather than age alone, reduces photosynthesis and growth by imposing hydraulic constraints.
The improved model was tested using long-term field data from two grand fir stands at the Mica Creek Experimental Watershed in northern Idaho, USA. Results showed that the updated 3-PG achieved a much stronger agreement with observations than previous model versions.
The model also improved predictions of annual variations in raw tree-ring widths and ring-width index (RWI). The inclusion of stored carbon helped dampen excessive annual variations in isotopic values, yielding more realistic simulations.
As addressed by Professor Liang Wei, "This modification strengthens 3-PG's physiological foundation and makes it a robust tool for studying forest growth, carbon cycling, and climate interactions. The enhanced ability to simulate tree rings provides a reliable benchmark for long-term ecological and climate research."
