Environ. Sci. Technol. 1987, 21, 606-608
Studies, SUNY-College of Environmental Science and Forestry, for their technical assistance and stimulating discussions and MaryGail Perkins for assistance in sample collection and drafting of X-ray spectra. We acknowledge Nancy Auer for enumeration of zooplankton. We also benefited from discussions with G. Russell-Hunter.
Table I. Ranges in Selected Chemical Conditions in the Upper 5 m of Onondaga Lake from Early Summer to Late Fall, 1985 condition PH [Ca2+],mrno1.L-’
range
7.3-8.0 11-15 1.2-2.2
alkalinity, mequiv-L-’ calcite saturation index (SI)” 0.3-1.1 ” SI = log [(ion activity product for calcite)/(thermodynamic solubility product for calcite)] ( 4 ) . SI > 0 corresponds to oversaturation.
greater weight of the zooplankton. The latter effect may have water-quality implications as it probably reduces grazing pressure on phytoplankton, and this may cause higher standing crops of phytoplankton and increased phytoplankton deposition. Effler et al. (12) hypothesized that the exceedingly high rates of oxygen depletion in the hypolimnion of Onondaga Lake are in part due to the high rate of phytoplankton deposition. The occurrence and extent of the calcium-coating phenomenon, documented here for Onondaga Lake, in other hard-water systems are unknown. The concentration of Ca2+in Onondaga Lake is unusually high as a result of the ionic waste received. However, the equilibrium conditions are not extraordinary with regard to the degree of oversaturation with respect to calcite solubility observed in other productive hard-water lakes (8). Thus, the coating phenomenon documented here may occur in other lakes that experience oversaturated conditions.
Acknowledgments We thank Robert Hanna, Arnold Day, and Jack McKeon of The N. C. Brown Center for Ultrastructure
Registry No. Calcite, 13397-26-7. Literature Cited (1) Effler, S. W. J. Great Lakes Res. 1984, 10, 3-14. (2) Strong,A.; Eadie, B. J. Limnol. Oceanogr. 1978,23,877-887. (3) Stumm, W.; Morgan, J. J. Aquatic Chemistry;Wiley: New York, 1970. (4) Effler, S. W.; Driscoll, C. T. Environ. Sci. Technol. 1985, 19, 716-720. (5) Jones, B. F.; Bowser, C. J. Lakes: Chemistry, Geology,and Physics; Lerman, A. Ed; Springer-Verlag: New York, 1978; pp 197-235. (6) Effler, S. W.; Field, S. D.; Meyer, M. A.; Sze, P. J. Enuiron.
Eng. Diu. (Am. SOC.Ciu. Eng.) 1981, 107, 191-210. (7) Meyer, M. A,; Effler, S. W. Enuiron. Pollut., Ser. A 1980, 23, 131-152. (8) American Public Health Association Standard Methods for Analysis of Water and Wastewater, 14th ed.; American Public Health Association: Washington, DC, 1975. (9) Wetzel, R. Limnology; Saunders: Philadelphia, 1975. (10) Garofalo, J. E.; Johnson, D. L.; Hassett, J. M. In Trace Substances in Environmental Health, 18th; Hemphill, D. D., Ed.; University of Missouri: Columbia, MO, 1984, pp 403-415. (11) Mudrock, A. J. Great Lakes Res. 1984, 10, 286-298. (12) Effler, S. W.; Perkins, M. G.; Brooks, C. M. Water,Air, Soil Pollut. 1986, 29, 93-108.
Received for review May 23,1986. Accepted December 24,1986.
CORRESPONDENCE Comment on “Regional Tree Growth Reductions due to Ambient Ozone: Evidence from Field Experiments” SIR Forests of the U.S.may well be adversely affected by ozone, but a case based on evidence for hybrid poplar cannot be made from the experiment of Wang et al. (I). The experiment that led the authors to conclude that ambient ozone causes regional tree growth reductions was confounded by a profound chamber effect as indicated by the significant improvement in growth in the ambient chamber vs. ambient no-chamber treatment. Even if there were no uncertainty in the methodology, 1 year of field data can never be considered definitive because of the yearly variability in growing conditions and in ozone episodes. In New Brunswick, NJ, we compared the growth of hybrid poplar “clone 388” in filtered and nonfiltered chambers for 5 years from 1975 to 1979 (2). No significant difference in linear growth was found between filtered and nonfiltered air treatments in any year or in biomass in the single year that it was measured, despite the occurrence 606
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of O3levels high enough to produce symptoms and growth reductions on sensitive herbaceous species. New Brunswick, NJ, is evidently a more polluted area than Millbrook, NY. Monitoring data for 1984 reveals 20 days in Millbrook compared to 46 in New Brunswick with a 1-h average of 0.08 ppm and 3 days in Millbrook compared to 10 in New Brunswick with a 1-h average of 0.12 ppm (3). We also conducted a greenhouse test in which hybrid poplar clone 388 was exposed to 0.17 ppm O3 for 5 h two, three, four, or five times during a 1-month period ( 4 ) . O3 did not significantly affect the growth of the seedlings. Although Wang et al. concluded that substantial growth reductions could occur in forests without accompanying visible symptoms, the notion is not supported by hybrid poplar data-Jensen and Dochinger exposed hybrid poplar to 1.0 ppm O3 for 2-8 h and found injury on 70% of the leaves, but no growth reduction. Similarly in our study cited above, injury occurred on 50% of the leaves without any growth reduction. Wang et al. cited five references (ref 20, 22, 23, 25, and 26) as evidence supporting the susceptibility of hybrid poplar to ambient ozone. Typically in these greenhouse and laboratory experiments, seedlings were exposed to 0.15
0013-936X/87/0921-0606$01.50/0
0 1987 American Chemical Society
ppm O3for 5 h/day, 5 days/week for 6 weeks. As illustrated by the Millbrook and New Brunswick monitoring data, such continuous, high-level exposures do not occur in ambient air in the eastern US. The published literature does not provide evidence for the sensitivity of this hybrid poplar to ambient ozone; rather, a rating of “tolerant” may even be justified. Registry No. 03,10028-15-6. Literature Cited (1) Wang, D.; Bormann, F. H.; Karnosky, D. F. Environ. Sci. Technol. 1986,20, 1122-1125. (2) Harkov, R.; Clarke, B.; Rhoads, A.; Brennan, E. Phytopathology 1980, 70, 463. (3) New Jersey Department of Environmental Protection, Trenton, NJ. (4) Harkov, R.; Brennan, E. Plant Dis. 1982, 66, 587-589.
Eileen Brennan* Rutgers, The State University of New Jersey New Brunswick, New Jersey 08903
Ronald S. Harkov New Jersey Department of Environmental Protection Trenton, New Jersey 08625
S I R Air pollution effects research is necessarily limited to controlled laboratory environments, less controlled greenhouse conditions, or to very site-specific field conditions. Extrapolation of results from these types of conditions to “natural” ecosystems is difficult. Our experiments reported in ES&T (1) follow up a 3-year series of field experiments on Populus tremuloides reported in the Canadian Journal of Forest Research (2). As a consequence of our findings, we did NOT “conclude that ambient ozone causes regional tree growth reductions” as Drs. Brennan and Harkov suggested we did. In our abstract we stated: “If this ‘invisible’ injury is typical in other tree species, the extent of ozone-induced forest damage may presently be greatly underestimated. Additional field studies on a regional basis are needed.” At the end of our results and discussion we stated: “While it is difficult to extrapolate to mature forests from controlled field experiments on saplings, our results suggest that there is a considerable potential for major growth suppression of Populus over much of the estimated 20 million ha where it is dominant in the United States.” We have taken the space to repeat these sentences because they reflect carefully considered wording, which, in fact, when compared with the surety with which a physicist experimentally determines the speed of light (to a precision of 0.004 parts per million) “concludes” nothing about the extent of regional tree growth reductions. We believe, and one of us has indicated in the literature (3),that conclusiue research on regional-scale issues, especially with respect to natural ecosystems, may never become available. Scientifically conducted laboratory, greenhouse, and site-specific field experiments, such as we conducted, can only contribute to the body of circumstantial evidence that may eventually lead us as a society to believe that air pollution is having regional effects on our surrounding landscape. The scientific method has no direct role in proving that regional ecosystems are impaired by air pollution. We cannot test the hypothesis with an experiment (with treatments and controls) over the vast complexity and diversity of ecosystems that comprise our
landscape. Even attempting such experiments for a single, full-sized forest ecosystem (containing 100+ plant taxa, not to mention animals, fungi, and microbes) is prohibitive in expense and time (with the life cycle of a typical tree >lo0 years). Epidemiological approaches can provide a “test” of sorts, but if our experience with smoking and cancer is any indicator, this approach promises to involve great quantities of research money (more than will probably be allocated to landscape health studies) and much controversy. We believe our experimental results contribute to the “case” that air pollution threatens our forest ecosystems. The experiments were conducted with a rigorous design and statistical analysis. They are only representative of one site, in one year, for one clone of Populus. On the other hand, substantial growth reduction actually occurred without artificial additions of pollutants and no visible signs of foliar injury. These two points make a major contribution to a conceptual framework (1)that impacts to trees occur under field conditions and (2) that effects can be “invisible” and therefore not evaluated over almost all of our landscape. In comparing our work with theirs, Drs. Brennan and Harkov raise another basic issue regarding effects research and the scientific method in general. There is substantial disagreement over the meaning of “failing to reject the null hypothesis”. Parkhurst ( 4 ) very succinctly points out the difference between “failing to reject the null hypothesis” and “accepting the null hypothesis”. Failure to disprove a null hypothesis does not prove that null hypothesis. He notes, however, that at least one popular biometry text (5) suggests that the two are equivalent. Failure to reject the null hypothesis can just as easily result from an inadequately replicated experiment as from the absence of an effect. Drs. Brennan and Harkov indicate that in their experiments no differences in linear growth were found in a 5-year period and no differences in biomass were observed in the single year that it was measured, despite higher concentrations of pollutants in New Brunswick, NJ, as compared with Millbrook, NY, (our study site). Because the reference they cite is only an abstract, we cannot evaluate the resolution of their experimental design. However, in our experiments, despite (1)use of clones, (2) careful selection and blocking of field plots, and (3) provision of uniform plant moisture conditions, the coefficient of variation for measured variables from tree to tree was approximately 40%. This high level of variation makes detection of differences less likely, and mandates numerous replicates. HOWshould we interpret the lack of significant differences in growth in Drs. Brennan and Harkov’s similar experiments with Populus? They also indicated that visible foliar injury occurred on 50% of the leaves “without any growth reduction”, thus suggesting that their experiments contradict our observation that growth reductions could occur without visible injury. We would suggest that there is no inherent conflict between our two sets of experiments. To begin with, the absence of a growth effect in New Jersey could easily be due to high interplot variation coupled with low numbers of replicates. In addition, we irrigated our plots (based on tensiometer readings) to maintain optimal growing conditions. Given the now well-known relationship between water stress, stomatal conductance, and ozone sensitivity, a simple difference in experiments (e.g., possibly the lack of irrigation in their experiment) could reduce the growth effect below the level of detection of their design. The absence of a growth effect despite visible injury on leaves might also be explained by a design of low resoluEnviron. Sci. Technol., Vol. 21, No. 6, 1987 607
