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Title: Gas exchange by Pinus Ponderosa in relation to atmospheric pollutants

Principal Investigator / Author(s): Helms, John

Contractor: UC Berkeley

Contract Number: a132-101

Research Program Area: Ecosystem & Multimedia Effects

Topic Areas: Acid Deposition, Ecosystem Impacts, Impacts


To assess potential impacts of pollutant exposure on plant performance, it is necessary to not only identify and quantify the various pollutant constituents, but also to determine the levels of deposition and uptake. Measures of foliage gas exchange rates are important to determining the ozone dosage received by forest trees. A common hypothesis is that plants with low gas exchange rates (low stomatal conductance) incur less ozone damage due to reduced oxidant uptake. Alternatively, plants having low gas exchange rates have reduced potential for carbon fixation and, therefore, a lesser potential for growth or damage repair. The purpose of the study reported here was to determine the extent to which realistic exposures to acid rain and ozone pollutants, acting singly and together, influence gas exchange rates by Pinus ponderosa (ponderosa pine), a tree of substantial economic, ecological, and aesthetic importance in California. As secondary objectives, the extent of response to pollutants as a function of life-stage (seedling or mature tree), foliage age (current-year or one-year-old foliage) and genotype were explored. Ponderosa pine seedlings and mature tree branches were exposed to combinations of ozone and acid rain using Branch Exposure Chambers (BECs) for a 15-month period beginning in September 1991. Acid rain treatments consisted of no rain or 17 weekly applications of 5 cm simulated rain of pH 5.1 or pH 3.0. The rainfall applications were made between January and May, 1992, to coincide with the natural rainfall season. Ozone treatments consisted of charcoal-filtered air, ambient ozone or twice ambient ozone applied for 14 h per day. The ozone and acid rain exposures were applied as a complete factorial treatment structure. Ambient ozone concentration (12-h average from 0900 to 2100 h) at the study site ranged from approximately 0.01 ppm in January to approximately 0.07 ppm in July and August. Relative to the 12-h average ozone concentration for the ambient treatment, ozone concentrations for charcoal-filtered and twice ambient treatments were approximately 55 and 190 percent, respectively, when averaged over the experiment. Measurements of mid-day photosynthesis and stomatal conductance were made on one-year-old and current-year foliage at monthly intervals from February through November. Estimates of foliar pigment concentrations were made in May and September to provide a relative measure of pollutant stress. The only significant effect of acid rain, relative to no rain, was a slight reduction in stomatal conductance for one-year-old seedling foliage exposed to pH 5.1 or pH 3.0 rain solutions. Relative to ambient ozone, exposure to twice ambient ozone resulted in significant reductions in photosynthesis and conductance for one-year-old mature branch foliage. This effect was most pronounced in the early summer and fall of 1992 as both stomatal conductance and photosynthesis were decreased by as much as 34 percent when exposed to twice ambient ozone. For the study period, gas exchange rates for seedling foliage were substantially affected by exposure to twice-ambient ozone and the degree of effect varied by genotype. Apparent reductions in conductance reached approximately 15 percent while reductions in photosynthesis were as high as 35 percent As with mature branches, the effects were more pronounced in one-year-old foliage. Seedling foliage tended to have higher rates of stomatal conductance and similar rates of photosynthesis compared to those of mature branches for most of the season. Late season gas exchange rates of one-year-old seedling foliage dropped off in contrast with increases for one-year-old branch foliage. Of the three seedlings genotypes, half-sibs of clone 3088 were most sensitive as gas exchange rates at twice ambient ozone were decreased, relative to ambient ozone exposure, for both current-year and one-year-old foliage. Declines in gas exchange for clone 3399 were present in one-year-old foliage only. Relative to ambient ozone, twice-ambient ozone exposure had no impact on gas exchange by clone 3087 for either age-class of foliage. When measured under controlled light and temperature conditions, mature branch and seedling foliage exposed to twice ambient ozone had net photosynthesis rates that were significantly decreased relative to those for the ambient ozone treatment The effect of twice ambient ozone was not only to alter the photosynthetic capacity of the exposed foliage, as indicated by a downward shift in the response surface, but it also resulted in change in the functional response of P D to temperature, and to a lesser extent, light intensity. Foliar pigment concentrations declined from May to September with the greatest decline occurring for foliage exposed to rain of pH 3.0 and twice ambient ozone concentrations. There was little statistical difference among treatments in May but by September, treatment effects were manifest as significant interactions among ozone, acid rain and genotype. In general, there were consistent declines in chlorophyll a, chlorophyll band carotenoids under the most severe acid rain and ozone treatments. When exposed to low and moderate levels of pollutants, pigmentation responses were inconsistent among the three genotypes. The observed decreases in chlorophyll concentration, indicating potential decreases in light harvesting capacity, are consistent with the observed declines in photosynthesis rates.


For questions regarding this research project, including available data and progress status, contact: Heather Choi at (916) 322-3893

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