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Onie of the most important forestry problems in the south concernis the tendency of pine stands to be succeeded by hardwoods. This occurs because pine seedlings usually fail to survive under forest canopies while the seedlings of many hardwood species survive and grow. It is often supposed that the failure of pine seedlings to grow under a forest canopy results from their high light requirements. Pine seedliings are assumed to be unable to carry on photosynthesis rapidly enough in the shade of a forest canopy to survive.Many hardwoods which presumably have lower light requirements and can therefore manufacture food more efficiently in the shade are able to grow vigorously under these coniditionis. Experiments of KORSTIAN aiid COILE (6) indicate, however, that pine seedlings will thrive in the shade of a pine or hardwood stand if protected fromii the root competition of the ov-er-story trees by trenching. Evidently pine seedlings can carry ol0 enougohl plhotosynthesis in the shade to provide food for growth, if supplied with sufficient water.These results raise somie questions coneerninig the actual effect of low light intensity on the photosynthesis of pine seedlings and the importaniee of shade in the competition of pine with hardwoods. Few miieasurements of the rate of photosynthesis of tree seedlings have been made. The only comliparison of American species of pines and hardwoods is based on deteriminationls of the compensation points of several species made by BURNS (1). Since nlo data are available for species native to the southeastern states several series of experiments were performed to determine the relative rates of plhotosynthesis of pine and certain competing hardwoods at various lighlt intensities. The species studied were loblolly pine (Pinus taeda L.), easterni red oak [Qtercus borealis miaxima (Marsh) Ashe], white oak (Qutercuts alba L.) anld dogwood (Corwuts florida, L.). It, was hoped that these experiments would vield information concerning the physiological differenees betweeni loblolly pine and the hardwoods which would aid in explaining tlle differeniees in their behavior when growing under forest stands. MethodsThe plants used in these experimients were one-or tw-o-year-old potted seedlings which had been grown out-of-doors, except the eastern red oak which was grown in a greenhouse at about half the intensity of full sunlight. The seedlings were kept in the laboratory only longc enough for the aetual tests, then returned to the greenhouse or out-of-doors. The plants wi-ere kept 350 www.plantphysiol.org on May 11, 2018 -Published by Downloaded from

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