Growth and xylem water potential of white oak and loblolly pine seedlings as affected by simulated acidic rain

Effects of simulated acidic rainfall on the growth and water relations of white oak (Quercus alba L.) and loblolly pine (Pinus taeda L.) seedlings grown under two fertility regimes were examined. Seedlings of each species grown in a loam soil were exposed to two simulated rains per week of pH 4.8, 4.2 or 3.6 for 26 wk. High and low fertility regimes were imposed by monthly application of one-half and one-quarter concentrations, respectively, of Hoagland's solution #2. Diameter growth of both species was reduced by exposure to rains of the higher acidities regardless of fertility treatment, and seedlings that received pH 4.2 and 3.6 rains also exhibited greater foliar chlorosis and necrosis than those that received rains of pH 4.8. The high fertility treatment stimulated diameter growth of white oak, but height growth, shoot and root dry weights and total root length were not substantially affected by acid rain or fertility treatments in either species. Following the final rain applications, drought was simulated by withholding irrigation for 2 wk, and seedling xylem pressure potential was then measured using the pressure chamber technique. Xylem pressure potential of white oak seedlings which had received rains of pH 3.6 was significantly lower than that of seedlings which had received rains of higher pH, a result that became more pronounced as soil water potential decreased. Rain acidity had little effect on the xylem pressure potential of loblolly pine, however. Soil pH analyses before initiation and after completion of the rain applications indicated that rainfall of pH 3.6 increased soil acidity more than rains of pH 4.2 or 4.8, although changes in soil pH were small overall.