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Hartikainen SM, Jach A, Grané A, et al. Assessing scale-wise similarity of curves with a thick pen: As illustrated through comparisons of spectral irradiance. Ecol Evol. 2018;8(20):10206-10218doi: 10.1002/ece3.4496.
Hartikainen, S. M., Jach, A., Grané, A., & Robson, T. M. (2018). Assessing scale-wise similarity of curves with a thick pen: As illustrated through comparisons of spectral irradiance. Ecology and evolution, 8(20), 10206-10218. https://doi.org/10.1002/ece3.4496
Hartikainen, Saara M, et al. "Assessing scale-wise similarity of curves with a thick pen: As illustrated through comparisons of spectral irradiance." Ecology and evolution vol. 8,20 (2018): 10206-10218. doi: https://doi.org/10.1002/ece3.4496
Hartikainen SM, Jach A, Grané A, Robson TM. Assessing scale-wise similarity of curves with a thick pen: As illustrated through comparisons of spectral irradiance. Ecol Evol. 2018 Sep 12;8(20):10206-10218. doi: 10.1002/ece3.4496. eCollection 2018 Oct. PMID: 30397459; PMCID: PMC6206219.
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Ecol Evol. 2018 Sep 12;8(20):10206-10218. doi: 10.1002/ece3.4496. eCollection 2018 Oct.

Assessing scale-wise similarity of curves with a thick pen: As illustrated through comparisons of spectral irradiance.

Ecology and evolution

Saara M Hartikainen, Agnieszka Jach, Aurea Grané, Thomas Matthew Robson

Affiliations

  1. OEB, Viikki Plant Science Centre University of Helsinki Helsinki Finland.
  2. Department of Finance and Statistics Hanken School of Economics Helsinki Finland.
  3. Department of Statistics Universidad Carlos III de Madrid Getafe Spain.

PMID: 30397459 PMCID: PMC6206219 DOI: 10.1002/ece3.4496

Abstract

Forest canopies create dynamic light environments in their understorey, where spectral composition changes among patterns of shade and sunflecks, and through the seasons with canopy phenology and sun angle. Plants use spectral composition as a cue to adjust their growth strategy for optimal resource use. Quantifying the ever-changing nature of the understorey light environment is technically challenging with respect to data collection. Thus, to capture the simultaneous variation occurring in multiple regions of the solar spectrum, we recorded spectral irradiance from forest understoreys over the wavelength range 300-800 nm using an array spectroradiometer. It is also methodologically challenging to analyze solar spectra because of their multi-scale nature and multivariate lay-out. To compare spectra, we therefore used a novel method termed thick pen transform (TPT), which is simple and visually interpretable. This enabled us to show that sunlight position in the forest understorey (i.e., shade, semi-shade, or sunfleck) was the most important factor in determining shape similarity of spectral irradiance. Likewise, the contributions of stand identity and time of year could be distinguished. Spectra from sunflecks were consistently the most similar, irrespective of differences in global irradiance. On average, the degree of cross-dependence increased with increasing scale, sometimes shifting from negative (dissimilar) to positive (similar) values. We conclude that the interplay of sunlight position, stand identity, and date cannot be ignored when quantifying and comparing spectral composition in forest understoreys. Technological advances mean that array spectroradiometers, which can record spectra contiguously over very short time intervals, are being widely adopted, not only to measure irradiance under pollution, clouds, atmospheric changes, and in biological systems, but also spectral changes at small scales in the photonics industry. We consider that TPT is an applicable method for spectral analysis in any field and can be a useful tool to analyze large datasets in general.

Keywords: canopy; cross‐dependence; multi‐scale; phenology; spectral irradiance; spectroradiometer; sunfleck; thick pen measure of association; thick pen transform; understorey

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