Terrestrial Laser Scanning for Evaluating Dutch Volume Functions for Douglas fir and Beech
By Jan Hooijmans
Accurate functions for estimating trunk volume of standing trees based on field measurements are essential for biomass calculations. It has been unknown how reliable volume functions used in the Netherlands are for estimating trunk volume and, for volume functions in general, how much deviations are caused by variation in trunk shape between trees. This research has focused on evaluating three Dutch volume functions for Beech (Fagus sylvatica) and Douglas fir (Pseudotsuga menziesii) by the use of Quantitative Structure Modeling (QSM) based on Terrestrial Laser Scanning data. The Beech sample was split in mature and young Beech, as it was expected that these groups could show differences in results. The estimations of the volume functions have been compared with the QSM derived trunk volumes and were further investigated for potential causes of deviations between the results. Besides, possibilities of estimating total tree volume based on trunk volume with volume expansion factors have been examined. The results demonstrated that the volume functions currently in use are reliable for predicting the volume of the sample as a whole for both Douglas fir and Beech. There was, however, more variation at the scale of individual trees for young and mature Beech. The results show that young and mature Beech could benefit from modifications of the volume functions. For mature Beech, the artificial form factor was shown to be the most important cause of deviations and could be corrected for based on a relationship with tree height. Young Beech was systematically overestimated and, therefore, use of a reduction factor was found helpful. Multiplying all trunk volumes with the average expansion factor of the sample provided a good estimation of the total volume on sample scale for both species, although, for Beech, there was again much variation on individual tree level. This research has shown that trunk volume estimations for individual young and mature Beech trees could benefit from corrections of the currently used volume functions and, therefore, it is recommended to collect more data on spatially spread locations. Additionally, it is recommended to collect data on Douglas fir trees with a greater height variation, as this could possibly reveal greater deviations.