- Physiological Effects
- Effect on Forest Trees
- Effective Dose
- Ozone Symptoms
- How We Use Exposure Indices
- Literature Cited
Ozone enters a leaf cell through openings, called stomates, that allow for gas exchange. Once inside the leaf the ozone can result in damage to the mesophyll cells in the center of the leaf. The mesophyll cells contain the chloroplasts where photosynthesis occurs. Available research does not clearly show what happens once the ozone contacts the mesophyll. The phytotoxic effects could be a result of:
- Direct contact of the ozone with the mesophyll.
- The chemical product of ozone's reaction with other gases within the leaf's air space.
- The chemical product of ozone's reaction with the cell membrane.
- Most likely, it is a combination of all three listed previously.
Ozone effects on forest vegetation are most pronounced when soil moisture and nutrients are adequate, and ozone concentrations are high. Under good soil moisture and nutrient conditions the stomates are likely to be open and the ozone will enter into the leaf and can damage the cells that produce the food for the plants through photosynthesis. Reductions in the photosynthesis will decrease the amount of carbohydrates produced and stored in the roots. Continual reduction in the annual production of carbohydrates has the potential to decrease the amount of root growth, tree height, and crown width. Individual trees with reductions in these three areas could be less competitive then neighboring trees for sunlight and nutrients, or the chronic stress could weaken the trees and make them more susceptible to insect attacks.
Scientists have determined the ozone parameters that explain most of the vegetation response are:
- Where a greater weight is placed on the higher ozone concentrations,
- Are cumulative throughout the growing season, and
- Take into account when the ozone enters the leaf through the stomata.
The amount of ozone measured with an ozone monitor is the concentration in the atmosphere. However, the amount of ozone that enters the leaf, called dose, depends on whether the stomates are open and other atmospheric conditions. The effective dose is the amount of ozone that enters the leaf during the growing season and has an impact to physiological process or causes cell death. The stomates are most likely to be open when there is adequate soil moisture and nutrients, and during sunny conditions. However, the stomates can be open during the nighttime hours, but the amount of ozone dose could be less than during the day. The impact of ozone entering the leaf will depend upon if the plant is able to neutralize the ozone before causing injury. It is possible the detoxification mechanism are most effective during the daytime when the greatest uptake (dose) occurs, however ozone entering the leaf at night could cause impacts because the defense mechanisms are not operating or are less efficient.
Research has developed means to estimate the dose of ozone entering the leaf, but the meteorological conditions across the area of concern along with the ozone concentrations need to be known to calculate the dose. Typically, the USDA Forest Service does not have adequate meteorological data to combine with the ozone monitoring data. Estimating the dose of ozone is not practical for National Forests or wildernesses at this time. Furthermore, it will be necessary for further research to be conducted in order to estimate the effective dose to account for the defense mechanisms to ozone for forest vegetation. Once adequate research has been conducted on effective dose then it will be necessary to translate the results to present what ozone concentrations or exposures caused injury or damage. This is likely to be necessary since the regulatory agencies show attainment of air pollution standards using data collected from ozone monitors.
Certain plant species have been used as "bioindicators" that ozone is causing phytotoxic impacts. One symptom used is the presence of a stippling (reddening or black) confined to the upper leaf surface between the veins. The presence of ozone symptoms indicates the plant had a physiological response to the ozone exposure. Symptoms are not an accurate indicator of how much growth loss has occurred to a sensitive plant from an ozone exposure. An adequate amount of soil moisture and nutrients at a specific site (microsite) appears to be one factor in determining if ozone will cause visible symptoms. Vegetation's sensitivity to ozone varies -- not only between species, but also within a species. For example, there may be two black cherry trees growing next to one another, and one will have severe ozone symptoms while the adjacent black cherry has no visible symptoms. Also, there is no relationship between the amount of ozone a plant is exposed to and the amount and severity of ozone symptoms.
Some air resource specialists rely upon measurements taken with ozone monitoring equipment at a site of interest, along with soil moisture estimates, to predict if a biomass (roots, shoots, and/or stem) reduction has occurred. Ozone monitors can provide over 4000 ozone readings from April through October. Researchers and technical specialists have examined ways to summarize and use this extensive information. The Ozone Calculator is one tool that has been developed to estimate if ozone exposures recorded at a monitoring site could cause a biomass reduction to the vegetation where fumigation studies have been conducted. The USDA Forest Service has compiled hourly average data which is compatible with the Ozone Calculator. Furthermore, a web-based spatial database has been developed which shows the location of the ozone monitoring sites where ozone data are available.
There are two exposure statistics used to estimate the growth loss to vegetation when summarizing data from an ozone monitor. The N100 is one statistic and is the number of hours when the measured ozone concentration is greater than or equal to 0.100 parts per million (ppm). Experimental trials with a frequent number of peaks (hourly averages greater than or equal to 0.100 ppm) have been demonstrated to cause greater growth loss to vegetation than trials with no peaks in the exposure regime (Hogsett et al., 1985; Musselman et al., 1983; and Musselman et al., 1986). The second statistic is the seasonal ozone exposure called the W126 (Lefohn and Runeckles, 1987). The W126 was developed as a biologically meaningful way to summarize hourly average ozone data. The W126 places a greater weight on the measured values as the concentrations increase. Thus, it is possible for a high W126 value to occur with few to no hours above 0.100 ppm. Therefore, it is also necessary to determine the number of hours the ozone concentrations are greater than or equal to 0.100 ppm. It should also be noted the lack of N100 values does not mean ozone symptoms will not be present when field surveys are conducted. The use of both the N100 and W126 is consistent with the recommendations of the Federal Land Manager Air Quality Related Values Workgroup (FLAG).
There are portions of the United States where a natural resource professional may want to perform an analysis to estimate what impact ozone may be having on vegetation, but there is no ozone monitoring data available at the site of interest. Spatial extrapolations (using ordinary kriging) of the available ozone monitoring for the W126 and N100 have been developed for the lower 48 United States.
FLAG. 2002. Federal Land Managers' Air Quality Related Values Workgroup. Phase I Report. December 2000.
Hogsett, W. E.; Plocher M; Wildman V.; Tingey, D. T. and Benett, J. P. 1985. Growth response of two varieties of slash pine seedlings to chronic ozone exposures. Can. J. Botany 63:2369-2376.
Lefohn, A. S.;Runeckles, V. C. 1987. Establishing a standard to protect vegetation - ozone exposure/dose considerations. Atmos. Environ. 21:561-568.
Musselman, R. C.; Oshima, R. J. and Gallavan, R. E. 1983. Significance of pollutant concentration distribution in the response of 'red kidney' beans to ozone. J. Am. Soc. Hort. Sci. 108:347-351.
Musselman, R. C.; Huerta, A. J.; McCool, P. M.; and Oshima, R. J. 1986. Response of beans to simulated ambient and uniform ozone distribution with equal peak concentrations. J. Am. Soc. Hort. Sci. 111:470-473.