External stem quality assessment in mixed-species tropical forest plantations using terrestrial laser scans

Abstract

This study aims to analyze the relationship between tree quality and diversity based on terrestrial laser scan (TLS) data of tree stems in a tropical mixed plantation. The study is divided into two parts. The first part examines the performance of the TLS to derive tree diameter at breast height (DBH) and tree height. TLS-based data allows to estimate both tree variables with a high accuracy (R²[subscript diameter] = 0.97, RMSE = 1.42 cm; R²[subscript height] = 0.96, RMSE = 1.05 m). These tree estimates allowed trusting further derived values of other external stem quality variables, needed for the second part of the thesis. This part analyses relationships between structural and species diversity and external stem quality. The species diversity is related to the species richness in a tree’s neighborhood. The structural diversity is computed as specific indices, based on manual measurements of the basal area and the tree heights. The relationships between these indices and variables were then analyzed for each tree species at the neighborhood scale. The results show that fast-growing to intermediate-growing tree species are most negatively affected by the structural diversity. Species diversity had a positive effect on branch dimensions for species of all ecological types. We also found out that, additional ecological factors such as tree growth and competition influence the external stem quality variables. Further studies in older stands should be carried out to analyze the longer-term influences of species and structural diversity on quality variables. Also, other ecological factors, such as climate, soil, or genetics, should be investigated to see if and how they are related to the diversity/stem quality relationship.