Estimating the Height and Basal Area at Individual Tree and Plot Levels in Canadian Subarctic Lichen Woodlands Using Stereo WorldView-3 Images

Benoît St-Onge,Simon Grandin

doi:10.3390/rs11030248

Abstract

Lichen woodlands (LW) are sparse forests that cover extensive areas in remote subarctic regions where warming due to climate change is fastest. They are difficult to study in situ or with airborne remote sensing due to their remoteness. We have tested a method for measuring individual tree heights and predicting basal area at tree and plot levels using WorldView-3 stereo images. Manual stereo measurements of tree heights were performed on short trees (2–12 m) of a LW region of Canada with a residual standard error of ≈0.9 m compared to accurate field or UAV height data. The number of detected trees significantly underestimated field counts, especially in peatlands in which the visual contrast between trees and ground cover was low. The heights measured from the WorldView-3 images were used to predict the basal area at individual tree level and summed up at plot level. In the best conditions (high contrast between trees and ground cover), the relationship to field basal area had a R2 of 0.79. Accurate estimates of above ground biomass should therefore also be possible. This method could be used to calibrate an extensive remote sensing approach without in-situ measurements, e.g., by linking precise structural data to ICESAT-2 footprints.

Highlights

Lichen woodlands (LW), and more generally northern open canopy forests, cover extensive areas in the circumpolar subarctic regions
Measuring the height of trees by spatial intersection of conjugate rays from VHSRI requires that the tree apex be clearly seen and visually matched in the two images
The DBH and basal area were predicted from the heights measured, and the plot level basal area was computed

Summary

Introduction

Lichen woodlands (LW), and more generally northern open canopy forests, cover extensive areas in the circumpolar subarctic regions. We will here adopt the latter definition and consider the LW as having a geographical distribution in Canada and Alaska as presented in the map proposed in Reference [4] (Figure 1) According to this definition, subarctic forest cover approximately 2 × 106 km in Canada alone. Subarctic forest cover approximately 2 × 106 km in Canada alone They are characterized by short growing seasons, temperatures varying from 20 ◦C in summer to −50 ◦C in winter, and a discontinuous permafrost. LW are characterized by short and open canopies, and a forest floor mostly covered by lichens (see Figure 2) Their above ground biomass density (t ha−1, thereafter termed “biomass”) is currently low, a lengthening of the growing season, CO2 fertilization, and the consequent rise in productivity of LW could have a significant global impact on carbon sequestration due to the Remote Sens. Their above ground biomass density (t ha−1, thereafter termed “biomass”) is currently low, a lengthening of the growing season, CO2 fertilization, and the consequent rise in productivity of LW could have a significant global impact on carbon sequestration due to the Remote Sens. 2019, 11, 248; doi:10.3390/rs11030248 www.mdpi.com/journal/remotesensing

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