Restoring Soil Calcium Reverses Forest Decline

Forest decline in the northeastern United States has been linked to the effects of acid deposition on soil nutrients. To test this link, we added a calcium silicate mineral to a paired watershed at the Hubbard Brook Experimental Forest, New Hampshire, in an amount designed to gradually replace the estimated amount of calcium lost as a result of human activity in the 20th Century (primarily because of acid deposition). The experimental restoration resulted in a recovery of tree biomass increment. The improved calcium nutrition also promoted higher aboveground net primary production and increased the photosynthetic surface area in the treated watershed relative to that in the reference watershed. These results demonstrated that soil acidification accelerated by acid deposition has contributed to the decline of forest growth and health on naturally acidic soil in the northeastern United States and that decline can be reversed by the addition of calcium.