Assessment of Canopy Health with Drone-Based Orthoimagery in a Southern Appalachian Red Spruce Forest

Ryley C Harris,Thomas J Pingel,Valerie A Thomas,Lisa M Kennedy

doi:10.3390/rs14061341

Abstract

Consumer-grade drone-produced digital orthoimagery is a valuable tool for conservation management and enables the low-cost monitoring of remote ecosystems. This study demonstrates the applicability of RGB orthoimagery for the assessment of forest health at the scale of individual trees in a 46-hectare plot of rare southern Appalachian red spruce forest on Whitetop Mountain, Virginia. We used photogrammetric Structure from Motion software Pix4Dmapper with drone-collected imagery to generate a mosaic for point cloud reconstruction and orthoimagery of the plot. Using 3-band RBG digital orthoimagery, we visually classified 9402 red spruce individuals, finding 8700 healthy (92.5%), 251 declining/dying (2.6%), and 451 dead (4.8%). We mapped individual spruce trees in each class and produced kernel density maps of health classes (live, dead, and dying). Our approach provided a nearly gap-free assessment of the red spruce canopy in our study site, versus a much more time-intensive field survey. Our maps provided useful information on stand mortality patterns and canopy gaps that could be used by managers to identify optimal locations for selective thinning to facilitate understory sapling regeneration. This approach, dependent mainly on an off-the-shelf drone system and visual interpretation of orthoimagery, could be applied by land managers to measure forest health in other spruce, or possibly spruce-fir, communities in the Appalachians. Our study highlights the usefulness of drone-produced orthoimagery for conservation monitoring, presenting a valid and accessible protocol for the monitoring and assessment of forest health in remote spruce, and possibly other conifer, populations. Adoption of drone-based monitoring may be especially useful in light of climate change and the possible displacement of southern Appalachian red spruce (and spruce-fir) ecosystems by the upslope migration of deciduous trees.

Highlights

Rare southern Appalachian spruce-fir forests (Picea rubens Sarg.-Abies fraseri (Pursh) Poir.) are represented by clusters of disjunct populations confined to the highest elevations of southwestern Virginia, eastern Tennessee, and western North Carolina.Thought to have originated during the last glacial episode [1–3], these Pleistocene relicts are isolated from similar spruce-fir forests characteristic of northern latitudes
Studies using drone-based RGB orthoimagery as a source of spruce forest health data [22] are still sparse, but our research aims to contribute to this body of research
This paper presents an analysis of red spruce forest health and structure in a large

Summary

Introduction

Rare southern Appalachian spruce-fir forests (Picea rubens Sarg.-Abies fraseri (Pursh) Poir.) are represented by clusters of disjunct populations confined to the highest elevations of southwestern Virginia, eastern Tennessee, and western North Carolina.Thought to have originated during the last glacial episode [1–3], these Pleistocene relicts are isolated from similar spruce-fir forests characteristic of northern latitudes. Recent genomic research on red spruce populations throughout its North American range suggests that the species comprises three geographically distinct phylogenetic subgroups, with only remnants still present on the landscape today [12], and cites reduced genetic diversity due to the dwindling exchange between disjunct populations as a driver of gradual species decline over the past several thousand years. This finding points toward the need for careful monitoring of the health and regeneration of the remaining red spruce populations

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