Surveys of the spectral photon distribution of light in the understory of eight boreal forest stands in northern Alberta were made during 1984. Two seasonal patterns of change in the red:far-red (R:FR) ratio and in the amount of photosynthetically active radiation (expressed as a percentage of full sun, PAR) were observed. In stands dominated by broadleaf deciduous trees, the R:FR ratio and PAR values declined rapidly in May and June during the time of leaf expansion and then increased in association with leaf senescence in September. In stands dominated by mature evergreen conifers, both the R:FR ratio and PAR values declined continuously from an early season high to a late season low. Several young evergreen stands exhibited no seasonal change in light characteristics. During mid-July, the R:FR ratios and PAR values were lower in older stands than in younger stands of the same successional sequence. The variability in light characteristics within mature forests was higher in mid-July than in September; the difference was attributed to the lower solar angle in September. The differences in light regime in early and late successional forests may have important implications for phytochrome-controlled development and succession in understory plant species.
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