Journal Article

Describing and Modelling Stem Form of Tropical Tree Species with Form Factor: A Comprehensive Review

Israel, Rebecca, Oluwajuwon, Tomiwa V., Lee, David J., Ogbuka, Chioma E., Ogana, Friday N., Hossain, Md. Sazzad

2025

Overview

The concept of tree or stem form has been central to forest research for over a century, playing a vital role in accurately assessing tree growth, volume, and biomass. The form factor is an essential component for expressing the shape of a tree, enabling more accurate volume estimation, which is vital for sustainable forest management and planning. Despite its simplicity, flexibility, and advantages in volume estimation, the form factor has received less attention compared to other measures like taper equations and form quotient. This review summarizes the concept, theories, and measures of stem form, and describes the factors influencing its variation. It focuses on the form factor, exploring its types, parameterization, and models in the context of various tropical species and geographic conditions. The review also discusses the use of the form factor in volume estimation and the issues with using default or generic values. The reviewed studies show that tree stem form and form factor variations are influenced by multiple site, tree, and stand characteristics, including site quality, soil type, climate conditions, tree species, age, crown metrics, genetic factors, stand density, and silviculture. The breast height form factor is the most adopted among the three common types of form factors due to its comparative benefits. Of the five most tested form factor functions for predicting tree form factors, Pollanschütz’s function is generally considered the best. However, its performance is often not significantly different from other models. This review identifies the “Hohenadl” method and mixed-effects modelling as overlooked yet potentially valuable approaches for form factor modelling. Using the form factor, especially by diameter or age classes, can enhance tree volume estimation, surpassing volume equations. However, relying on default or generic form factors can lead to volume and biomass estimation errors of up to 17–35%, underscoring the need to limit variation sources in form factor modelling and application. Further recommendations are provided for improving the statistical techniques involved in developing form factor functions.

• Publisher: MDPI AG
• Subject: Age determination / Analysis / Biomass / Carbon / Climate change / Climatic conditions / Environmental protection / Forest management / Forestry research / Forests / Form factors / Genetic factors / Modelling / Parameterization / Plant species / Silviculture / Soil types / Statistical analysis / Statistical models / Stems / Sustainability management / Sustainable forestry / Trees / Variation