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Light intensity, measured by the anthracene–benzene technique, significantly increased in the montane boreal forest of Camels Hump mountain, Vermont, since the mid- 1960's. The region of increased light corresponds to those elevations where red spruce (Picearubens Sarg.) has declined significantly over the same time period. Soil temperature measured at 15 cm has increased and there is higher week-to-week variability than in the 1960's, especially where canopy decline is most severe.
Perkins et al. (Mon,) studied this question.