Automobiles burning fossil fuels and power plants burning fuels containing sulfur or nitrogen release acid-forming particles into the air. These particles collect in the atmosphere in clouds, where they may travel thousands of miles before falling to the ground in pollution called acid rain, although it may be in the form of hail, fog, mist, sleet or snow. Stands of red spruce trees in the northeastern U.S. have been particularly devastated by acid rain.
Red Spruce Loss
More half of the red spruce in New York's Adirondack Mountains and Vermont's Green Mountains plus a quarter of the red spruce in New Hampshire's White Mountains have died because of acid rain since the 1960s. The losses have been especially large at higher elevations.
Loss of Calcium
Calcium and magnesium are important plant nutrients. Acid rain leaches calcium from the soil. Scientists monitoring the effects of acid rain in the Hubbard Brook Experimental Forest in New Hampshire's White Mountains say that over the past several decades, available calcium in the soil there has dropped by 50 percent. The loss of calcium makes the needles of red spruce less able to tolerate low temperatures. Trees freeze more easily in winter, injuring some and killing others. A study by the U.S. Forest Service found that adding calcium to the soil of declining stands of red spruce trees significantly increased the health of the trees.
Acid rain increases the amount of dissolved inorganic aluminum in the soil. High concentrations of aluminum hinders the ability of the roots of red spruce to take in water containing essential nutrients.
Sulfur and Nitric Acid
Rain containing high levels of nitric and sulfuric acids causes high levels of nitrogen and sulfur to accumulate in the soils where red spruce trees grow. These concentrations hinder the ability of the soil to accept other acids. This slows the growth of red spruce and delays the ability of the ecosystem to recover from the damage done by acid rain.
Research conducted U.S. Forest Service suggests that loss of calcium caused by acid rain harms the biological ability of red spruce and other trees to recognize and respond to various forms of stress. The plants suffer exaggerated injury as a result of environmental stress, including pests, pathogens, ozone depletion and climate change.