Influence of Vegetation Period on Personal Laser Scanning Accuracy for Tree Attribute Estimation in Pedunculate Oak and European Beech Forests

Abstract. Estimating the main tree attributes using hand-held personal laser scanning (PLS) instruments has been an ongoing area of research. So far, the vast majority of the studies have been conducted, during non-vegetation (leaf-off) period when the scanning view is not obstructed by foliage (Jurjević et al. 2020, Tupinambá-Simões et al. 2023, Vandendaele et al. 2024, Kokeza et al. 2024). Contrary, the number of studies conducted in the vegetation (leaf-on) period is negligible, and therefore, the more detailed comparison PLS studies between leaf-off and leaf-on estimates are still missing. This study investigates the impact of vegetation occlusion on the accuracy of the main tree attributes (diameter at breast height, tree height) in two different forest areas located in Central Croatia, i.e. in the lowland pedunculate oak (Quercus robur L.) forests and in the hilly European beech (Fagus sylvatica L.) forests. PLSHH scanning and collection of reference ground-truth data (detailed field measurements, static terrestrial laser scanning) in both forest areas were conducted during vegetation (leaf-on) and non-vegetation (leaf-off) periods within the same year, using consistent methodology to ensure comparability. The results show great potential of hand-held personal laser scanning technology in forest inventory. Diameter at breast height can be estimated with great accuracy in both study areas and both vegetation periods. However, the scanning in the leaf-off period produced slightly higher accuracy, which can be attributed to the presence of understorey vegetation in the leaf-on period. The more significant influence of the vegetation period is observed for tree heights, especially in European beech forest on hilly terrain with a greater presence of understorey vegetation. Further research should investigate whether the application of a more appropriate scanning scheme could improve the estimation accuracy of tree heights in such demand and complex forest environment.