Biotic control of calcium cycling in northern hardwood forests: acid rain and aging forest. Ecosystems

Observations of declining base saturation in soils and declining calcium (Ca) in streamwater have contrib-uted to concerns that prolonged exposure to acid rain threatens forest health and productivity. We suggest that these changes could be caused, in part, by aging of the forests. To test this possibility, we characterized Ca cycling in previously harvested, variously aged northern hardwood stands over 15–18 years. The Ca content and concentrations in the forest floor and the density of snails, which require Ca for growth, in-creased in young stands (less than 30 years old) and decreased in older stands (more than 30 years old) over the measurement period. Similarly, the concen-trations of Ca in litterfall decreased with stand age, and hydrologic export of Ca from a young stand was higher than that from an old stand. Ecosystem bud-gets suggest that the supply of Ca from the mineral soil to other parts of the ecosystem is large (3.3–4.7 g Ca m2 y1) in young forest stands but negligible or negative in older stands (–1 g Ca m2 y1). This difference in Ca mobilization between young and old stands is large compared to the changes in soil Ca that can reasonably be attributed to acid precipitation (less than 1 g m2 y1). We conclude that changes in soil and streamwater Ca in maturing forests do not nec-essarily indicate an important loss of bioavailable Ca. Trace amounts of apatite in the mineral soil may be the source of Ca needed for forest regrowth. Key words: calcium; forest succession; acid rain; snails; litterfall; forest floor; northern hardwoods