Abstract
Key MessageThis study showed that digital terrestrial photogrammetry is able to produce accurate estimates of stem volume and diameter across a range of species and tree sizes that showed strong correspondence when compared with traditional inventory techniques. This paper demonstrates the utility of the technology for characterizing trees in complex habitats such as boreal mixedwood forests.ContextAccurate knowledge of tree stem taper and volume are key components of forest inventories to manage and study forest resources. Recent developments have seen the increasing use of ground-based point clouds, including from digital terrestrial photogrammetry (DTP), to provide accurate estimates of these key forest attributes.AimsIn this study, we evaluated the utility of DTP based on a small set of photos (12 per tree) for estimating stem volume and taper on a set of 15 trees from 6 different species (Populus tremuloides, Picea glauca, Pinus contorta latifolia, Betula papyrifera, Picea mariana, Abies balsamea) in a boreal mixedwood forest in Alberta, Canada.MethodsWe constructed accurate photogrammetric point clouds and derived taper and volume from three point cloud–based methods, which were then compared with estimates from conventional, field-based measurements. All methods were evaluated for their accuracy based on field-measured taper and volume of felled trees.ResultsOf the methods tested, we found that the point cloud–derived diameters in a taper curve matching approach performed the best at estimating diameters at the lowest parts of the stem (< 30% of total tree height), while using known DBH and height provided more accurate estimates for the upper parts of the stem (> 50% of total height). Using the field-measured DBH and height as inputs to calculate stem volume yielded the most accurate predictions; however, these were not significantly different from the best point cloud-based estimates.ConclusionThe methodology confirmed that using a small set of photographs provided accurate estimates of individual tree DBH, taper, and volume across a range of species and size gradients (10.8–40.4 cm DBH).
Highlights
The key to sustainably managing the world’s 4 billion hectares of forested area (Bahamondez et al 2010) is undertaking detailed and accurate inventories at varying scales (Gillis et al 2005)
We evaluate a methodology for digital terrestrial photogrammetry (DTP) estimation of diameter at breast height (DBH), upper stem diameter (> 1.3 m), and volume of individual trees in a boreal mixedwood forest
A separate analysis using only six images at a single location for point cloud reconstruction yielded an root-meansquared error (RMSE) of 2.00 cm, or 8.10%, for DBH estimation
Summary
The key to sustainably managing the world’s 4 billion hectares of forested area (Bahamondez et al 2010) is undertaking detailed and accurate inventories at varying scales (Gillis et al 2005). While some components of a forest inventory are distribution-based, such as species mixtures and age, the size of individual tree stems remains a critical measurement that provides the basis for the remainder of a forest inventory One such measurement is the diameter at breast height (DBH), which is a key input to allometric equations calculating, for example, volume (Huang 1994) or biomass (Lambert et al 2005). A relative frequency distribution of DBHs in a given area yields a stem size distribution (Taubert et al 2013), which can be used directly to estimate forest attributes such as its structure, successional stage, or volume (Gobakken and Næsset 2004; Hetemäki et al 2010; Nduwayezu et al 2015) These inventory attributes provide valuable insights to inform stand- and landscape-level forest management decisions; their complete and accurate estimation is critical for maximizing a range of both economic and ecological forest values
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