The impact of ozone, water stress, and acid rain on the growth and physiology of fraser fir seedlings

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Virginia Polytechnic Institute and State University

Three studies were conducted to determine the effects of ozone, water stress, and simulated acidic rain on the growth and physiology of Fraser fir seedlings. In Experiment I, seedlings were first exposed to 2 levels of moisture stress conditioning (MSC) for four weeks, and 3 levels of ozone ( < 0.02, 0.05, and 0.10 ppm) and three levels of water stress (control, moderate, and severe) for 10 wks. The 2 MSC levels were a control (well-watered at all times) and a stress treatment in which seedlings were not watered until pre-dawn needle water potential measurements ('I') fell below -1.0 MPa. The 3 levels of water stress were a control (well-watered at all times); a moderately stressed treatment (seedlings watered when 'I' levels fell between -0.8 and -1.0 MPa); and a severely stressed treatment (seedlings watered when 'I' fell below -1.2 MPa). Fraser fir seedlings were exposed to 3 levels of simulated rain (pH 3.0, 4.3, and 5.6) and 2 levels of ozone ( < 0.02 and 0.10 ppm) in Experiment II for 10 wks. Fraser fir seedlings in Experiment III were exposed to ozone levels of < 0.02, 0.05, and 0.10 ppm ozone and control, moderate, and severe water stress (as specified above) for 10 wks during the first year. Seedlings were then exposed to 2 levels of ozone ( < 0.02 and 0.10 ppm) for 10 wks in the following year. Analysis of data indicate that ozone of 0.10 ppm significantly decreased net photosynthesis (Pn) in one study but was not significant in reducing Pn in the other two studies. Ozone also failed to reduce growth significantly; however, decreasing trends were often apparent. Fraser fir biomass, transpiration (Ts), and needle conductance (Cs) were significantly decreased by water stress. Water-use efficiency was also improved as a result of prior moisture stress. Simulated acidic rain did not result in any significant changes in biomass accumulation, height, or diameter increment over the ten week period of the study. A solution of pH 3.0 did result in significantly higher root surface area and significantly reduced Ts after 5 wks of exposure; these responses are possibly due to a fertilization effect. No treatment interactions were significant. However, the combined, cumulative effect of all of these stresses may contribute to an overall decline in forest ecosystem productivity.