Ectomycorrhizal colonization of loblolly pine seedlings during three growing seasons in response to ozone, acidic precipitation, and soil Mg status

A three-year study was initiated in 1987 to evaluate the impact of O3, acidic precipitation, and soil Mg on ectomycorrhizal colonization of loblolly pine (Pinus taeda L.) seedlings. Thirty-six open-top chambers equipped with a rainfall exclusion/addition system were utilized to administer three levels of O3 (subambient, ambient, or twice ambient) and two precipitation acidity levels (pH 3.8 or 5.2) to seedlings growing in 24-liter plastic pots containing soil having either 35 or 15 mg kg(-1) of exchangeable Mg. Seedlings exposed to the twice ambient O3 treatment exhibited smaller percentages of total ectomycorrhizal short roots at the end of each year of the study, but trends were statistically significant in 1989 only. Changes in number of specific ectomycorrhizal morphotypes in response to O3 were not consistent from year to year. Acidic precipitation treatments had no effect on number or percent of mycorrhizal short roots, and responses of two morphotypes to soil Mg treatments were probably due to differences in the soil environment rather than a result of changes in aboveground processes. Temporal shifts in morphotype frequencies were observed for seedlings in all treatments and indicate that mycorrhizal succession occurred during the study period.