Form factors and volume models for Falcataria moluccana in smallholder plantations, Central Kalimantan, Indonesia

Abstract

ABSTRACT Falcataria moluccana (sengon) is grown extensively for timber production in smallholder plantations in Indonesia. However, stem form and volume information necessary for its sustainable forest management, yield estimation and carbon assessment are largely unavailable. Therefore, this study developed form factor and stem volume models for sengon trees in Central Kalimantan, Indonesia. Data were collected using a destructive method from 24 trees across six diameter classes. Two types of form factors, artificial and absolute, were determined, and their relationship with tree growth was established. Four form factor functions were fitted, and the accuracy of volume estimation using a default form factor was compared to the functions. Additionally, 11 stem volume models were developed, and their performance was compared with the commonly used published models. The form factor for sengon was estimated at 0.48, which was higher and more accurate than the absolute form factor. A linear relationship was observed between sengon tree form and growth, particularly with the artificial form factor (R 2 = 0.55–0.62). The Pollanschütz function was identified as the best form factor function, accurately predicting form factors for precise tree volume computations. Using the default form factor of 0.5 led to an overestimation of volume by up to 17%. Among the volume models, the power models with diameter as the sole predictor (MD3: V = 0.00018 * DBH2.3817) and with both diameter and height variables (MDH4: V = 0.000095 * DBH1.9974 * H0.6327) were the most accurate. Importantly, the models developed in this study offer improved accuracy compared to existing models from Java, which tend to overestimate stem volume. Therefore, this study provides suitable models and valuable insights for better valuation and management of smallholder sengon plantations in Central Kalimantan and other areas with similar site and stand conditions.