Accumulation and depletion of base cations in forest floors in the northeastern United States

Loss of base cations from forest soils can be accelerated by acid rain, by forest regrowth following harvest removals, and by declining inputs of base cations from atmospheric deposition. Calcium losses from forest Boors have been reported at several sites in the northeastern United States. To test for loss of base cations from forest Boors at the Hubbard Brook Experimenta l Forest in New Hampshire (USA), we analyzed samples collected on seven dat es between 1976 and 1997. Calcium and magnesium contents of the forest floo r did not decline significantly; a change >0.9%/yr would have been detectab le. Concentrations of Ca were 40% higher in 1969-1970 than in the current s tudy, but the difference is partly due to changes in collection methods. Ma gnesium concentrations were too variable to detect a loss of <47% over the 21-yr interval. Td determine whether base-cation losses were associated wit h forest growth, we resampled a chronosequence of northern hardwood stands in the White Mountains of New Hampshire. The 13 stands did not show consist ent changes in Ca, Mg, and potassium over the 15-yr interval. Losses of the se cations were most pronounced in stands logged more than 25 yr earlier. Y ounger stands, contrary to our expectation that rapid forest growth should cause cation depletion, all gained base cations in the forest Boor Early in stand development these forest floors appeared to accumulate biomass along with living vegetation, rather than serving as a net source of nutrients. Finally, in a regional survey of 28 mature stands in the northeastern Unite d States, some lost significant forest-floor Ca and Mg between 1980 and 199 0, while others gained. The average change in Ca and Mg content was not sig nificant; a loss of 1.4%/yr would have been detectable. Forest Boors in the region are not currently experiencing rapid losses of base cations, though losses may have preceeded the onset of these three studies.