Agroforestry vs. Agrivoltaic: spectral composition of transmitted radiation and implications for understory crops

Abstract In both agroforestry and agrivoltaics, crops are cultivated under the shade of a top story layer of trees and photovoltaic (PV) panels, respectively. However, the quality (i.e. spectral composition) of the transmitted radiation might differ between the two systems. Tree canopies are green and absorb different spectra selectively, while panels are black and, thus, should not alter the spectral composition of transmitted radiation. Consequently, plant growth and yield may differ depending on the spectral composition of light. In this study, the spectral composition of transmitted radiation (at ground level) was measured with a spectrometer along transects between adjacent rows of trees and PV panels. The transects crossed both sunlit and shaded areas. The radiation transmitted in sunlit areas was nearly identical, qualitatively and quantitatively, to the incident radiation above both systems. However, in the shaded areas transmission was strongly reduced and the spectral composition changed. Under tree canopies the percentage of green (G) and red (R) radiation decreased, while blue (B) and near ultraviolet (NUV) remained similar to the sunlit areas, and far-red (FR) increased sharply. Under the PV panels, both R and FR decreased, G remained similar, while B and NUV increased gradually from the edge of the shade towards the center of the shaded area. This dramatically changed the ratios between different spectra. For instance, the R:FR ratio under the panels varied with the position but remained close to the incoming radiation value (1.35), while under the trees it decreased to 0.35. The R:FR ratio decreased in close correlation (R2 = 0.98) with the fraction of transmitted radiation, under the trees, but not under the panels. The R:B ratio decreased in the shade in both systems, but more so in the panel system. R:G and B:G ratios also changed between and within systems, but less dramatically, while the B:FR ratio decreased at decreasing transmittance under the trees, but increased under the panels. The results indicate that even when transmitted radiation quantity is similar, radiation quality differs substantially between agroforestry and agrivoltaic systems. Possible effects of these changes on understory plant growth and yield are discussed.