Flux determinations and physiological response in the exposure of red spruce to gaseous hydrogen peroxide, ozone, and sulfur dioxide

We report on the 3-week exposure of a branch of a forest-grown red spruce (Picea rubens) sapling to the combination of gaseous hydrogen peroxide, ozone, and sulphur dioxide. The exposure was conducted continuously using concentrations of H₂O₂, O₃, and SO₂ that have been observed during the summertime on the summit of Whiteface Mountain, New York. Fluxes of H₂O, CO₂, and the three pollutants were determined throughout the exposure. At weekly intervals, measurements of chlorophyll fluorescence, stomatal conductance, and hydrocarbon emissions were made. The response of the branch was compared to an equivalent branch of the same tree which received no pollutants but was otherwise treated identically. The exposure produced no visible injury symptoms but did produce an increase in dark respiration; the respiration rate more than doubled during the 21-day exposure period. Net photosynthesis was unaffected for both the experimental and the control branches. Nighttime fluxes of SO₂ and H₂O₂ to external plant surfaces were significant. The stomatal component of O₃ uptake by the branch displayed a linear increase during the experiment, and showed no evidence of saturating. Daytime and nighttime fluxes of H₂O₂ were increasing at the end of the experiment. We observed that isoprene is emitted from red spruce, but saw no clear-cut change in emission rate in response to the exposure experiment. Possible relationships between our experimental results and observations of red spruce decline in the forests of the northeastern United States are suggested for further investigation.