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

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.

Seasonal Patterns in Spectral Irradiance and Leaf UV-A Absorbance Under Forest Canopies

“…Four "measurement points" approximately equidistance between the nearest trees were established in each stand. We considered this the minimum number of replicate patches required to describe small scale variation in irradiance (Hartikainen et al, 2018) and in the plant community in each stand, while remaining feasible to measure close to solar noon.…”

Understory light quality affects leaf pigments and leaf phenology in different plant functional types

“…Understory spectral irradiance in canopy shade during spring leaf out (25/04/2015). The measurements were taken in a deciduous B. pendula stand, following methods described in Hartikainen et al (2018). The different filter treatments are represented by line colours & types, whereby grey = ambient understory irradiance, black = transparent control filter, yellow = attenuation of UV radiation below 350nm, orange = attenuation of total UV radiation, and blue= attenuation of blue and UV radiation.…”

“…The effect of canopy closure on understory spectral composition in the stands was measured in 2015 (Hartikainen et al 2018), and during canopy leaf-out between April and June in 2016, 2017, and 2018. Spectral irradiance was recorded with a Maya 2000 Pro spectrometer (Ocean Optics, Dunedin, FL, USA) calibrated for accuracy in the solar UV and PAR regions of the spectrum.…”

“…Spectral irradiance was recorded with a Maya 2000 Pro spectrometer (Ocean Optics, Dunedin, FL, USA) calibrated for accuracy in the solar UV and PAR regions of the spectrum. The same device and protocol as Hartikainen et al (2018) were used for measurements and processing of the irradiance data. Ambient solar PAR above the canopy at Lammi Biological Station between 2015-2019 was also recorded to show the effects of changing cloudiness on incoming irradiance (PQS1 PAR Quantum Sensor, Kipp & Zonen, Delft, Netherlands, Fig.…”

“…Once the canopy closes, the understory light environment is mostly shade (Chazdon and Pearcy, 1991). Not only does the amount of irradiance change in the understory, but so does its spectral composition, the light quality (Federer and Taner 1966, Vezina et al 1966, Ross and Flanagan 1986, Messier et al 1998, Leuchner et al 2005, Navrátil et al2007, Hartikainen et al 2018). Far-Red (FR) light is mostly reflected or transmitted from canopy leaf surfaces and scattered into the understory shade, reducing the ratio of red(R): FR light (Ballaré and Pierik 2017).…”