The long-term effects of carbon dioxide on natural systems: issues and research needs

Research on the responses of plants to increasing levels of carbon dioxide has largely assessed physiological, phenotypic, and community-level effects. Little attention has been directed to investigating the possibility that escalating levels of carbon dioxide may serve as a selection pressure altering the genetic diversity of plant populations. Plant populations exposed to elevated levels of heavy metals or ozone have been shown to undergo selection, and it is reasonable to consider that populations experiencing long-term exposure to escalating levels of carbon dioxide may show similar responses. Selection of this nature could be particularly significant because of the global extent of the effect.Genetic selection occurs when plants are subject to an agent of selection and three conditions for a property responsive to the agent are satisfied at the population level. In the population, variation must exist in the property, part of the variation must be genetically controlled, and variation in the property must affect reproductive fitness. If these conditions are satisfied, the frequency distribution of the property, and the gene frequency associated with it, will change over time in response to the agent of selection.Research on the selection pressure effects of carbon dioxide involves assessments that integrate across temporal, spatial, and biological scales, and embrace variation in the environment and genetics. To be effective, the research will have to adopt approaches that have not been commonly employed in previous air quality studies. The questions posed are biologically complex, and new research approaches and methods are required to answer them. Some of the new approaches that can be used to assess changes in gene frequency include use of natural carbon dioxide gradients, model plant systems, molecular markers, and DNA microarray technology.     

Evaluation of the effects of ozone and acidic precipitation,alone and in combination,on the photosynthesis,nutrition, and growth of red spruce and sugar maple

Substantial and widespread morbidity and mortality of red spruce have been observed in high elevation forests of the northeast under circumstances indicative of a stress-related disease. Whether red spruce at lower elevations are experiencing a more subtle loss of growth and vigor is uncertain. In addition, sugar maple has exhibited decline of varying extent and intensity for several decades. Forests in the northeast are exposed to two air pollutants, ozone (O₃) and acidic precipitation, that are widespread in occurrence and have the potential, both individually and collectively, to produce impacts to forest trees. the roles, if any, of these two stress agents in the tree declines found in the northeast are not known.In 1986, a five-year study was initiated to evaluate the effects of O₃ and acidic precipitation on red spruce and sugar maple. The trees will be exposed to controlled levels of O₃ and acidic precipitation in the field using open-top chambers. The experiment is a 4×3 factorial conducted in split plots with O₃ treatments as whole plots and simulated rain treatments comprising the split plots. Broadly stated, the research will evaluate the effects of the pollutants on the processes, fluxes, and pools associated with carbon, water, and nutrients in the soil/tree/atmosphere system. These evaluations will be conducted on a systems level and will be integrated through the development of mechanistic simulation models.Assessment of the effects of the treatments on carbon fixation by photosynthesis, the loss of carbon through respiration, and the allocation of carbon in growth will be a central focus of the study. Whole-tree cuvettes will be used to assess net photosynthesis, respiration, transpiration, and stomatal conductance.Considerable emphasis will be placed on determining the influences of the treatments on the biogeochemistry of the system. These studies will focus on the leaching of nutrients from the tree canopy, the mobilization and loss of nutrients from the soil, soil solution chemistry, and the alteration of tree nutrition by the input of additional nitrogen in precipitation.Statistical and simulation modeling will be used to assess and describe the effects of the treatments. The modeling approaches are different in technique, but complementary. Statistical models will be used to describe the responses of growth and physiological variables to the ozone and acidic precipitation treatments. Simulation models will be built to describe the relationships between photosynthesis, respiration, nutrition, and water use, how these processes are affected by the treatments, and how these effects ultimately result in altered growth. The simulation models will initially provide a framework for the formulation of hypotheses regarding the interrelationships of plant components and processes and how they are affected by the treatments.Contribution from Fourth World Wilderness Congress—Acid Rain Symposium, Denver (Estes Park), Colorado, September 11–18, 1987.     

Response of sugar maple to multiple year exposures to ozone and simulated acidic precipitation

Potted sugar maple seedlings were exposed to ozone and acidic precipitation in open-top chambers for three consecutive growing seasons. Periodic measurements of photosynthesis, dark respiration, through-fall and soil solution chemistry, and annual measurements of the weight of plant parts were made. Experimental treatments caused few and minor effects on above- or below-ground growth of the seedlings, even after three growing seasons. There were trends for reduced photosynthesis in trees exposed to elevated concentrations of ozone and increased photosynthesis in those exposed to the lowest pH simulated rain treatment. The chemistries of soil-solutions and through-fall were not altered significantly by treatment. Although major effects were not observed, sugar maple may respond to exposures that take place over a significant part of its life cycle.     

Assessing the risk of foliar injury from ozone on vegetation in parks in the U.S. National Park Service's Vital Signs Network

The risk of ozone injury to plants was assessed in support of the National Park Service's Vital Signs Monitoring Network program. The assessment examined bioindicator species, evaluated levels of ozone exposure, and investigated soil moisture conditions during periods of exposure for a 5-year period in each park. The assessment assigned each park a risk rating of high, moderate, or low. For the 244 parks for which assessments were conducted, the risk of foliar injury was high in 65 parks, moderate in 46 parks, and low in 131 parks. Among the well-known parks with a high risk of ozone injury are Gettysburg, Valley Forge, Delaware Water Gap, Cape Cod, Fire Island, Antietam, Harpers Ferry, Manassas, Wolf Trap Farm Park, Mammoth Cave, Shiloh, Sleeping Bear Dunes, Great Smoky Mountains, Joshua Tree, Sequoia and Kings Canyon, and Yosemite.     

The influence of ozone exposure dynamics on the growth and yield of kidney bean

A field experiment was conducted in open-top chambers to assess the importance of peak exposure concentration and exposure frequency on the responses of kidney bean plants to O3. There were five treatments in the study: charcoal-filtered air, constant exposure to 0.05 ppm O3 (131 microg m(-3)) daily. fluctuating exposure to 0.08 ppm O3 on three alternate days, cluster exposure to 0.08 ppm O3 on three consecutive days, and peak exposure to 0.12 ppm O3 on two consecutive days. Exposures lasted 4 h and produced an average weekly exposure-period concentration of approximately 0.05 ppm in the O3-addition treatments and 0.025 ppm in the charcoal-filtered treatment. Exposures began on June 23 and terminated on September 8. Plants were harvested weekly and assessed for the number, area, and dry mass of leaves; dry mass of stems; dry mass of roots; the number of pods; and the incidence of foliar O3 injury. Yield was assessed at the end of the study. There were no consistent differences between the plants receiving charcoal-filtered air and those receiving O3 exposure. Significant differences were detected among the treatments for several of the growth variables assessed at the interim harvests, but in the final two harvests these differences had mostly disappeared. There were no significant effects of the O3-addition treatments on yield when compared to the plants receiving charcoal-filtered air. This indicates that there were no cumulative impacts on plants exposed to 0.12 ppm O3 for 4 h on two consecutive days followed by filtered air compared to plants receiving charcoal-filtered air. The seasonal 7-h average concentrations of O3 in the peak and filtered air treatments were approximately 0.040 and 0.025 ppm, respectively.