The Spectral Composition of Shade Light in Woodlands

Abstract

It is well established in general terms that the leafy canopy of a woodland is a highly selective filter of the light falling on it, and there is a considerable measure of agreement in detail (Knuchel, 1914; Carter, 1934; Seybold, 1934, 1936; Egle, 1937; Atkins et al., 1937; Evans, 1939). Knuchel showed that a canopy of conifers is practically a neutral filter of the diffuse sky light from near the zenith over the range of wavelengths he studied (440-652 mv), while for the same conditions the percentage transmission of thin-leaved deciduous canopies is considerably higher in the yellow-green than in those red and blue regions of the spectrum which he had investigated. Seybold (1936) established the distinction between 'blue shade' and 'green shade' light: the former in sites where diffuse sky light alone has access through gaps in the canopy, so that on a cloudless day the shorter wavelengths tend to predominate in the light reaching the woodland floor (excluding sunflecks); the 'green shade', or more correctly the 'dark red-green shade', in deciduous woodlands where the reflections and transmissions from the leaves contribute largely to the shade light, which therefore is relatively rich in the darkand infra-red, with much green but less orange-red and blue light. Egle, who worked with Seybold, gives data which show very clearly the differences between the transmissions (shade light/diffuse sky light in the open) of a Quercus-Carpinus canopy and a Picea canopy (Egle, 1937, p. 551, Fig. 5); the curve of the spectral composition (intensity against wavelength) of the shade light in the Picea wood runs more nearly level than that in the Quercus-Carpinus wood; since the relative spectral composition of the diffuse sky light on the top of the wood is the same in both cases, the transmission in the red in the coniferous wood is only about half, while in the blue-violet it is about double, that in the deciduous wood. Atkins et al. (1937) also point out that 'the dark green foliage of Quercus ilex gives a curve which is decidedly different from that of the beech wood', the curve referred to being of an estimate of the spectral composition of the shade light, giving evidence that the evergreen Quercus has less highly selective foliage than the Fagus. Egle (loc. cit.) points out that the coniferous canopy only approaches a neutral filter of the incident light over the limited range of wavelengths (excluding the far red) investigated by Knuchel. Ecologists who have been aware of these phenomena have often questioned whether they are of any significance for the growth of woodland plants compared, say, with the great variations in light intensity from place to place and time to time. Shirley (1945) perhaps states the consensus of opinion when he writes: 'Measurements made beneath forest canopies during the past ten years tend to confirm the conclusion reached in the previous review (Shirley, 1935) that changes in light quality resulting from passage through a leafy canopy are of minor ecologic significance (Atkins et al.,