Growth response and drought susceptibility of forest trees exposed to simulated acidic rain and ozone

One-year-old seedlings of red spruce, loblolly pine, yellow-poplar, and sweetgum were exposed to ozone (0.0 or 0.1 ppm, 4 hr d⁻¹, 3 d wk⁻¹) in combination with simulated acidic rain (pH 5.6 or 3.0, 1 h d⁻¹, 2 d wk⁻¹, 0.75 cm hr⁻¹) for ten weeks. After the ten-week treatment with ozone and simulated acidic rain, the seedlings were submitted to two drought cycles, and water potential, net photosynthesis (Pn), stomatal conductance (Cs), and transpiration (Ts) were measured.

Whole-plant fresh weight increment (FWT) and dry weight were significantly reduced in red spruce seedlings after the ten-week treatment with ozone. Ozone also significantly reduced shoot height growth (SHG) and increased the apparent plastochron duration (APD) of sweetgum seedlings. Treatment with simulated rain at pH 3.0 significantly increased FWT and SHG of red spruce compared to pH 5.6 as well as FWT and relative growth rate (RGR) of loblolly pine. The APD was significantly reduced in sweetgum seedlings exposed to simulated acid rain at pH 3.0 compared to 5.6. Significant interactions between ozone and simulated acidic rain occurred in all species except red spruce, and they were additive. The SHG was significantly lower in loblolly seedlings exposed to 0.1 ppm ozone + pH 5.6 than in seedlings exposed to either 0.1 ppm ozone + pH 3.0 or 0.0 ppm ozone + pH 5.6. The APD was significantly increased in yellow-poplar and sweetgum seedlings exposed to 0.1 ppm ozone + pH 5.6 compared to any other treatment.

Visible symptoms appeared on the adaxial leaf surface of yellow-poplar seedlings submitted to acid rain at pH 3.0 regardless of ozone treatment. Visible foliar symptoms were also observed on the adaxial surface of sweetgum seedlings exposed to 0.1 ppm ozone. The symptoms were characterized by premature red pigmentation with small brown necrotic lesions.

Foliar concentrations of P and S were significantly increased in all seedlings exposed to simulated rain at pH 3.0 compared with pH 5.6 except for red spruce which exhibited the increases of K and S. In general, there were neither significant effects of ozone nor interactions between ozone and rain pH on foliar nutrient concentrations.

There were no significant effects of ozone on Pn, Cs, Ts, or water-use efficiency (WUE) prior to the drought cycles for all species. However, after the first drought cycle, Pn and Cs were significantly changed in loblolly pine and sweetgum pre-exposed to 0.1 ppm ozone compared with controls. The ten-week treatment of simulated acidic rain at pH 3.0 significantly increased Pn and Ts of loblolly pine and reduced Cs of yellow-poplar. After the first drought cycle, yellow-poplar treated with pH 3.0 rain showed lower Pn and WUE than seedlings exposed to pH 5.6 solution.

A 0.5 MPa shift in the response of net photosynthesis (Pn) to decreasing water potential occurred in red spruce seedlings across all air pollutant treatments after the drought cycles indicating photosynthetic acclimation to drought stress. During the second drought cycle, Pn was more sensitive to water potential in red spruce and loblolly pine seedlings exposed to 0.10 ppm ozone + pH 3.0 solution compared with seedlings exposed to 0.0 ppm ozone + pH 5.6 solution. This increased sensitivity of Pn to water potential might be explained through a change in root hydraulic conductivity (Lp).

In general, Lp showed significant effects of ozone and simulated acidic rain after moisture stress. After the first drought cycle, Lp was decreased in yellow-poplar and sweetgum seedlings exposed to 0.10 ppm ozone. After the second drought cycle, Lp was decreased in yellow-poplar and red spruce seedlings exposed to 0.1 ppm ozone. Significant interactions between ozone and simulated acidic rain occurred in the Lp of red spruce and yellow-poplar seedlings after the first drought cycle.

The results from this dissertation indicate that ozone and/or simulated acidic rain can alter the growth and drought susceptibility of forest tree species under laboratory conditions. Forest trees which are periodically exposed to ambient ozone and acidic rain in natural forest ecosystems probably have a response similar to the results from this study. Additional research is necessary to determine if such effects are occurring in natural forest ecosystems.