Ozone Pollution and White Pines: Phase II

Part of the Young Naturalist Awards Curriculum Collection.

by Megan, Grade 12, Massachusetts - 2006 YNA Winner


When I started a new school in eighth grade, I thought the 40-minute commute each way would be a headache I would have to endure. I never imagined that it would in fact spark my curiosity, leading me on a two-year search to discover how air pollution was affecting the environment in my town, which is on Cape Cod. My head propped against the window, I would stare at the trees as they rushed by until they formed a green, brown, and rust blur on the side of the highway. Why wasn't this vista a pure green panorama, though? I leaned over and asked my dad why the trees looked so mottled and burned on the side of the road. He explained patiently that all of the salt from the roads and the air pollutants released from the cars were burning their needles and leaves. Interesting, I thought. And so I set off on a search through the research—by scientists on the East Coast who are studying jack and ponderosa pines; in the annals of Forest Watch, a group in New Hampshire that monitors air pollution damage to pine trees; and in the published work of people from all over the world, all asking the question, how is air pollution affecting our environment?

A leafy Eastern White Pine tree in a forest
An Eastern White Pine (Pinus strobes)

As I discovered, ozone pollution at the tropospheric level is increasing rapidly, caused by increased smokestack and tailpipe emissions from combusting gas and burning coal, as well as many other sources. The ozone produced is very harmful to living things, leading to various diseases and ailments in humans, and death or damage in plants. In 1990 the EPA set new guidelines to limit the production of ground-level ozone. However, many areas greatly exceed these regulations. I realized that is necessary to start exploring the detrimental effects this ozone pollution is having on the surrounding environment so that people can see the necessity of tighter ozone-emission standards.

The purpose of this project was to compare the health of white pine trees in various areas on the Cape to determine if there were visible correlations between the amount of damage to the trees and the area's proximity to ozone-producing sites such as busy intersections, power plants, or in the middle of forests. I hypothesized that white pine trees closer to areas with increased levels of ozone pollution, or closer to objects emitting ozone-forming pollutants, would display higher levels of ozone damage.

To understand how ozone pollution is different from the ozone layer we also hear about, I started off by researching the basics about how ozone is formed, what it does, and how it affects us and our planet. The sun both creates and destroys ozone, with photochemical reactions involving shortwave sunshine in the stratosphere (Scorer, 2002). Ozone (O3) is formed naturally when UV radiation (less that 243 nanometers long) breaks the bond between two oxygen atoms (O) that are bonded together in an oxygen molecule (O2). One of these oxygen atoms then bonds with M, a molecule of another element, usually nitrogen. The remaining oxygen atom then bonds with an oxygen molecule, producing an ozone molecule.

The stratosphere, the layer of our atmosphere 12 to 50 kilometers above the Earth's surface, contains 90 percent of all of the atmospheric ozone (Krupa, 1997; Reid, 2000). Without the ozone layer, it is very unlikely that life on Earth ever would have developed, because the ozone in our atmosphere absorbs the ultraviolet radiation from the sun (Krupa, Sagar V. 1997). This biologically harmful radiation, specifically the UV-B rays (280–320 nanometers in length), damages living cells and causes ailments such as skin cancer, cataracts, and wrinkles. The ozone layer keeps a large amount of this radiation from reaching the Earth's surface, where it would harm life (Krupa, 1997; Reid, 2000). However, when large amounts of ozone are present in the troposphere (the layer of our atmosphere from the surface of the Earth to about 12 kilometers in altitude), it can be harmful and is labeled "bad ozone" (Reid, 2000).

Scientists initially thought that the amount of ozone in the troposphere existed in a state of dynamic equilibrium, that ozone formation and destruction happened at equal rates, and that the ozone level in the troposphere remained constant. They could not figure out an explanation when it was found that tropospheric ozone levels were rising. It wasn't until the 1950s that chemists discovered that the major contributors to ozone formation in the troposphere were volatile organic compounds (VOCs) and nitrogen oxides. These originate chiefly from the burning of gasoline and other fossil fuels (USDA-ARS, 1998).

Volatile organic compounds, such as hydrocarbons, derive from the evaporation of liquid fuels, solvents, and organic chemicals. VOCs also come from gasoline combustion and are emitted naturally by some plants and bacterial processes in soil (Chemistry in the Sunlight). Between 1900 and 1970, as the use of automobiles skyrocketed, the production of VOCs increased 690 percent (Delnore, April 3, 2002). Nitrogen oxides can be created naturally by lightning, forest fires, burning biomass, and chemical processes in soils. However, nitrogen oxides also derive from the combustion of fossil fuels at high temperatures: from the smokestack and tailpipe emissions of automobiles, diesel trucks and buses, farming and construction equipment, boats and trains. The primary source of nitrogen oxides in the United States is coal-fired power plants (Chemistry in the Sunlight).

Ozone is the most widespread pollutant in Europe and North America, and at the tropospheric level it is very damaging to the tissues of living things (Finnish Environment Institute, 1998). In humans, exposure to ozone can aggravate or increase susceptibility to respiratory conditions and illnesses such as asthma, bronchitis, and pneumonia. It also reduces lung function and the capacity for exercise, and can trigger symptoms such as wheezing, coughing, and pain while taking deep breaths. It does this by irritating the lung airways and causing inflammation, much like a sunburn. It can also cause chest pains and coughing. Ozone particularly affects young children and the elderly, but high ozone levels can affect healthy people who are active outdoors. Permanent lung damage can occur if a person spends several months with repeated exposure to ozone pollution, even though the levels are not that high (Delnore, 2002).

The corrosive nature of ozone can damage plant tissue and destroy forest and agricultural vegetation (Delnore, 2002). Ozone causes more damage to vegetation than all other air pollutants combined (USDA-ARS, 1998). It hinders the ability of plants to produce and store food, which in turn makes them more susceptible to insects, other pollutants, disease, and harsh weather. Ozone reduces crop yield and damages the leaves and needles of trees, ruining the gorgeous appearance of forests and national parks (U.S. Department of Environmental Protection, Office of Air Quality, Planning & Standards, 2000).

When a plant is exposed to ozone, its visible symptoms are classified as either acute or chronic. Acute responses happen within a few hours or days and are usually a result of exposure to high levels of ozone pollution. The symptoms include the death of cells, resulting in stippling, flecking, bleaching, and bifacial necrosis. Chronic injury is a characteristic response to long-term low-concentration exposure. The symptoms develop more slowly, and may include chlorosis, pigmentation, and necrosis. In conifers, damage due to ozone pollution is most commonly expressed by chlorotic mottle and tip necrosis (or tip burn), along with needle shortening (Finnish Environment Institute, 1998).

I focused on ozone pollution's damage to the Eastern White Pine, or Pinus strobes. This tree is the largest of the northeastern conifers and grows up to 100 feet high, though it used to reach heights of 150 feet or more. The pines have rows of horizontal branches, forming a broad and irregular crown around each trunk, which can reach two to four feet in diameter. The evergreen needles of the white pine come in blue-green bundles of five and are whorled, slender, and 2.5 to 5 inches long; the cones are 4 to 8 inches long, yellow-brown, and narrowly cylindrical (Mirov, 1967; Sutton, 1992).

In white pines, whorl retention and fascicle retention cannot be measured, because white pines do not grow in a whorl formation, or have a specific number of fascicles in bundles. The percent of living crown is also not relevant on a white pine. The symptoms that indicate ozone damage for a white pine are the average needle length, tip necrosis, and chlorotic mottle.

Two hand-drawn images, of a White Eastern Pine fascicle cross-section, and a pine needle with chlorotic mottle.
Figures 1 and 2

Chlorotic mottle occurs on the stomatal sides of an evergreen needle. It is the key symptom in determining ozone damage. Figure 1 illustrates a white pine fascicle, with its characteristic bundle of five needles. Chlorotic mottle is characterized by yellow blotches around the two white stomatal stripes on the needle that gradually transition into the healthy, green color of the needle as seen in Figure 2.

When the stomatas open, generally in the early morning (Air Quality and Ozone Injury to Forests), ozone enters through them and damages the sensitive mesophyll tissue surrounding the stomatal opening (Forest Watch). The mesophyll tissue is vital in photosynthesis; the ozone reduces the net photosynthesis of pines by degrading the chlorophyll within the cells. It also reduces growth and makes trees more vulnerable to natural pests such as the pine bark beetle (Development and Application of the Ozone Injury Index, 1996).

Another characteristic of ozone-damaged white pines is tip necrosis, or death of the end of the needles. White pine needles tend to have brown tips as they age, but ozone damage accentuates this. When tip necrosis is combined with symptoms such as chlorotic mottle, the occurrence of ozone damage is almost certain (Forest Watch).

It is important to understand that one can be exposed to high levels of ozone even if one doesn't live in a highly industrialized area. Industrial and urban areas release ozone-forming pollutants that are blown long distances by the wind, and then heat and sunlight cause them to form ozone. The concentration of ozone can actually increase the further it gets from urban centers. Hours and sometimes days after pollutants are released into the air, they form ozone in downwind areas (U.S. Dept. of Environmental Protection, Office of Air Quality, Planning & Standards, 2000).

Therefore, it is necessary to measure local levels of ozone to get an accurate representation of the ozone pollution for a certain area. Millions of Americans live in places with ozone concentrations that exceed the EPA's air-quality standards. Areas with extremely high levels of ozone pollution are the Northeast, the Lake Michigan area, parts of the Southeast, southeastern Texas, and parts of California (U.S. Dept. of Environmental Protection, Office of Air Quality, Planning & Standards, 2000).

The purpose of this project was to concentrate on the ozone pollution of Cape Cod. I would evaluate the health of white pines in specific areas—such as near busy intersections, near parking lots, near power plants, and in the middle of forests—to determine the level of ozone damage to the trees. Then I would compare the results in these different areas to determine if there is a correlation between tree damage and their proximity to ozone-producing sites.

Two photos, side-by-side, of the Mirant Power Plant and the trees near it.
Figure 3: The Mirant Power Plant and Trees Near It

I hypothesized that white pine trees in areas with high levels of ozone pollution would display higher levels of ozone damage. I hypothesized that the trees to the north and northeast (downwind) of the local power plant would have more ozone damage than the trees south and southwest (upwind) of the plant. I also hypothesized that the trees on the southern side of the Cape and moraine would have more ozone damage than trees on the north side of the Cape and moraine. Finally, I hypothesized that pines closest to main roads, intersections, and parking lots would show more ozone damage than the trees in more remote forests.

A map of Cape Cod with the collection sites starred.
Map of Cape Cod and Collection Sites

Three trees were found in each of 11 locations around the Cape: the Christmas Tree Shop parking lot in Hyannis, a residential area in Craigville, a condo parking lot in Hyannis, conservation land in Centerville, a residential area in Bourne southwest of the power plant, a shopping-plaza parking lot in Bourne just northeast of the power plant, a residential area in Bourne just northeast of the power plant, a forest in rural West Barnstable, Beebe Woods in Falmouth, the Falmouth Academy parking lot, and the intersection in front of Roche Brothers in Mashpee.
The trees I selected were of relatively similar age, which I determined by counting the layers of branches, an indicator of age for white pines. Ten needles were selected from each tree, and the average needle length, tip necrosis length, and percent of the needle covered in chlorotic mottle were measured to come up with an average for each of these three variables for each location. The average-needle and tip-necrosis lengths were measured using a millimeter ruler.

Against a flat black surface, a human finger pointing to three green pine needles with tip necrosis, shown in brown.
Needles showing tip necrosis

The percent of the needle covered in chlorotic mottle (a yellow spotting cause by ozone entering the needle's stomatas and killing the mesophyll tissue) was measured by viewing the needle under a large magnifying apparatus with a graph paper transparency and counting the total number of squares a needle took up, as well as the total number of squares containing chlorotic mottle. For the needles with tip necrosis, the amount of tip necrosis was not counted in either of the totals. From those two data pieces, a percentage of the needle covered in chlorotic mottle was found. Thus, 1,400 needle measurements were made. The three variables were averaged for each tree, and a standard deviation was found for each of these averages. The average for each location was then found. The standard deviation for this average was found by using the following equation:

A mathematical equation.
(Centurino, pers. comm., 2/7/05)
A young woman taking measurements with a tape measure at a desk.
Megan taking measurements

where S1 stands for the standard deviation of the average of Tree 1, S2 stands for the standard deviation of the average for Tree 2, etc. For the condo parking lot in Hyannis and the residential area northeast of the power plant where four trees were measured, the standard deviation of the fourth tree was squared and added to the numerator of the fraction, and the denominator was changed to 4. The averages for each of the three variables measured for each area were then compared to the relative ozone pollution in the area, as determined by the proximity of the location to ozone-producing emitters.

I hypothesized that the trees in areas where there were lots of cars, starting and stopping as well as just driving by, would be exposed to more ozone and thus more prone to showing ozone damage. I also hypothesized that the trees downwind (northeast) of the power plant and its smokestack emissions would show damage. The amount of ozone in the troposphere peaks about 150 miles away from its source, and thus Cape Cod bears the brunt of many of the emissions from New York City and other ports along the East Coast. The air travels up the coast, bringing pollutants with it, and then hits Cape Cod, so our air is more polluted than many big cities. I hypothesized that the pines on the south side of the Cape would show more damage than those on the north side, because they would get higher emissions from further down the coast.

A bar chart.
Chart 1 (Click to Enlarge)

Chart 1 shows the average needle length for the pine needles in each location, in order of increasing length. I hypothesized that areas with more ozone pollution would have smaller needle lengths. This held true for the areas with four of the five smallest average needle lengths: the busy intersection in Mashpee, the neighborhood downwind of the power plant, the parking lot in Hyannis, and the Falmouth Academy parking lot. Pines in the conservation land, the West Barnstable woods, Falmouth Woods, and upwind of the power plant had longer average needle lengths. However, the data that did not fit with my hypothesis included the Centerville conservation land, which had the fourth-smallest average needle length, 81.3mm, and the parking lot upwind of the power plant, which had the second-longest average needle length, 101.6mm.

From my observations and the data I collected, I determined that the average needle length for a tree is a very variable statistic. Even when the needles are sampled randomly, there are still huge discrepancies in length between the needles on an individual tree. This is because there are so many variables in the growth and development of a tree. New-year growths, the whorls on the end of the branch, are perhaps not yet fully grown and thus seem stunted; the needles in toward the trunk may better indicate the length of the tree's needles. Needles farther up the trunk also tend to be shorter, particularly in older trees where the lower branches have died off and only the higher branches have needles. Therefore, even though it has been shown that short needle length is an indicator of ozone damage, needle length is so variable that it is an unreliable indicator as a symptom of damage.

A bar chart.
Chart 2 (Click to enlarge)

I also hypothesized that pines in areas with higher ozone levels would show greater tip necrosis, or death at the tips of their needles. The results, shown in Chart 2, completely supported my hypothesis, with trees in areas downwind of the power plant and near busy roads showing the greatest tip necrosis. Trees in areas with the least pollution, such as woods and residential areas, showed the least tip necrosis.

Tip necrosis is an excellent indicator of ozone pollution damage, although the nature of its occurrence causes there to be large error bars, all of which overlap but do not by any means invalidate the data. Tip necrosis does not affect all needles equally; it is the rate of occurrence, its proliferation, and the length of the damaged needles that really matter. I did not expect that all the needles on a particular fascicle or tree would have equal amounts of tip necrosis. An increased average tip necrosis length is indicative of ozone pollution damage, not the closeness of the data points.

Chart 3 Megan
Chart 3 (Click to Enlarge)

I further hypothesized that trees in areas of high ozone pollution would show a greater percent of each needle covered in chlorotic mottle. The areas are shown in Chart 3, by order of increasing percentage of the needle covered in chlorotic mottle. My hypothesis was once again fully supported by the data. Trees in areas of high ozone pollution, such as downwind of the power plant and along busy intersections and roads, had the highest percentage of mottle, and trees in the areas with the least ozone pollution, such as the woods and residential areas, had the smallest percentage.

Chlorotic mottle is the most concrete of the characteristics indicating ozone pollution, as it has been produced in laboratory studies, and its only known cause is ozone. It can, however, be easily mistaken for chlorotic flecking if not carefully evaluated. Flecking is caused by chemical and salt spray burning the needle. This leaves distinct, orange-yellow legions on all surfaces of the area, whereas the mottle is a light yellow spotting on the stomatal sides of the needle, and the edges of the lesions blend softly into the natural green of the needle. During sampling, needles found to have chlorotic flecking were discarded so as to not affect the data.

My hypothesis that the trees on the south side of the Cape Cod moraine would have more ozone damage than the trees on the north side was not supported by the data. The trees were more affected by their direct proximity to ozone emitters than by their position relative to wind patterns. However, I had hypothesized that trees near busy roads or intersections would show greater ozone pollution damage than those in more rural areas. This hypothesis was strongly supported by all of the data. The woods in West Barnstable, for example, had an average needle length of 90.6mm, tip necrosis of 0.1mm, and 3 percent chlorotic mottle. Conversely, the trees at the busy intersection in Mashpee had an average needle length of 62.3mm, tip necrosis of 3.6mm, and 49 percent chlorotic mottle. The pines in Beebe woods in Falmouth had an average needle length of 106.0mm, a tip necrosis length of 0mm, and 7 percent chlorotic mottle. The conservation land pines in Centerville had similarly healthy values of 81.8mm, 0.1mm, and 3 percent, respectively. Despite a weak correlation between average needle length and proximity to ozone pollution, these values for needle length all fall within the normal healthy range for a white pine needle length, and thus signify a healthy needle.

My final hypothesis, that the trees downwind of the power plant would show higher levels of ozone pollution damage than the trees upwind of the power plant, was very strongly supported by the data and the observations I made while collecting the data. The needles collected from a residential area in Bourne were just southwest (upwind) of the power plant, easily within sight of it. Their average needle length was 85.5mm, tip necrosis was .9mm, and chlorotic mottling covered 9 percent of the needle. Overall, in was noted in the field that all of the trees found were a dark, healthy green, with long, supple, unblemished needles and just a light spotting of chlorotic mottle. The appearance of some chlorotic mottle is understandable since there is some traffic in the area. Needles found barely half a mile away, however, just downwind of the power plant and once again well within sight of it, were horribly deformed, browned, stunted, and extremely unhealthy-looking. It was doubtful that many of the pines would survive the year. The three trees sampled at the shopping plaza had an average needle length of 101.6mm, with 21.3mm of tip necrosis, and 32 percent of the needle covered in chlorotic mottle. The four trees in a residential area had an average needle length of 72.2mm, tip necrosis of 46.1mm, and 58 percent of the needle covered in chlorotic mottle.

The main sources for possible error in the project came from the nature of the project itself; trees in general are very variable things, as they are affected by the nutrients in their soil, the amount of sunlight they receive, their age, the chemicals they are exposed to, the wind and weather, as well as many other factors. I took steps, however, to find pines of a similar age with no visible deformities. Other sources of error might have existed in my collection technique; despite my random method for picking needles, the ones picked may not have accurately represented the actual averages for each tree. It would be impossible, however, to attempt to measure a hundred needles or so from each tree.

This project, which started out as a mere question on a very long commute, turned into a fascinating and eventually very persuasive report on how ozone pollution is affecting the environment of Cape Cod. My hypotheses were supported, and I found definite trends that indicated increased tip necrosis and chlorotic mottling of pine needles in areas such as intersections, parking lots, and downwind from a power plant. The pines that were in woody areas, or upwind of the power plant, were found to have much lower levels of tip necrosis and chlorotic mottling.

This project is very relevant when considering the health of humans, animals, and plants. As the population of the Earth grows, so does the amount of pollution we are producing. Every year, huge amounts of VOCs and nitrogen oxides are pumped into the air through the burning of fossil fuels. These products are reacting to form vast amounts of ozone in the troposphere, ozone that severely damages living tissue. Though I only showed how ozone is affecting white pine trees, these trees are indicators of how the pollution is starting to hurt humans.

According to an article in the Cape Cod Times on January 24, 2004, the expected additional pollution projected from a two-year delay in stricter emission standards at the Mirant Canal electric plant in Bourne will result in over 15.75 million more pounds of nitrogen oxide emissions. A recent Harvard University study cited in the article linked more than 43,000 asthma attacks and 300,000 incidents of upper respiratory illnesses in residents who live near Massachusetts' power plants. The results expected from delaying the emission standards on the Bourne plant include 87 premature deaths, 940 emergency room visits, 23,800 asthma attacks, and 164,000 incidents of respiratory illness for Cape-area residents.

The more I learn about ozone and its negative effects on the Earth, the more I can work to help people see the need to tighten emission standards before the environment and the health of the population is irreversibly damaged. This project gave me more than just some data about the trees in my town: I came away from it with a passionate concern for the environment and a desire to work to educate the public about the problems the environment is facing. When I started at that new school back in eighth grade, I thought maybe I wanted to be writer when I grew up. This project, however, has led me to apply to college to become an environmental science major, so that one day I can be a lobbyist for tighter environmental protection laws. Right now, I am informing local officials about the data I collected in my research, and trying to help spark change in my community. When I won the Massachusetts State Science Fair with this research, people actually started to listen. I hope that through my research, I can translate my passion to other people and help protect the environment in the process.



Delnore, Vic.  TRACE: Seasonal Variability of Tropospheric Ozone . NASA, Global Tropospheric Experiment (GTE). Retrieved from the World Wide Web on 3 April 2002.

Earth Observatory Reference, NASA.  Chemistry in the Sunlight . Retrieved from the World Wide Web on 17 February 2004.

Environment Canada.  Ozone: A Direct (and Growing) Threat . Retrieved from the World Wide Web on 7 September 2004.

Finnish Environment Institute (SYKE).  ICP IM Manual: Damage Parameters . 1998. Retrieved from the World Wide Web on 13 October 2005.
http://www.vyh.fi/eng/intcoop/projects /manual/chap7_16.htm

Hendrickson, Ole.  A Decade of Forest Health Monitoring in Evidence of Air Pollution Effects.  2003. Retrieved from the World Wide Web on 24 October 2005.

Krupa, Sagar V.  Air Pollution, People, and Plants—An Introduction.  St. Paul: The American Phytopathological Society, 1997.

Mirov, N.T.  The Genus  Pinus .  New York: The Ronald Press Company, 1967.

Reid, Stephen J.  Ozone and Climate Change.  Amsterdam: Gordon and Breach Science Publishers, 2000.

Scorer, Richard S.  Air Pollution Meteorology . Chichester: Horwood Publishing, 2002.

Sutton, Anne, and Myron Sutton.  The Audubon Society Nature Guides: Eastern Forests.  New York: Chanticleer Press, 1992.

U.S. Department of Agriculture, Forest Service.  Development and Application of the Ozone Injury Index. Pacific Southwest Research Station. 1996. Retrieved from the World Wide Web on 30 October 2004.

U.S. Department of Environmental Protection, Office of Air Quality, Planning & Standards.  Health and Environmental Impacts of Ground Level Ozone . 2000. Retrieved from the World Wide Web on 12 September 2004.

University of New Hampshire and NASA.  Forest Watch . Retrieved from the World Wide Web on 30 October 2004.