Quantifying vegetation indices using terrestrial laser scanning: methodological complexities and ecological insights from a Mediterranean forest
<p>Accurate measurement of vegetation density metrics including plant, wood and leaf area indices (PAI, WAI and LAI) is key to monitoring and modelling carbon storage and uptake in forests. Traditional passive sensor approaches, such as digital hemispherical photography (DHP), cannot separate...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2023-07-01
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| Series: | Biogeosciences |
| Online Access: | https://bg.copernicus.org/articles/20/2769/2023/bg-20-2769-2023.pdf |
| _version_ | 1797780508880404480 |
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| author | W. R. M. Flynn H. J. F. Owen S. W. D. Grieve S. W. D. Grieve E. R. Lines |
| author_facet | W. R. M. Flynn H. J. F. Owen S. W. D. Grieve S. W. D. Grieve E. R. Lines |
| author_sort | W. R. M. Flynn |
| collection | DOAJ |
| description | <p>Accurate measurement of vegetation density metrics including
plant, wood and leaf area indices (PAI, WAI and LAI) is key to monitoring
and modelling carbon storage and uptake in forests. Traditional passive
sensor approaches, such as digital hemispherical photography (DHP), cannot
separate leaf and wood material, nor individual trees, and require many
assumptions in processing. Terrestrial laser scanning (TLS) data offer new
opportunities to improve understanding of tree and canopy structure.
Multiple methods have been developed to derive PAI and LAI from TLS data,
but there is little consensus on the best approach, nor are methods
benchmarked as standard.</p>
<p>Using TLS data collected in 33 plots containing 2472 trees of 5 species
in Mediterranean forests, we compare three TLS methods (<i>lidar pulse</i>, <i>2D intensity image</i> and
<i>voxel-based</i>) to derive PAI and compare with co-located DHP. We then separate leaf and
wood in individual tree point clouds to calculate the ratio of wood to total
plant area (<span class="inline-formula"><i>α</i></span>), a metric to correct for non-photosynthetic material
in LAI estimates. We use individual tree TLS point clouds to estimate how
<span class="inline-formula"><i>α</i></span> varies with species, tree height and stand density.</p>
<p>We find the lidar pulse method agrees most closely with DHP, but it is limited to single-scan data, so it cannot determine individual tree properties, including <span class="inline-formula"><i>α</i></span>. The voxel-based method shows promise for ecological studies as it can be applied to
individual tree point clouds. Using the voxel-based method, we show that species
explain some variation in <span class="inline-formula"><i>α</i></span>; however, height and plot density were
better predictors.</p>
<p>Our findings highlight the value of TLS data to improve fundamental
understanding of tree form and function as well as the importance of rigorous
testing of TLS data processing methods at a time when new approaches are
being rapidly developed. New algorithms need to be compared against
traditional methods and existing algorithms, using common reference data.
Whilst promising, our results show that metrics derived from TLS data are
not yet reliably calibrated and validated to the extent they are ready to
replace traditional approaches for large-scale monitoring of PAI and LAI.</p> |
| first_indexed | 2024-03-12T23:45:07Z |
| format | Article |
| id | doaj.art-122e6fd8218043c6a2e4ecda65cb0300 |
| institution | Directory Open Access Journal |
| issn | 1726-4170 1726-4189 |
| language | English |
| last_indexed | 2024-03-12T23:45:07Z |
| publishDate | 2023-07-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Biogeosciences |
| spelling | doaj.art-122e6fd8218043c6a2e4ecda65cb03002023-07-14T08:42:08ZengCopernicus PublicationsBiogeosciences1726-41701726-41892023-07-01202769278410.5194/bg-20-2769-2023Quantifying vegetation indices using terrestrial laser scanning: methodological complexities and ecological insights from a Mediterranean forestW. R. M. Flynn0H. J. F. Owen1S. W. D. Grieve2S. W. D. Grieve3E. R. Lines4School of Geography, Queen Mary University of London, Mile End Rd, Bethnal Green, London, E1 4NS, United KingdomDepartment of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, United KingdomSchool of Geography, Queen Mary University of London, Mile End Rd, Bethnal Green, London, E1 4NS, United KingdomDigital Environment Research Institute, Queen Mary University of London, New Road, London, E1 1HH, United KingdomDepartment of Geography, University of Cambridge, Downing Place, Cambridge, CB2 3EN, United Kingdom<p>Accurate measurement of vegetation density metrics including plant, wood and leaf area indices (PAI, WAI and LAI) is key to monitoring and modelling carbon storage and uptake in forests. Traditional passive sensor approaches, such as digital hemispherical photography (DHP), cannot separate leaf and wood material, nor individual trees, and require many assumptions in processing. Terrestrial laser scanning (TLS) data offer new opportunities to improve understanding of tree and canopy structure. Multiple methods have been developed to derive PAI and LAI from TLS data, but there is little consensus on the best approach, nor are methods benchmarked as standard.</p> <p>Using TLS data collected in 33 plots containing 2472 trees of 5 species in Mediterranean forests, we compare three TLS methods (<i>lidar pulse</i>, <i>2D intensity image</i> and <i>voxel-based</i>) to derive PAI and compare with co-located DHP. We then separate leaf and wood in individual tree point clouds to calculate the ratio of wood to total plant area (<span class="inline-formula"><i>α</i></span>), a metric to correct for non-photosynthetic material in LAI estimates. We use individual tree TLS point clouds to estimate how <span class="inline-formula"><i>α</i></span> varies with species, tree height and stand density.</p> <p>We find the lidar pulse method agrees most closely with DHP, but it is limited to single-scan data, so it cannot determine individual tree properties, including <span class="inline-formula"><i>α</i></span>. The voxel-based method shows promise for ecological studies as it can be applied to individual tree point clouds. Using the voxel-based method, we show that species explain some variation in <span class="inline-formula"><i>α</i></span>; however, height and plot density were better predictors.</p> <p>Our findings highlight the value of TLS data to improve fundamental understanding of tree form and function as well as the importance of rigorous testing of TLS data processing methods at a time when new approaches are being rapidly developed. New algorithms need to be compared against traditional methods and existing algorithms, using common reference data. Whilst promising, our results show that metrics derived from TLS data are not yet reliably calibrated and validated to the extent they are ready to replace traditional approaches for large-scale monitoring of PAI and LAI.</p>https://bg.copernicus.org/articles/20/2769/2023/bg-20-2769-2023.pdf |
| spellingShingle | W. R. M. Flynn H. J. F. Owen S. W. D. Grieve S. W. D. Grieve E. R. Lines Quantifying vegetation indices using terrestrial laser scanning: methodological complexities and ecological insights from a Mediterranean forest Biogeosciences |
| title | Quantifying vegetation indices using terrestrial laser scanning: methodological complexities and ecological insights from a Mediterranean forest |
| title_full | Quantifying vegetation indices using terrestrial laser scanning: methodological complexities and ecological insights from a Mediterranean forest |
| title_fullStr | Quantifying vegetation indices using terrestrial laser scanning: methodological complexities and ecological insights from a Mediterranean forest |
| title_full_unstemmed | Quantifying vegetation indices using terrestrial laser scanning: methodological complexities and ecological insights from a Mediterranean forest |
| title_short | Quantifying vegetation indices using terrestrial laser scanning: methodological complexities and ecological insights from a Mediterranean forest |
| title_sort | quantifying vegetation indices using terrestrial laser scanning methodological complexities and ecological insights from a mediterranean forest |
| url | https://bg.copernicus.org/articles/20/2769/2023/bg-20-2769-2023.pdf |
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