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No. 95-7 |
| May 1995 | |
| RESEARCH NOTES |
| California Environmental Protection Agency | Brief Reports to the Scientific and Technical |
| Air Resources Board | Community |
| Research Division, John R. Holmes, Ph.D., Chief | P.O. Box 2815, Sacramento CA 98512 |
In California, species of pine that are sensitive to air pollution are exposed to harmful levels of ozone and to elevated levels of nitrogen-derived pollutants in acidic deposition. Both of these pollutants can disrupt physiological processes, such as gas exchange by leaves. The objectives of the companion studies on which this Research Note is based were to characterize the effects of ozone, in combination with acidic deposition, on a suite of growth and physiological responses in mature branches and seedlings of ponderosa pine grown under conditions representative of north central California. Exposure to twice-ambient ozone caused moderate reductions in photosynthesis and stomatal conductance in seedlings and mature branches. Exposure to acidic rain had no consistent impact. The results indicate that exposure to elevated levels of ozone inhibits pine tree gas exchange rates throughout the growing season. After several years of exposure, this decrease in gas exchange rates could diminish the ecological and economic value of forest resources. While acid rain alone had no effect after one year of exposure, its long-term effect, in combination with ozone, is not well characterized. The studies were performed by the University of California, Berkeley, and the U.S.D.A. Forest Service.
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Background: |
In the 1960s, field surveys in the San Bernardino mountains documented the presence of pine trees with varying degrees of ozone-caused needle injury. Other researchers found that gas exchange processes, stomatal conductance, and photosynthesis, were depressed in ozone-injured pine needles. In the 1980s, field surveys in the Sierra Nevada found widespread ozone-caused needle injury on Jeffrey and ponderosa pine in Sequoia and Kings Canyon National Parks. These findings suggest that ambient ozone may also be affecting gas exchange processes in pines in the Sierra Nevada. In forests, trees range in age from very young seedlings (<5 years old) to mature trees (>70 years old). To date, studies on the effects of air pollution on forest species have focused on seedling responses, largely because of the difficulty in exposing mature trees to controlled atmospheres. In most cases, the effects of air pollutants on older trees have been assessed by extrapolating results obtained on seedlings, in consideration of known structural, physiological, and microclimatic differences. Few studies, if any, have collected concurrent, direct measurements of plant responses in trees of different ages, and measurements of this kind are needed to determine whether young and old trees are similarly affected by ambient air pollutants. |
| Methods: | Ponderosa pine (Pinus ponderosa) seedlings and mature branches were exposed to ambient ozone or twice-ambient ozone and acidic rain (pH 5.1 or 3.0) for 15 months in branch exposure chambers. Results of these exposures were compared with those from charcoal-filtered (ozone-free) air. Three genotypes of ponderosa pine were examined in this study. The "mature branches" were branches from trees produced by grafting buds from 70-year-old trees onto rootstock seedlings that were grown for over 15 years in a seed production orchard at the U.S.D.A. Forest Service Tree Improvement Center in Chico, California. The trees that were the source of the buds for mature branches were also the source of seed for the seedlings used in the study. Ozone exposures were applied for 14 hrs per day from September 1991 through December 1992. The acidic rain treatments were applied weekly as 5-cm events from January through May 1992. Acidic gases and particles were measured over a three-day period in summer to quantify acidic depositions during dry periods. Measurements of mid-day photosynthesis and stomatal conductance were made at monthly intervals from February through November 1992, using a portable gas exchange system. |
| Results: |
Daytime 12-hr average ozone concentrations were ~20, ~40, and ~80 ppb in the charcoal-filtered air, ambient ozone, and twice-ambient ozone treatments, respectively. The pH 5.1 acidic rain treatment provided wet-deposited hydrogen ion, nitrogen, and sulfur inputs of 6.6, 25, and 29 meq per square meter, while inputs in the pH 3.0 rain treatment were 128, 18, and 12 times higher, respectively. In July, mean concentrations of nitric acid vapor were 4.4 µg per cubic meter during the day and 1.7 µg per cubic meter at night. Mid-day gas exchange rates were measured in mature branches and seedlings from February through November 1992. Mean rates of stomatal conductance and photosynthesis, and the effects of ozone, differed as a function of plant life stage (mature branch or seedling) and needle age (current-year or one-year-old). Gas exchange rates were consistently higher in seedlings than in mature branches. In seedlings, mean stomatal conductance and photosynthesis ranged from 0.06 to 0.12 mol per square meter per second and 3.0 to 5.1 µmol CO2 per square meter per second, respectively. In mature branches, corresponding values ranged from 0.05 to 0.09 mol per square meter per second and 3.2 to 4.5 µmol CO2 per square meter per second. In terms of needle age, mean gas exchange rates were higher in current-year (emerged in 1992) than one-year-old needles (emerged in 1991). Differences between needle age classes were more pronounced in seedlings than in mature branches; rates in current-year needles were 40 to 70 percent higher than in one-year-old needles in seedlings and 20 to 40 percent higher in mature branches. Plants exposed to twice-ambient ozone exhibited consistent decreases in gas exchange rates, but the effects of ambient ozone were very slight. Statistically significant decreases were observed in one-year-old needles of mature branches, which experienced 15 and 20 percent decreases in stomatal conductance and net photosynthesis, respectively, relative to plants exposed to charcoal-filtered air. Decreases of similar magnitude (~15 percent) were observed in one-year-old needles in seedlings, but the differences were not statistically significant. In current-year foliage, decreases in gas exchange caused by twice-ambient ozone levels ranged from 3 to 12 percent. Among genotypes, decreases in gas exchange caused by twice-ambient ozone levels tended to be less pronounced in the clone of intermediate vigor than in the clones of low and high vigor. |
| Significance and Application: |
The availability of mature branches and trees with the same genetic background, and extensive facilities for exposing pines to ozone and acidic rain, provided an uncommon opportunity to collect concurrent, direct measurements of gas exchange on trees of different ages. Strictly speaking, although the study did not examine mature whole trees, the branches used in the study are structurally and physiologically similar to branches from mature whole trees. Studying mature branches in conjunction with seedlings enables direct comparisons to be made between tree responses to air pollutants at young and older life stages. The results of these studies indicate that gas exchange rates in older trees may be more sensitive to ozone-caused decreases than in seedlings. Moreover, one-year-old needles exhibit greater ozone-caused depressions than current-year needles; this is consistent with the cumulative nature of ozone injury to pine tree needles. If air pollution-related decrements in fundamental growth processes are occurring at levels found in forests outside the South Coast Air Basin, improvements in air quality may be needed statewide to prevent the gradual deterioration of our ecological resources. |
| Related Projects: | From 1989 to 1991, the Air Resources Board sponsored a field chamber study to examine the effects of ozone and dry acidic deposition on ponderosa pine seedlings at Shirley Meadow in the southern Sierra Nevada. After two years of exposure, twice-ambient ozone caused reductions in gas exchange rates, foliar starch reserves, and growth of foliage, stems, and roots. |
| This research was conducted under contract with the University of California, Berkeley, and the U.S.D.A. Forest Service (ARB Contract No.s A132-101 and A132-174). Comments or questions can be directed to the contract manager, Brent Takemoto, by mail, FAX (916) 322-4357, phone (916) 324-2981, or e-mail: btakemo@arb.ca.gov. For an index of Research Notes, call (916) 445-0753 or FAX (916) 322-4357. |
| Copies of the research report upon which this Note is based can be ordered from: |
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National Technical Information Service
5285 Port Royal Rd Springfield VA 22161 Request NTIS No. PB94-109063 for the UC Berkeley report. The NTIS number for the USDA Forest Service report is pending. |
| Reports: Gas Exchange by Pinus ponderosa in Relation to Atmospheric Pollutant (A132-101) by J.A. Helms, P.D. Anderson, and J.L.J. Houpis and Determination of Acidic Gas and Particle Concentrations in Open-Top Field Chambers (A132-174) by A. Bytnerowicz. |