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.
Highlights
A measurement of spectral irradiance can be viewed as a curve made up of rough and smooth components: The rough parts are linked with variations in the spectrum occurring over small scales, while the smooth parts are linked with large-scale variations
We demonstrate the utility of this approach in addressing the following questions related to spectral irradiance in forest understoreys: (a) How is the spectrum modified by the time of year during spring canopy leaf-out? (b) Do stands comprising different canopy types affect the spectrum differently? (c) To what extent is the solar irradiance spectrally different at distinct sunlight positions in the understorey of a forest stand? (d) Are the impacts of sunlight position, stand, and time of year on the rough part of the spectrum different from the impacts of these factors on the baseline part of the spectrum?
Of the various factors examined, position had the strongest influence on spectral irradiance: That is, spectra from sunflecks, semi-shade, and shade positions compared by thick pen measure of association (TPMA) were least similar overall and became ever more dissimilar as the spring progressed (Figure 9)
Summary
A measurement of spectral irradiance can be viewed as a curve made up of rough and smooth components: The rough parts are linked with variations in the spectrum occurring over small scales, while the smooth parts are linked with large-scale variations (scale is defined as the number of units of the x-variable, here wavelength, scale equals the number of nm). Different regions of the solar spectrum are differentially affected due to selective absorption, transmittance, and reflectance by, for example, atmospheric gases, particles, or organic material In addition to these fundamental considerations, the time of year and nearby physical structures both interact with radiation and can significantly alter the incident irradiance at a given location (Federer & Tanner, 1966; Grace, 1983; Smith, 1982). Technological advances in the manufacture of small portable spectroradiometers, that use linear array detectors to simultaneously measure a spectrum, allow multiple successive snapshots of highly dynamic environments such as forest understoreys to be captured within seconds or even faster This contrasts with a scanning spectroradiometer which would scan across the wavelength range over a period of time taking up to minutes to record a single solar spectrum. We demonstrate the utility of this approach in addressing the following questions related to spectral irradiance in forest understoreys: (a) How is the spectrum modified by the time of year during spring canopy leaf-out? (b) Do stands comprising different canopy types (structure and tree species) affect the spectrum differently? (c) To what extent is the solar irradiance spectrally different at distinct sunlight positions (sunfleck, semi-shade, and shade) in the understorey of a forest stand? (d) Are the impacts of sunlight position, stand, and time of year on the rough part of the spectrum different from the impacts of these factors on the baseline part of the spectrum?
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