Selective filtering of light by coniferous forests and minimum light energy requirements for regeneration

Abstract

Spectroradiometric analyses were made to examine the light-filtering capacity of coniferous forests and to establish the lower limits of light energy for growth of Pseudotsuga menziesii (Mirb.) Franco, Abies concolor (Gord. and Glend.) Lindl., and Pinns ponderosa Dougl. Light energy (400–750 nm) was recorded periodically on clear days in June and July at 48 points under four mixed conifer stands and segregated into four spectral bands (blue, 400–450; green, 500–550; red, 650–700; far-red, 700–750 nm). Minor changes in the proportion of light in one band to that in another occur throughout the day, but in general the 400- to 700-nm range appears rather uniformly absorbed by a coniferous forest canopy. Averaged for an entire day, only the far-red band showed a significant proportional increase under a coniferous canopy compared to unfiltered sunlight.The relation between the energy received in each of the spectral bands and the total recorded in the 400- to 700-nm range was linear. The sampled canopies transmitted up to 25% of full sunlight. Beneath the canopies, the ratio of blue to total energy (400–700 nm) was 0.1540. Green, red, and far-red had ratios of 0.1853, 0.1220, and 0.1207.Terminal growth of 34 seedlings was measured at the light-sampling points to provide a means of establishing lower limits of light energy for survival. The lower limit for Abies and Pseudotsuga was 1.85 langleys (ly) per day (400–700 nm). Pinus grew only where the light energy exceeded 36.8 ly/day.An interaction with moisture appears to influence the minimum requirements for light energy of a species. Where moisture was adequate throughout the growing season, the lower limit of light for Pseudotsuga seedling establishment was 1.85 ly/day; where moisture became limiting, the minimum light requirement increased to 6.64 ly/day.