The Impact of Simulated Acid Rain and Fertilizer Application on a Mature Sugar Maple (Acer Saccharum Marsh.) Forstt in Central Ontario Canada

The effects of two year's addition of simulated acid precipitation, with and without added fertilizer, on mycorrhizae, litter decomposition and soil and tree chemistry in a sugar maple (Acer saccharum Marsh.) dominated forest were investigated. The forest floor beneath mature sugar maple trees was irrigated at monthly intervals between May and September with local lake water acidified to pH 3, pH 4 or untreated lakewater of pH 4.9. In addition, a commercial organic slow-release fertilizer (Maple Gro) was added to the soil prior to irrigation with pH 3 spray. Trees to which no experimental spray was applied were also included as controls. Eight trees were used for each of the five treatments. Application of the acid spray alone did not acidify surface soil nor cause visible symptoms of decline in trees. The pH of the soil solution and soil leachate was increased by addition of acidified lakewater. An increase in the concentration of sulphate (SO₄ ^2-), calcium (Ca), and magnesium (Mg) in soil leachate was only recorded in plots irrigated with water acidified to pH 3 + Maple Gro. The initial rate of litter decomposition tended to be higher following application of the acid sprays, although mycorrhizal infection of sugar maple roots was reduced in the pH 3 and pH 4 treatments. Concentrations of zinc (Zn), chromium (Cr) and nickel (Ni) were highest in wood formed during the period irrigated with water acidified to pH 3. Foliar nitrogen (N) concentrations tended to be higher in all irrigated treatments, although there were no differences between treatment in any of the other nutrients measured in foliage after two years of treatment. It is concluded that the application of simulated acid rain under field conditions results in a complex interaction of events which are not reproduced in pot trials and must be fully understood before the impact of acid rain on sugar maple forests can be evaluated.