FIELD-MEASUREMENTS OF ISOPRENE EMISSION FROM TREES IN RESPONSE TO TEMPERATURE AND LIGHT

The atmospheric hydrocarbon budget is important for predicting ozone e pisodes and the effects of pollution mitigation strategies. Isoprene e mission from plants is an important part of the atmospheric hydrocarbo n budget. We measured isoprene emission capacity at the bottom, middle , and top of the canopies of a white oak (Quercus alba L.) tree and a red oak (Quercus rubra L.) tree growing adjacent to a tower in the Duk e University Forest. Leaves at the top of the white oak tree canopy ha d a three- to fivefold greater capacity for emitting isoprene than lea ves at the bottom of the tree canopy. Isoprene emission rate increased with increasing temperature up to about 42 degrees C. We conclude tha t leaves at the top of the white oak tree canopy had higher isoprene e mission rates because they were exposed to more sunlight, reduced wate r availability, and higher temperature than leaves at the bottom of th e canopy. Between 35 and 40 degrees C, white oak photosynthesis and st omatal conductance declined, whereas red oak (Quercus rubra) photosynt hesis and stomatal conductance increased over this range. Red oak had lower rates of isoprene emission than white oak, perhaps reflecting th e higher stomatal conductance that would keep leaves cool. The concent ration of isoprene inside the leaf was estimated with a simplified for m of the equation Used to estimate CO2 inside leaves.