Correlation between ozone exposure and visible foliar injury in ponderosa and Jeffrey pines

PII:

Atmospheric Environment Vol. 32, No. 17, pp. 3001—3010, 1998 ( 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain S1352–2310(97)00513–X 1352—2310/98 $19.00#0.00

CORRELATION BETWEEN OZONE EXPOSURE AND VISIBLE FOLIAR INJURY IN PONDEROSA AND JEFFREY PINES DAVID H. SALARDINO* and JOHN J. CARROLL Department of Land, Air and Water Resources, University of California, Davis, CA 95616, U.S.A. (First received 21 May 1997 and in final form 12 November 1997. Published June 1998) Abstract — Ozone exposure was related to ozone-induced visible foliar injury in ponderosa and Jeffrey pines growing on the western slopes of the Sierra Nevada Mountains of California. Measurements of ozone exposure, chlorotic mottle and fascicle retention were collected during the years 1992—1994 for 11 sites located throughout the Sierra Nevada. From these data, summer season (1 June—15 October) hourly ozone concentrations were used to calculate various ozone exposure indices. Injury scores based on fascicle retention, chlorotic mottle and an index containing both were correlated with exposure. It is shown that the combined injury index correlates with exposure better than either single variable index. For the relatively low levels of injury measured, exposure indices which included 14 h of daylight exposure (06:00—19:59) or full-day exposure consistently correlated better with injury than did exposure indices in which only 7 h (09:00—15:59) exposures were included. Conversely, the W95 index and the 7 h indices were less well correlated with injury. ( 1998 Elsevier Science Ltd. All rights reserved Key word index: Ozone exposure, foliar injury, exposure indices, Sierra Nevada, pines.

INTRODUCTION

In the 1950s exposure to air pollution was determined to cause visible injury in vegetation. In pines, ozone enters the needles during transpiration via the stomata and the resulting injury causes a subsequent reduction in carbon fixation during photosynthesis (Beyers et al., 1992). The research of Miller et al. (1963) revealed that observed chlorosis in ponderosa pines of the San Bernardino Mountains was principally caused by exposure to photochemical oxidants, primarily ozone. Temple et al. (1992) exposed ponderosa pine seedlings in the Sierra Nevada to ambient and 1.5 times ambient ozone concentrations. The resulting injury included observable chlorosis and premature abscission of needles experiencing multiple-year exposure. At present, ozone-related injury to vegetation is evident in many areas throughout the United States (MacKenzie and El-Ashry, 1989) and Europe (Karenlampi and Skarby, 1996). Methods for ozone exposure classification include the time of day, and the duration and magnitude of the exposure episode. Lee et al. (1988) evaluated 613 different ozone exposure indices in relation to the response of soybean, wheat, cotton, corn, and sorghum. These authors concluded that ‘‘those (exposure) measures emphasizing peak concentrations and accumulating concentrations over time perform better * Author to whom correspondence should be addressed: 409 Grand Avenue, Pasadena, CA 91030, USA.

than those averaging concentrations’’, and indices in which the peak concentrations were deemphasized did not perform well. As explained by Musselman et al. (1988) however, selecting a threshold concentration at which ozone becomes important to plant vitality may be difficult because the threshold concentration at which a plant is sensitive to injury may not be constant, and may be dependent upon such variables as plant age and time of exposure. Tropospheric ozone has the ability to be transported over several 100 km prior to removal from the atmosphere by chemical reaction or deposition onto surfaces such as soil and vegetation. Daytime winds result in the transport of pollutants such as ozone from the Central Valley of California eastward to the forests of the bordering Sierra Nevada Mountains (Van Ooy and Carroll, 1995). However, the diurnal ozone pattern described by the hourly average concentrations varies at different mountain locations. This spatial variability is in part due to the scatter locations of multiple source areas and the control of local vertical mixing and wind flow by local topography. The purpose of this study is to determine relationships between observed injury to ponderosa (Pinus ponderosa) and Jeffrey (Pinus jeffreyi) pines of the Sierra Nevada Mountains and several measures of ozone exposure. Ponderosa and Jeffrey pines are among the most sensitive pine trees to ozone-induced injury (Miller and Millecan, 1971), and are the major pine species located within the forests of the Sierra Nevada Mountains. Data used in this study are:

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ozone concentrations and meteorological variables at six University of California at Davis ( UCD) operated sites, ozone concentrations at five National Park Service (NPS) sites and visible foliar injury data collected from plots located at each of the ozone measurement sites. This paper reports correlations between two foliar injury measures (chlorotic mottle and fascicle retention) both separately and together, and several estimates of ozone exposure.

DATA COLLECTION

Ozone and meteorology Six of the monitoring stations were established by the University of California at Davis in 1991 in accordance with the Sierra Cooperative Ozone Impact Assessment Study (SCOIAS) funded by the California Air Resources Board (Carroll, 1992; Van Ooy and Carroll, 1995). Ambient ozone concentrations and meteorological variables including ambient temperature, relative humidity, wind speed, wind direction, and solar radiation were measured at each site. Variables were sampled each second and stored as 5 min averages on a PC-based system located at each site. Following data quality assurance tests, hourly averages for each of the variables were then calculated. If less than 75% of the 5 min averages were available for an hour, that hour of data was flagged as missing. In

addition, hourly ozone data were obtained for the years 1991—1994 from the National Parks Service in Denver, Colorado for five NPS monitoring stations. Site locations are shown in Fig. 1. Needle injury Injury data for the pine needles of ponderosa and Jeffrey pines were collected at the six UCD and five NPS operated sites during the years 1991—1994 by trained field observers using the protocols specified by Miller et al. (1996a), as part of project FOREST (Forest Ozone Response Study), a parallel project to SCOIAS, involving the U.S. Forest Service, U.S. National Parks Service, and the California Air Resources Board. In accordance with the specified protocols, three vegetation plots located within three miles and 500 ft in elevation of each monitoring station were randomly selected. Within each vegetation plot 50 trees were randomly chosen and injury data were collected for five branches per tree each year. Needle injury evaluation occurred yearly at each site sometime between late July and late September, with most of the data collected in August and September, representing months near the end of the currentyear-needle growth. A list of dates for the evaluation of the injury was provided by the FOREST project. The collection period for any particular site ranged from days to a few weeks. In order for an analysis to be completed using these data, one particular day for

Fig. 1. Three-dimensional map of central California, showing the location of each site. Vertical scale is exaggerated.

Ozone exposure and injury in pines

each year at each site had to be chosen from this range of days to represent the actual evaluation date. The mid-point between the first and last day of collection at any particular site and year was chosen as the collection date for that site and year. The 1992 evaluation dates were not available for Mountain Home, Jerseydale, and White Cloud. For these sites the 1992 collection dates were chosen to be midway between the collection dates for 1991 and 1993. The 1991 and 1992 evaluation dates were not recorded for Mount Lassen and therefore it was assumed that the collection date for this site during these years was the same as the 1993 collection date at this site. Pine trees retain up to eight whorls on a branch (Miller et al., 1996a), with each whorl representing one year’s growth; therefore, injury symptoms on older whorls represent the results of multiple years of exposure. During evaluation, each whorl on a chosen branch received a separate needle length, fascicle retention and chlorotic mottle rating. Fascicle retention was scored as the percent of fascicles retained in a whorl and was based on the following category system: category 0"0%, category 1"1—33%, category 2"34—66%, and category 3"'67% fascicle retention. Chlorotic mottle (tissue discoloration resulting from the death of chlorophyll cells, presumed to be caused by ozone exposure) was evaluated based on a category system similar to the fascicle retention scoring system in which, category 0"0%, category 1"1—6%, category 2"7—25%, category 3"26—50%, category 4"51—75%, and category 5"'75% of the surface area of the needles within a whorl exhibited chlorotic mottle. The field observer estimated the percent chlorotic mottle and percent needle fascicle retention, which were then coded on the data sheets 0, 1, 2, 3, 4 or 5, and 0, 1, 2 or 3, respectively. These scale values were then used for the computations reported here. It is noted that Jeffrey and ponderosa are two separate pine species, however they have similar physiological characteristics and sensitivity to ozone (Miller and Millecan, 1971). Therefore, in completing this analysis the two species were grouped together.

ANALYSIS

Ozone exposure Due to the different diurnal ozone patterns observed among the sites (cf. Van Ooy and Carroll, 1995), it is reasonable to assume that seasonally averaged concentrations would not be good indicators of the degree to which plants located at any particular site are exposed to ozone. For example, both Giant Forest and Jerseydale have similar seasonal averages, but have very different diurnal ozone concentration patterns. Giant Forest has a strong diurnal pattern, in which vegetation is exposed to ozone concentrations typically ranging up to 80 ppbv during the daylight

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hours. At Jerseydale however, vegetation is typically exposed to a more constant amount of ozone over 24 h, but at lower concentrations, between 55 and 70 ppbv. Three types of exposure measures were utilized in this analysis; HRSnn, SUMnn, and sigmoidal. The HRSnn index represents the total number of hours in a specified exposure period for which the hourly ozone concentration was greater than or equal to a particular threshold concentration; nn (pphm). The SUMnn index represents a sum of the ozone concentrations for hours during a specified exposure period when ozone concentrations were greater than or equal to the particular threshold concentration. The threshold concentration used here for both indices was 60 ppbv (i.e.; HRS06 and SUM06). The SUM0 index was also utilized, representing a sum of all hourly ozone concentrations within the exposure period. The W95 and W126 (sigmoidal) indices of Lefohn and Runeckles (1987) were also used which include a weighting factor which gives greater significance to higher ozone concentrations. Ozone is believed to enter plant tissue by diffusion through stomata that are open during the photosynthetic process; therefore, the hours for which the stomata are open are, in theory, the times when the vegetation is most sensitive to injury. Some workers have suggested ozone indices be calculated using the seven mid-day hours only (09:00—15:59), so as to include only the peak photosynthesis periods of the day. However, in summer, daylight periods last up to 14 h. Late afternoon periods in the mountains, when ozone concentrations often peak, may be very significant to ozone induced vegetative injury. In this study 7 h, 14 h (06:00—19:59) and 24 h periods were used to compute ozone exposures. The period 1 June—15 October was chosen for analysis because it is representative of the growing season, and is also the period when average monthly ozone concentrations are highest (U.S. Department of Health, Education, and Welfare, 1970). For a given year, the exposure index for a particular site includes the hours from midnight 1 June of that year through midnight of the needle injury data collection date of the same year. A 2 yr exposure index includes the hours from 1 June—15 October of the previous year and 1 June through the needle injury data collection data of the current year. Similarly, the 3 yr exposure indices include the exposure from the previous two summer seasons (i.e. 1 June—15 October) through to the current-year-needle injury data collection date. Needle injury For each year average chlorotic mottle (CM) and fascicle retention (FR) scores were calculated for each year’s needle whorl at each site. Multiple-year exposure to ozone was represented by injury to needles other than current-year growth. Plant susceptibility was treated as independent of plant age and any missing data were neglected.

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By virtue of the method by which injury to the pines was evaluated, there exist two methods for expressing average CM and FR. Considering that in the field CM and FR were estimated as percentages but coded on the data sheets (i.e. 0—5 for CM and 0—3 for FR), one procedure uses the midpoints of the percentage ranges to represent injury amount (Miller et al., 1996b). These percentage ranges are based on the Horsfall and Barratt (1945) scale which is constructed to adjust to the performance of the human eye which perceives the intensity of visual pattern on a logarithmic scale, i.e. smaller increments of change can be distinguished at the extremes of the scale. The second procedure, used in this paper, was to average coded scale values (0, 1, 2, 3, etc.) for CM and FR. The results from the procedure using mid-range percentages (Miller et al., 1996b) differed only in that when used in combination with a curvilinear fit, the correlation value was higher for FR but about the same for CM, as compared to our method. The average CM and FR scores for each site and age of whorl were separately paired with the corresponding SUMnn exposure index value. For example, for injury data collected in 1994 for a 2 yr old whorl, the average chlorotic mottle or fascicle retention was paired with an exposure that included the hourly ozone concentrations for 1 June—15 October 1992, 1 June—15 October 1993, and 1 June to the date at which the whorl was evaluated for injury in 1994. During the injury evaluation, the chlorotic mottle score given to a particular whorl was based only on those needles present in the whorl and obviously could not include needles that had abscised. Assuming that the most injured needles would be those that

senesced early, the chlorotic mottle of those needles were not included in the whorl score and therefore the chlorotic mottle score would not represent the true whorl injury. In order to account for missing needles in the chlorotic mottle score, a fascicle retention adjusted average chlorotic mottle score was calculated assuming that the missing needles would have had a chlorotic mottle score of category 5, or greater than 75% coverage by mottle, i.e.: CM +N [(CM M )#5(1!M )] i i i " i/1 N FR where CM/FR is the fascicle retention adjusted average chlorotic mottle score for a particular year’s growth at a site, M represents the percentage of i needles remaining in whorl i, such that a fascicle retention score of 0, 1, 2 or 3 results in an M value of 0, 0.33, 0.66 and 1, respectively, CM is the chlorotic i mottle category of whorl i, and N is the total number of whorls for a particular year’s growth for which data were collected. This new injury index results in a score of 0 for whorls with no chlorotic mottle coverage and complete fascicle retention, to five for whorls with complete chlorotic mottle coverage and 100% fascicle retention or 0% fascicle retention or a combination of the two. The CM/FR index was correlated with the HRS06, SUM0, SUM06, W126, and W95 ozone exposure indices.

RESULTS AND DISCUSSION

The eleven sites differ in their diurnal pattern of hourly averaged ozone concentrations (Table 1 and

Table 1. Average daily ozone variations Site

Mount Lassen White Cloud Sly Park Five-Mile Camp Mather Wawona Jerseydale Shaver Lake Grant Grove Giant Forest Mountain Home

Seasonal average hourly concentration

Average daily maximum hourly concentration

Hour the average daily maximum occurs

Average daily minimum hourly concentration

Hour the average daily minimum occurs

Diurnal variation average concentration!

40.8

52.3

17:00

27.6

07:00

24.7

63.0

68.1

21:00

54.6

08:00

13.5

52.4 65.3 48.8

69.7 69.7 61.9

17:00 17:00 17:00

35.1 56.6 36.2

07:00 08:00 07:00

34.6 13.1 25.7

39.6 65.6 54.8

64.6 71.8 88.4

16:00 14:00 17:00

13.1 56.3 29.1

07:00 08:00 07:00

51.5 15.5 59.3

62.9

79.8

17:00

50.2

07:00

29.6

65.0

83.7

18:00

47.6

07:00

36.1

70.8

95.4

17:00

49.3

07:00

46.1

! Difference between the average daily maximum and minimum hourly concentrations.

Ozone exposure and injury in pines

cf. Van Ooy and Carroll, 1995). These contrasts are illustrated in Fig. 2 which shows high—low-average bars for the hourly average ozone concentrations over the years 1991—1994 for the most diurnal site (Mountain Home) and a strongly diurnal site, Five Mile. Mountain Home, Shaver Lake, Giant Forest and Grant Grove all exhibit strong diurnal variations in hourly ozone concentrations, with seasonally averaged hourly concentrations increasing during the morning and reaching hourly peaks between about 80 to 95 ppbv in the late afternoon. At these sites, concentrations decrease dramatically during the evening hours. Wawona, Camp Mather and Mount Lassen display similar diurnal patterns in ozone concentrations, but these sites have lower maximum average

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hourly concentrations of between 50 to 65 ppbv. Sly Park exhibits strong diurnal variations in ozone concentrations with an average maximum hourly concentration near 70 ppbv. Jerseydale, Five Mile and White Cloud exhibit little diurnal variation, with average hourly ozone concentrations typically varying less than 20 ppbv over 24 hr, and with maximum hourly average concentrations approaching 70 ppbv. Generally, ozone concentrations decrease with distance to the north and with distance eastward from the Central Valley. The relatively low concentrations observed at Wawona and Camp Mather are believed to be due to their position, which is farther from the San Joaquin Valley than the Jerseydale and FiveMile, allowing for greater dilution of transported

Fig. 2. Minimum and maximum range (vertical bar), and average (horizontal dash) seasonal (June—September) mean ozone concentration by hour of day for the years 1991—1994 for a strongly diurnal site (Mountain Home) and a weakly diurnal site (Five Mile).

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pollutants. Low concentrations observed at Mount Lassen are atributed to its location far removed from major pollutant sources. Seasonal (1 June—30 September) average 24 h daily ozone concentrations for the years 1991—1994, at the eleven sites are depicted in Fig. 3a. Average concentrations range from 40.8 ppbv at Wawona to 70.8 ppbv at Mountain Home.

Correlation coefficients (r) were calculated for chlorotic mottle and fascicle retention scores for the various sites vs the 24 and 7-h SUM0 and SUM06 exposure indices. The 24 h SUM0 exposures for the eleven sites ranged between 50,000 and 600,000 ppbv h, while the SUM06 exposures ranged between 0 and 500,000 ppbv h. The 7 h SUM0 exposures range

Fig. 3. (a) Minimum and maximum range (vertical bar), and average (horizontal dash) seasonally averaged ozone concentration (June—September) for each station for the years 1991—1994 vs latitude. Wawona is offset #0.1° for clarity. (b) Fascicle retention adjusted chlorotic mottle scores for the 11 sites plotted with the number representing the number of years the needles were exposed, versus latitude. Wawona is offset #0.1° for clarity.

Ozone exposure and injury in pines

between 10,000 and 200,000 ppbv h, while the SUM06 exposures ranged between 0 and 190,000 ppbv h. Yearly chlorotic mottle scores ranged between 0 and 1.4, while yearly fascicle retention scores ranged between 2.2 and 3. The fascicle retention adjusted chlorotic mottle scores ranged between 0 and 1.8 among sites. Figure 3b depicts the average CM/FR scores given at each site between 1992 and 1994 for the various ages of whorls. It is important to note that at certain sites (e.g. Jerseydale, Shaver Lake and Mount Lassen) the average injury scores for some whorls decreased over time and exposure. It is possible that lightly injured needles may recover during periods when ozone concentrations are low or that observer bias exists in the sampling procedure. Each year field crews were formed and trained on how to evaluate needles for injury. In accordance with the experimental protocol, random checks of field crew scores were completed by crew managers. However, because different field crews were used each year and because the data collected are based on visual estimates of injury, interyear scoring was susceptible to observer bias. This is especially true at sites with low levels of injury, or where the actual range of injury is small. Several plots of injury versus exposure are shown in Fig. 4, along with best fit lines of regression. These demonstrate that the data clearly scatter about an apparent straight line, at least for the levels of injury detected. While higher-order polynomials or other forms of fit may result in slightly better correlation

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coefficients (Miller et al., 1996b), given the narrow range of injury, linear regression would preserve any significant dependence of injury on exposure. The correlation coefficients for the three measures of injury (chlorotic mottle, fascicle retention, and fascicle retention adjusted chlorotic mottle) and the various exposure indices are listed in Table 2. The correlation coefficients between selected exposure measures and chlorotic mottle scores range between 0.71 for the 7 h SUM06 index to 0.77, for the 24 h SUM0 and SUM06 indices. While this indicates a strong correlation between both indices and CM, the longer time base (i.e., 24 h) is clearly a better predictor of CM injury. Correlation coefficients for exposure vs fascicle retention are negative, i.e. poorer retention with increasing exposure and are weaker compared to CM, with coefficients ranging between 0.62 and 0.65. Here the 7 h SUM06 is a slightly between predictor than the other indices shown. For the combined CM/FR scores, the correlation coefficients among all indices tested range from 0.65 to 0.83. The highest correlation observed is with the HRS06 index and is essentially the same for the 24, 14, and 7 h sampling. Similarly, the 24 and 14 hour SUM0 and SUM06 correlation coefficients are also quite high (0.80—0.81) but the 7 h sampled period indices are slightly lower. For the W126 index, the correlation coefficients increase from 0.77 to 0.81 when changing from 7 to 24 h sampling. The W95 index has the lowest correlation coefficients among those tested.

Fig. 4. Scatter plots with lines of regression of 24 h exposure vs injury for the SUM0 and (a) chlorotic mottle injury alone, and (b) fascicle retention injury alone, and (c) fascicle retention adjusted chlorotic mottle.

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Fig. 4. (continued )

For the data obtained, all indices examined are good predictors of injury as described by the combined CM/FR index. There is a slight improvement seen in using 14 h per day or longer as the sampling time over the seven midday hours. We attribute this partially to the fact that the length of daylight is much longer than 7 h per day. Perhaps more important is the fact that for those sites at which diurnal variations in ozone concentrations are large, high ozone concen-

trations persist into the late afternoon hours that are excluded from the midday centered 7 h sample. Except for the clearly lower correlation coefficients for the W95 index, there is no compelling evidence in this analysis that thresholds matter. One reason for this may be that the range of injury is relatively low, and hence subtle differences are masked by the scatter in the data which in turn is partly due to the qualitative nature of the injury classification methods.

Ozone exposure and injury in pines Table 2. Correlation coefficients (r) between exposure and injury indices. Means for the 11 sites Exposure index!

CM"

FR

FR adjusted CM

24-SUM0 14-SUM0 7-SUM0 24-SUM06 14-SUM06 7-SUM06 24-HRS06 14-HRS06 7-HRS06 24-W126 14-W126 7-W126 24-W95 14-W95 7-W95

0.77 — 0.72 0.77 — 0.71 — — — — — — — — —

!0.63 — !0.62 !0.62 — !0.65 — — — — — — — — —

0.81 0.80 0.78 0.81 0.81 0.79 0.82 0.83 0.81 0.81 0.79 0.77 0.74 0.68 0.65

! The number preceding the type of exposure index is the duration of daily exposure (hours) taken into consideration when computing the index. " CM"chlorotic mottle, FR"fascicle retention, and FR adjusted CM"fascicle retention adjusted chlorotic mottle.

A second explanation is that the indices for different thresholds applied to the same site are highly correlated with each other: i.e. if there are frequent periods of ozone concentrations higher than 60 ppbv at site A and infrequent occurrences at site B, then the SUM0, SUM06 and HRS06 indices for site A will all be higher than the same indices at site B.

CONCLUSIONS

Eleven monitoring stations were established in national parks and forests along the western slopes of the Sierra Nevada Mountains. The ozone exposure patterns were found to vary from site to site. Most sites experience a strong diurnal variation in ozone concentrations in which relatively high ozone concentrations are observed during the daylight hours, with maximum hourly concentrations occurring during the afternoon, while relatively low ozone concentrations are observed at night. The strongly diurnal sites also tend to experience the highest hourly maximum concentrations among all sites, namely Mountain Home, Shaver Lake, Giant Forest and Grant Grove with seasonally averaged hourly ozone maxima approaching 90 ppbv. Sly Park and Wawona also exhibit diurnal ozone patterns, however, these sites do not reach such high daily maximums, with maximums between 64 and 70 ppbv. The sites with a moderately strong diurnal pattern: Camp Mather and Mount Lassen experience maximum hourly concentrations between 60 and 70 ppbv. The remaining sites, Jerseydale, Five Mile and White Cloud experience very little variation in ozone concentrations over the course of the average day, with average hourly maxima between 60 and

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70 ppbv during the daylight periods, and descreasing slightly during the night. The use of latitude alone is not a good indicator of ozone exposure. Topography and distance from the San Joaquin Valley also are important factors in determining local ozone concentrations. A combined symptom injury index, which is comprised of a chlorotic mottle score that is adjusted for fascicle retention was determined to better estimate injury than using either fascicle retention or chlorotic mottle separately. Fascicle retention, when separately analyzed as an injury index, correlated poorly with the various ozone exposure indices. Chlorotic mottle correlated better with ozone exposure but worked best when abscised needles were considered in the chlorotic mottle score. The performance of all exposure indices in correlating with fascicle retention adjusted chlorotic mottle was similar, ranging between 0.77 and 0.83 except for the sigmoidally weighted index W95 with coefficients between 0.65 and 0.74. In all cases, 24 h exposure indices had larger correlation coefficients with the combined injury score than did their 7 h counterparts. We conclude that the 7 h period is too short, and therefore does not adequately represent the total time for which the needles are absorbing ozone and in areas removed from pollution sources may miss late afternoon periods when local ozone concentrations peak. A second possibility is that nocturnal exposure is important, at least for the onset of injury. In summary, many factors, which could not be accounted for in this study, influence plant responses to ozone exposure, including the micro-environment of the plant, plant age, and even the history of ozone exposure. Temple et al. (1992) found that water availability plays a major role in determining the extent of ozone damage to pine seedlings by affecting the plants uptake of ozone. Plants that were water-stressed incurred less injury caused by a decrease in ozone uptake. Our results show that increased exposure to ozone does lead to increased observable injury to pine needles. For the small range of injury observed, most exposure indices performed equally well in predicting injury, and no distinct threshold effects can be demonstrated. We note however, that indices with different thresholds computed for the same sites will be highly correlated with each other. There is a slight improvement in correlation values using 14 and 24 h sampling periods over using mid-day centered seven hour samplings. Acknowledgements—The authors wish to thank Paul Miller of the Pacific Southwest Forest and Range Experiment Station, US Forest Service for sharing data and insights with us, and Alan Dixon of UC Davis for maintaining the instruments at the UCD monitoring stations and evaluating data quality. We also thank John Pronos, USDA, Forest Pest Management, Kenneth Stolte, USDA, Forest Health Monitoring, Dan Duriscoe and Diane Ewell, Sequoia-Kings Canyon National Park, Deborah Mangis, USEPA, Brent

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Takemoto, CARB, and Trent Proctor, USDA, Sequoia National Forest, for their assistance as advisors or field crew trainers. Likewise, we wish to thank the field crews who received training and gathered tree injury data annually and who represented the National Park Service at SequoiaKings, Yosemite and Lassen Volcanic National Parks and the USDA Forest Service at the Eldorado, Sequoia, Sierra, Stanislaus and Tahoe National Forests. Support for this work was provided by the University of California, Division of Agriculture and Natural Resources, by the California Air Resources Board under agreement No. A92-346 and the U.S. EPA (R819658) Center for Ecological Health Research at UC Davis. This support is gratefully acknowledged. Although the information in this document has been funded in part by the United States Environmental Protection Agency, it may not necessarily reflect the views of the Agency and no official endorsement should be inferred.

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Lefohn, A. S. and Runeckles, V. C. (1987) Establishing a standard to protect vegetation-ozone exposure/dose considerations. Atmospheric Environment 21, 561—568. MacKenzie, J. J. and El-Ashry, M. T. (1989) Tree and crop injury: A summary of the evidence. In Air Pollution ¹oll on Forests and Crops pp. 1—21. Yale University Press, New Haven. Miller, P. R., Pameter, J. R., Taylor, O. C. and Cardiff, E. A. (1963) Ozone injury to the foliage of Pinus ponderosa. Phytopathology 53, 1072—1076. Miller, P. R. and Millecan, A. A. (1971) Extent of oxidant air pollution damage to some pines and other conifers in California. Plant Disease Reporter 55(6), 555—559. Miller, P. R., Stolte, K. W., Duriscoe, D. M. and Pronos, J. (1996a) Evaluating ozone air pollution effects on pines in the western ºnited States. USDA Forest Service General Technical Report PSW-GTR-155. Miller, P. R., Guthrey, D. R., Schilling, S. L. and Carroll, J. J. (1996b) Ozone injury responses of ponderosa and Jeffrey pine in the Sierra Nevada and San Bernardino Mountains in California. In Proceedings of the International Symposium on Air Pollution and Climate Change Effects on forest Ecosystems, 5—9 February, 1996. Eds A. Bytnerowicz, M. J. Arbaugh, S. L. Schilling, technical coordinators. Riverside, CA, GO-164 (in press.) Musselman, R. C., McCool, P. M. and Younglove, T. (1988) Selecting ozone exposure statistics for determining crop yield loss from air pollutants. Environmental Pollution 53, 63—78. Temple, P. J., Riechers, G. H. and Miller, P. R. (1992) Foliar injury responses of ponderosa pine seedlings to ozone, wet and dry acidic deposition, and drought. Environmental and Experimental Biology 32 (2), 101—113. U. S. Department of Health, Education, and Welfare. (1970) Air quality criteria for photochemical oxidants. National Air Pollution Control Admnistration, Washington, D.C. Van Ooy, D. J. and Carroll, J. J. (1995) The spatial variation of ozone climatology on the western slope of the Sierra Nevada. Atmospheric Environment 29 (11), 1319—1330.