Abstract:
Spectroradiometric analyses were made to examine the light filtering capacity of coniferous forests and to establish the lower light energy limits for growth of Pseudotsuga menziesii (Mirb. ) Franco, Abies concolor (Gord. and Glend.) Lindi. and Pinus ponderosa Dougi. Visible energy(400-750 nm) was recorded at 48 points under four mixed conifer stands and segregated into four spectral bands (blue, 400-450; green, 500-550; red, 65O-700; far-red, 700-750). Each energy band was expressed as a proportion of the total visible energy using a linear regression. An analysis of unfiltered solar radiation. compared with filtered revealed a significant difference at the 99 percent level for each of the four bands, indicating that a coniferous canopy is a selective filter. Close linear relationships between the bands and the total
energy were established for a range of canopy densities which allowed penetration of 0.4 to 25 percent of the total energy (400-700 nm) received under the canopy per day. The ratio of blu to total energy (400-700 nm) was 0.l597. Green, red, and far-red had ratios of 0.1919, 0.1240, and 0.1300 respectively. Terminal growth of 34 seedlings was measured at the light sample points to provide a means of establishing lower light energy limits for survival. The limits for Abies and Pseudotsuga were 2.0
Clear Day Index (CDI) which is equivalent to 2,000 W cm -2 day -1 (400-700 nm). Pinus was found only where the light energy exceeded
40.0 CDI. An interaction with moisture appears to influence the minimum light energy requirements of a species. Where moisture was adequate throughout the growing season, the light limit for Pseudotsuga
seedling establishment was 2.0 CDI; where moisture became limiting the minimum light requirement increased to 7.0 CDI.
