Krupa et al. 2001

Background

Starting in the 1950s, researchers recognized the detrimental effects of smog on several plant species. In the past 50 years, the effects of tropospheric ozone have been identified for several crops, deciduous trees, and conifers. Ozone can induce negative responses in plants that are subjected to surface ozone in over short and long periods of time. Chronic exposure occurs when plants are exposed to low levels of O3 over an entire lifetime. Acute exposure refers to periods of relatively high O3 exposure (>80 ppb) over several hours or days. Chronic and acute exposure can negatively impact sensitive species and result in foliar injury. The effects of ozone vary by genus, species, variety, and genotype. Sensitive species include several varieties of tobacco and white clover.

Although ozone is often thought as an urban problem, it can travel long distances to agricultural and forested areas (Chappelka and Samuelson 1998).

Ozone affects plants through diffusion into leaves and by entering the leaf through the stomata. The amount of ozone entering the stomata depends on the stomatal opening which may be affected by environmental and cultural factors, including increased CO2 or irrigation (Krupa et al. 2001).

Ozone affects wild plants and agricultural crops. A 1999 estimate suggests that a 25 percent reduction in ambient ozone could create benefits for the United States equivalent to $1-2 billion annually. Today the ozone damage to the eight major U.S. crops is approximately $2-3 billion. Additionally, the same model predicts that the benefits in the agricultural sector could reach $3.5-6.1 billion annually if the production of all ozone precursor pollutants (VOC and NOx) were eliminated (Murphy et al. 1999).

Individual plants can be affected in several ways:

1) Physiology

Several studies have found that stratospheric ozone can reduce carbon fixation, increase respiration, alter the partitioning of carbon, and change nutrient allocation patterns (Chappelka and Samuelson 1998). Ozone can cause direct cellular damage to the leaf after it enters through the stomata (Felzer et al. 2004).

2) Growth

Many tree species have demonstrated reduced growth under ambient and elevated levels of surface ozone. However, many of the species experiencing stunted growth did not exhibit foliar ozone injury (Chappelka and Samuelson 1998). The biomass and yield of agricultural crops may also be reduced.

3) Plant disease

Plants exposed to acute or chronic levels of O3 may be more susceptible to disease. But ozone can decrease the effects of disease caused by facultative parasites. But the incidence of disease by facultative parasites may increase. Ozone is not expected to directly affect fungal pathogens but rather hosts plants could become more susceptible. Disease may also alter the foliar response of trees to ozone (Krupa et al 2001).

4) Visible, Foliar Injury

One of the main questions is whether visible foliar injury is directly associated with decreased growth and biomass production. Research has indicated that various species and varieties demonstrate very different responses to ozone. It has been difficult to separate the effects of ozone from many other environmental factors that affect plant growth. The images on the left are examples of ozone-induced damage to plants (Chappelka and Samuelson 1998; Krupa et al 2001).

Changes in plant chemistry result in secondary effects on pests and ecosystem processes:

The effects of chronic and acute exposure of ozone on plants has been widely studied over the past four decades. However, there has been little investigation of the effects of tropospheric ozone on higher tropic levels. Recent study has indicated that plants affected chemically or physically by stratospheric ozone may be more susceptible to disease and pest outbreaks. The development of free air carbon dioxide enrichment (FACE) technology has allowed large-scale investigations in the field to examine the impacts of elevated ozone on insects and plant diseases (Percy et al. 2002).

Additionally, ecosystem models have been used to create continental and global scale estimates of changes in net primary production and carbon sequestration due to increased surface ozone concentrations. Until recently, the effects of ozone had been studied for specfic species or regions. Global biogeochemical models have allowed researchers to setimate the effects of ozone for the entire United States over the 20th century.

The results and conclusions of selected experiments are presented in the case studies for pest species and for net primary production & the carbon cycle.

Potential for Future Work:

The Aspen Free Air CO2 Experiment (Aspen FACE) in Wisconsin has been exposing large portions of forest to elevated concentrations of carbon dioxide and ozone. These experiments have demonstrated that the effects of elevated CO2 and O3 can have cumulative or canceling effects on forest trees. Topics of future research can include (1) separating the effects of ozone from other environmental variables, (2) continuing to examine the effects of ozone on a variety of different species, and (3) expanding research to the Southern Hemisphere, (4) examining pest and ecosystem responses to changing chemistry in plants due to tropospheric ozone.

For example, Agrell et al. (2005) found that nitrogen levels birch was only affected under elevated levels of CO2 and ozone, where nitrogen levels decreased 22% compared with the other treatments (CO2 or O3 separately). However, nitrogen levels in the two aspen genotypes showed no response under any of the treatments. Further research could further explain the variable responses of different tree species and the additive effects of CO2 + ozone.

Addtionally, Morgan et al. (2003) found that elevated carbon dioxide significantly decreased the effects of ozone which is believed to be associated with a decrease in stomatal conductance. In this case, CO2 works to counteract the effects of ozone. Additional research could provide more information on additive effects of increasing CO2 and ozone concentrations.