A Consumer Grade UAV-Based Framework to Estimate Structural Attributes of Coppice and High Oak Forest Stands in Semi-Arid Regions

Arvin Fakhri,Hooman Latifi

doi:10.3390/rs13214367

Arvin Fakhri, Hooman Latifi

Open Access

https://doi.org/10.3390/rs13214367

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Abstract

Semi-arid tree covers, in both high and coppice growth forms, play an essential role in protecting water and soil resources and provides multiple ecosystem services across fragile ecosystems. Thus, they require continuous inventories. Quantification of forest structure in these tree covers provides important measures for their management and biodiversity conservation. We present a framework, based on consumer-grade UAV photogrammetry, to separately estimate primary variables of tree height (H) and crown area (A) across diverse coppice and high stands dominated by Quercus brantii Lindl. along the latitudinal gradient of Zagros mountains of western Iran. Then, multivariate linear regressions were parametrized with H and A to estimate the diameter at breast height (DBH) of high trees because of its importance to accelerate the existing practical DBH inventories across Zagros Forests. The estimated variables were finally applied to a model tree aboveground biomass (AGB) for both vegetative growth forms by local allometric equations and Random Forest models. In each step, the estimated variables were evaluated against the field reference values, indicating practically high accuracies reaching root mean square error (RMSE) of 0.68 m and 4.74 cm for H and DBH, as well as relative RMSE < 10% for AGB estimates. The results generally suggest an effective framework for single tree-based attribute estimation over mountainous, semi-arid coppice, and high stands.

Highlights

Published: 29 October 2021Forests significantly contribute to the global carbon cycle by providing the largest reserves of terrestrial carbon [1]
Sparsely available field estimates in Zagros is exacerbated by weak technical infrastructure, e.g., infeasibility of using Terrestrial LiDAR, GeoSLAM, or airborne LiDAR, which entail relying on methods based on a combination of limited field data and passive photogrammetry or remote sensing data
We suggested a Unmanned Aerial Vehicles (UAVs)-assisted workflow to measure and estimate a range of primary and secondary structural attributes on single tree-level across multiple forest sites, located along the latitudinal gradient of Zagros Forests

Summary

Introduction

Forests significantly contribute to the global carbon cycle by providing the largest reserves of terrestrial carbon [1]. The structural analysis of such vegetation is a prerequisite to assess their current condition and evaluate their ecosystem services such as carbon stocks, CO2 uptake [4,5], and aboveground biomass (AGB) [6] as affected by disturbances and land use changes. Tree height and diameter at breast height (DBH) are among the basic forest inventory attributes [8,9], which are integrated by many allometric equations for estimating the AGB [10,11] and are essential information for quantifying forest carbon cycle [12], carbon stock [13], and global climate change [14]. One of the main obstacles of these measurements is the lack of sufficient visibility to the Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations

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