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Net photosynthesis of the three dominant tree species of the oak-pine forest of central Long Island was monitored intensively throughout one growing season to test the practicality of field measurements of net photosynthesis for estimating gross production of a forest. White oak (Quercus alba) leaves absorbed 16.9 g C0 2 /g of leaf during the growing season; scarlet oak (Q. coccinea) absorbed 9.9 g/g; pitch pine (Pinus rigida), 7.5 g/g. Previously published data integrated with the C0 2 exchange rates over the growing season gave a new estimate of gross production for the forest of 2,950 g dry matter/m 2 per year. This value is approximately 10-22o/o higher than previous estimates which were based principally on harvest techniques. The discrepancy between estimates emphasizes the difficulty of the measurements and the scale of the errors one must expect even with elaborate techniques.

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