Books

Responses of Northern U.S. Forests to Environmental Change
ISBN 0-387-98900-5

Chapter 5: Interacting Effects of Multiple Stresses on Growth and Physiological Processes in Northern Forest Trees

Judson G. Isebrands, Richard E. Dickson, Joanne Rebbeck, and David F. Karnosky
Two pollutants that have substantial impacts on plant growth and are increasing in the atmosphere as a consequence of human activity are CO₂ and O₃. Plant responses to CO₂ and O₃ are complex, and become even more difficult to interpret when other known stressors are considered, including nitrogen limitation, temperature and moisture extremes, and pests. In general, increasing atmospheric CO2 increases photosynthetic rates, height growth, and biomass production, while increasing atmospheric O₃ decreases photosynthetic rates and biomass production, and increases leaf senescence. The amount and sometimes the direction of change depends on internal plant factors such as age and genotype. Higher CO₂ concentrations may compensate for some other environmental stresses. For example, most studies show that CO₂ enrichment increases growth even though light and/or nutrients are limiting. It is becoming clear that both increasing CO₂ and O₃ impact fundamental plant processes which in turn affect susceptibility to plant-feeding insects.

Studies on trembling aspen show that O₃ usually decreases growth although the effect varies significantly with genotype. Root growth appears particularly sensitive to O₃. In contrast, substantial increases in relative below-ground C allocation were found in response to elevated CO₂. Experiments with both elevated CO₂ and O₃ suggest that elevated CO₂ does not compensate for reduced growth caused by elevated O₃. When N limits growth, there appears to be no response to elevated CO₂. Because CO₂ and O₃ change the chemical composition of the foliage, resistance to insect attack and nutritional value of foliage are altered. Elevated O₃ appears to increase insect growth and elevated CO₂ decreases insect growth. Under field conditions, these changes in insect physiology may offset increases or decreases in biomass production that are associated with a changing atmosphere.

Consistent growth responses of yellow poplar to O₃ have not been reported even though the species shows visual foliar symptoms of exposure. One study reported an increase in biomass production during the first year of exposure in open-top chambers, and a decrease after two seasons. Elevated CO₂ appears to increase yellow poplar growth regardless of level of exposure to O₃. In general, research on yellow poplar suggests that under field conditions, this species will increase biomass production even when nutrients and moisture are limited and in the presence of O₃.

In summary, there appear to be major differences in the response of species and genotypes to multiple interacting stresses. The mechanisms causing the responses are largely unknown. Experimental methods have a major impact on how results from these studies should be interpreted. Significant chamber effects are common, limiting extrapolation of many experimental results to field conditions. Open-air exposure experiments and field physiological studies will help alleviate this concern over time, as will improvements of physiological models.

Below: Changes in total stem dry weight of aspen clones exposed to elevated O₃ and O₃ + CO₂ for 3 years in open-top chambers.

Previous: Chapter 4 | Next: Chapter 6