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 floors have been reported at several sites in the northeastern United States. To test for loss of base cations from forest floors at the Hubbard Brook Experimental Forest in New Hampshire (USA), we analyzed samples collected on seven dates between 1976 and 1997. Calcium and magnesium contents of the forest floor did not decline significantly; a change >0.9%/yr would have been detectable. Concentrations of Ca were 40% higher in 1969-1970 than in the current study, but the difference is partly due to changes in collection methods. Magnesium concentrations were too variable to detect a loss of <47% over the 21-yr interval. To determine whether basecation losses were associated with forest growth, we resampled a chronosequence of northern hardwood stands in the White Mountains of New Hampshire. The 13 stands did not show consistent changes in Ca, Mg, and potassium over the 15-yr interval. Losses of these cations were most pronounced in stands logged more than 25 yr earlier. Younger stands, contrary to our expectation that rapid forest growth should cause cation depletion, all gained base cations in the forest floor. 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 United States, some lost significant forest-floor Ca and Mg between 1980 and 1990, while others gained. The average change in Ca and Mg content was not significant; a loss of 1.4%/yr would have been detectable. Forest floors in the region are not currently experiencing rapid losses of base cations, though losses may have preceeded the onset of these three studies.