Tree-ring cation response to experimental watershed acidification in West Virginia and Maine

The impact or experimental watershed acidification on xylem cation chemistry was evaluated in eight tree species at two sites in West Virginia (Clover Run and Fernow) and one site in Maine (Bear Brook). All sites had received regular additions of (NH₄)₂SO₄ equivalent to twice the ambient annual wet plus dry atmospheric deposition of N and S. Multiple wood cores were extracted from tree boles in five trees of each species on treatment and control areas at each site with increment borers. Cores were divided into several age segments and composited for each tree. Ground wood samples were destructively analyzed for Ca, Mg, Mn, and Al concentrations using inductively coupled plasma emission (ICP) methods. All tree species sampled at the two West Virginia sites exhibited significant Ca and/or Mg concentration decreases and Mn concentration increases in sapwood on the treated relative to control areas after 8 yr of treatment. At Bear Brook, tree-ring concentrations in three species showed similar trends after 5 yr of treatment, but differences were generally not significant. Sapwood molar ratios of Ca/Mn and Mg/Mn were better indices to soil acidification than Ca/Al, due to low Al concentrations and insensitivity of sapwood Al concentrations to treatments. Overall, sapwood chemistry appeared to be a reliable indicator of the current nutrient status of trees; but, except for Japanese larch (Larix leptolepis Sieb. and Zucc.), sapwood chemistry did not preserve a record of the chronology of past changes due to treatments.