NET PHOTOSYNTHESIS AND STOMATAL CONDUCTANCE OF RED SPRUCE TWIGS BEFORE AND AFTER TWIG DETACHMENT

Rates of net photosynthesis and stomatal conductance of red spruce (Pi cea rubens Sarg.) twigs were measured in field conditions before and a fter twig detachment on clear midsummer days in New Brunswick. Althoug h these variables did not change significantly within about 6 min afte r twig detachment, they did drop continually after detachment. Regress ion models for the rates of net photosynthesis and stomatal conductanc e were developed that can be used to simulate this drop. Photosyntheti cally active radiation, leaf temperature, relative humidity, and CO2 c oncentration were the principal covariates in these models. The regres sions improved by including simulated leaf moisture values in the anal ysis of data obtained after twig detachment.