Photosynthetic Action Spectra of Trees

Abstract

The relative reflectance spectra for control and treated (surface wiped) current-year foliage of Douglas fir, and Sitka, Colorado, and Blue spruce (Pseudotsuga menziesii [Mirb.] Franco, Picea sitchensis [Bong.] Carr., Picea pungens Engelm., and Picea pungens Engelm. var. hoopsii, respectively) were obtained from 220 to 700 nm. The green color of the control foliage of both Douglas fir and Sitka spruce was unaffected by the treatment whereas the blue-green and blue-white foliage of control Colorado and Blue spruce, respectively, became “green” as a result of the wiping. The relative reflectance curves for all green foliage, including the treated Colorado and Blue spruce, were all very similar with a peak in the green (540-560 nm), minima in the red (660-680 nm) and blue (450-500 nm), and very low reflectivities in the ultraviolet (λ < 400 nm). In contrast, the control foliage for Colorado and Blue spruce both showed a generally higher relative reflectance over most of the visible spectrum (400-700 nm) with a marked increase in the blue region (400-500 nm). At wavelengths below 420 nm, their relative reflectances increased sharply with decreasing wavelength, the reflectance at 220 nm for Blue spruce being over four times that at 540 nm.

Scanning electron microscope examination of the needles' surfaces revealed a system of wax filaments whose complexity correlated with the degree of ultraviolet and blue reflectance.

It is concluded that both the bluish appearance (glaucous bloom) and the low relative efficiencies of blue light in photosynthesis of Colorado and Blue spruce result from the selectively enhanced reflection of blue light caused by the presence of the epicuticular wax deposits. The enhanced blue light reflection was shown to be the shoulder of a scattering effect which appeared to peak in the short ultraviolet region below 200 nm. The ecological implications of the results are discussed.