Influence of phenology on waveform features in deciduous and coniferous trees in airborne LiDAR

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Volume Title

A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä

Date

2023-08-01

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Language

en

Pages

19

Series

Remote Sensing of Environment, Volume 293

Abstract

Information on forest structure is vital for sustainable forest management. Currently, airborne LiDAR remote sensing has been well established as an effective tool to characterize the structure of canopies and forest inventory variables. Radiometry and geometry are highly intertwined in LiDAR remote sensing of forest vegetation and phenology influences the geometric-optical properties of deciduous and evergreen trees causing seasonal variation in LiDAR observations. This variation may be considered as a nuisance or exploited in for example tree species identification. Airborne LiDAR data are also influenced by sensor functioning, acquisition settings, scan geometry and the atmosphere. Reliable estimation of subtle phenological effects calls for data in which the impact of the external factors is minimal. We experimented with such data and explored LIDAR waveforms (WFs) in boreal trees in winter, early summer and late summer. Our objectives were to i) assess the match of the multitemporal LiDAR data for observing true changes in vegetation; ii) quantify the influence of phenology in deciduous and evergreen trees; iii) study the effect of varying scan zenith angle (SZA) and canopy age on WF features in different phenostates; iv) assess the temporal feature correlation in individual living and dead standing trees. A WF-recording pulsed LiDAR sensor unit operating at the wavelength of 1550 nm was used in repeated acquisitions. WF attributes such as energy, peak amplitude and echo width were derived for each pulse and were localized vertically to crown, understory and ground components. Silver and downy birch, black alder, European aspen, Siberian larch, Scots pine, Norway spruce and dead standing spruce formed our strata. Results showed that phenology caused more variation in WF features of deciduous trees compared to evergreen conifers. Deciduous trees displayed substantial between-species variation that was linked with differences in branching pattern, leaf orientation and bark reflectance. Pine displayed a possible winter-early summer anomaly in canopy backscattering that may be linked with changes in foliage clumping or with the role of stamens in early summer trees. Trees displayed positive temporal correlation in WF features and correlations were the strongest in evergreen and deciduous conifers and decreased with time. SZA had minor influence on WF features whereas age exercised a strong effect on many features with parallel variation between species and phenostates. Structural changes following death, i.e. ‘aging’ changed the geometric WF features of dead standing trees. Our results provide new insights for enhancing tree species identification by using WF LiDAR and for LiDAR time-series analysis of vegetation.

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| openaire: EC/H2020/771049/EU//FREEDLES Funding Information: Academy of Finland (AF) (project ‘Time-stamped and free photons') and Suomen Luonnonvarain Tutkimussäätiö provided funding for the LiDAR data and early processing. Personnel at Finnmap were helpful in finding ways to provide the WF data, which was never asked by any other customers. Dr. Andreas Roncat kindly gave us his Matlab code to access Riegl proprietary sdf-format. Dr. Pekka Kaitaniemi from Hyytiälä made phenological observations during the campaigns. Dr. Antti Uotila helped in finding the aspens in Hyytiälä. Crown photographs were provided by Dr. Minna Blomqvist and forestry student Albert Häme. The work of AH received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 771049 ). The text reflects only the authors' view, and the Agency is not responsible for any use that may be made of the information it contains. SJ's AF grant numbers were 330422 and 337127 . Funding Information: Academy of Finland (AF) (project ‘Time-stamped and free photons') and Suomen Luonnonvarain Tutkimussäätiö provided funding for the LiDAR data and early processing. Personnel at Finnmap were helpful in finding ways to provide the WF data, which was never asked by any other customers. Dr. Andreas Roncat kindly gave us his Matlab code to access Riegl proprietary sdf-format. Dr. Pekka Kaitaniemi from Hyytiälä made phenological observations during the campaigns. Dr. Antti Uotila helped in finding the aspens in Hyytiälä. Crown photographs were provided by Dr. Minna Blomqvist and forestry student Albert Häme. The work of AH received funding from the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation programme (grant agreement No 771049). The text reflects only the authors' view, and the Agency is not responsible for any use that may be made of the information it contains. SJ's AF grant numbers were 330422 and 337127. Publisher Copyright: © 2023 The Author(s)

Keywords

1550 nm, Branching structure, Change detection, Leaf orientation, Mortality, Radiometric match, Seasonality, Species identification, Time-series

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Citation

Korpela, I, Polvivaara, A, Hovi, A, Junttila, S & Holopainen, M 2023, ' Influence of phenology on waveform features in deciduous and coniferous trees in airborne LiDAR ', Remote Sensing of Environment, vol. 293, 113618 . https://doi.org/10.1016/j.rse.2023.113618