Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Natural Resources

Department name when degree awarded

Soil Science and Biometeorology


Inge Dirmhirn


The spectral distribution of the global radiation from 0.4 to 2.5 microns penetrating deciduous and coniferous canopies were measured during clear days between 10 a.m. and 2 p.m. using a double-quartz monochromator.

In the visible region (0.4 to 0.7 micron) the average relative spectral transmissions under both canopies are about one percent beginning at 0.4 micron and decreasing to about half a percent at 0.67 micron. There is only a small peak in the green (0.55 micron) transmission under deciduous stands while there is none under coniferous canopies. The slightly higher transmission in the blue (0.4 micron) is attributed to the direct sky radiation penetrating through the gaps in the canopies. There is a steep increase in the transmission at about 0.7 micron. The increase is relatively higher under deciduous stands compared to coniferous stands.

In the infrared region from 0.8 to about 1.4 microns, the average relative spectral transmission under deciduous stands is about 10 percent which is double the transmission under coniferous canopies. The transmission under deciduous stands is about twice that of the coniferous stands throughout the near infrared with very low transmission in the water absorption band at 1.45 and practically no transmission at all in the 1.90 micron-band.

The absolute spectral transmission exhibit a somewhat different distribution, especially in the visible region. Since the highest intensity of the solar spectrum in the open is located in the 0.5 micron-band, this is also reflected in the absolute values. The small peak in the green under deciduous stands is now indicated as a slight shift of the peak to the 0.55 micron-band. The water absorption bands at the 0.95 and 1.15 microns are also distinct, with hardly no transmission at all beyond 1.7 microns.

The spectral transmittance of forest canopies differ from those reported for single leaves in the proportion of radiation transmitted in the visible and infrared regions. For example, the ratio of the transmission at 0.55 micron to that at 1.10 micron-band is about one to twelve compared to about one to five in single leaves.

A deciduous canopy consisting of several layers of leaves wi ll only allow a very small amount of transmission, mostly in the green portion and somewhat more in the infrared region between 0.72 and 1.40 microns. Under natural conditions in the forest, there exists a very weak "green" shadow and a somewhat stronger "infrared" shadow. The altered spectral composition may influence the understory vegetation as regards photosynthesis, seed germination, and the photoperiodic responses in the forest floor.