Abstract
Abstract. One fundamental metric to characterize trees and forest stands is the diameter at breast height (DBH). However, the vertical geometry of tree stems hampers a direct measurement by means of orthographic aerial imagery. Nevertheless, the DBH in deciduous forest stands could be measured from UAV-based imagery using the width of a stem´s cast shadow projected on the ground. Here, we compare in-situ measured DBH of 100 trees with the DBH visually interpreted from cast-shadows derived in UAV-based aerial imagery. Then, based on simulated datasets, we determine suitable DBH sampling sizes for a robust and efficient retrieval of stand diameter distributions. The UAV-based DBH estimation resulted in an r2 of 0.74, RMSE of 7.61 cm, NRMSE of 12.8 % and approximately unbiased results. According to our simulations it can be assumed that a sample size of 25–50 individual DBH measurements per forest stand allows estimating reliable diameter distributions. The presented pilot study gives a first insight on the potential of such an approach for operational assessments of diameter distribution in deciduous forest stands and might be particularly interesting for stands in difficult terrain situations. The presented approach can be extended to estimate the basal area, timber stock or biomass.
Highlights
Sustainable forest management requires repeated assessments of forest inventory metrics including stand attributes, as e.g. tree density, basal area or standing timber volume
The explicit locations of the tree shadow sections that correspond to the diameter at breast height (DBH) height (1.3 m) were projected on the digital terrain model incorporating the sun orientation corresponding to the time of the unmanned aerial vehicle (UAV) flight as well as the local terrain conditions (DTM)
The results show that cast shadows in UAV-based orthomosaics can be used to manually estimate the DBH of individual trees with reasonable accuracy in deciduous forest stands during leaf-off state
Summary
Sustainable forest management requires repeated assessments of forest inventory metrics including stand attributes, as e.g. tree density, basal area or standing timber volume. New remote and proximate sensing technologies along with unmanned aerial vehicle (UAV) technology emerge at an unprecedented pace and offer new possibilities to develop applications for foresters What makes these systems interesting for practitioners is that recent UAV systems equipped with RGB cameras and functions such as automatic flight planning (through waypoints) have become readily available as off-the-shelf-products. Until now applications using UAV-based photogrammetry to retrieve the diameter at breast height (DBH), diameter distributions or the basal area remain limited This contrasts the great importance of these parameters for practitioners to describe individual trees or stands, respectively. Orthographic remote sensing data such as orthophotos or canopy height models might not depict trees in the understory which are obscured by overlapping tree crowns This is particular relevant for uneven aged deciduous or mixed stands, meaning that smaller trees are less likely to be considered in the estimated DBH distribution and basal area for a given stand.
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