Global radiation beneath the canopy and in a clearing of a suburban hardwood forest

Three recording instruments were used over a 1-year period to gather global radiation data under clear skies in a small (375 m 2) pine-sapling inhabited clearing, under the canopy of a pine-seedling free hardwood forest, and in the open. Curves for the annual cycle of shortwave radiation available to the clearing and under the canopy are bimodal due to the superposition of the out-of-phase cycles of deciduous vegetation leafing and the amount of solar energy above the canopy of the forest. Ratios of the daily amount of solar radiation beneath the canopy and in the clearing to that in the open have annual ranges of 3–28% and 7–43%, respectively. Actual energy available to the pines on clear days in the clearing was above 150 ly/day from March to June, but never reached this level under the canopy. These data suggest that 125–150 ly/day may be the clear-sky solar energy required for pine survival at the Piedmont study site.