Sulfuric and nitric acids are detrimental to all plant life. Even if the soil is well-buffered, forests can be damaged by acid fog. Acid deposition of all sorts ruins the waxy coatings of leaves, harming the tree's ability to exchange water and gases with the atmosphere. Trees weakened by acid experience slower growth or injury and are more vulnerable to stresses such as pests or drought. Acid-damaged plants are easily identified: the leaves of leafy plants turn yellow, and damaged pine needles become reddish-orange at the tips before they die.
Acid rain also leaches soil nutrients, which stunts tree growth. When trees are deficient in calcium, they are less able to withstand freezing. "As with immune-compromised humans, plants may appear and function as if they were healthy, until exposed to even a routine stress or disease, then experience declines far more exaggerated than expected," said Professor Donald DeHayes of the University of Vermont in Medical News Today in 2005. DeHayes' study documented the depletion of calcium by acid rain that weakened high-altitude red spruce (Picea rubens) trees, making them more vulnerable to winter freezing. Other trees, such as the balsam fir (Abies balsamea), white pine (Pinus strobes), sugar maple (Acer saccharum), flowering dogwood (Cornus florida), and eastern hemlock (Tsuga canadensis) are also damaged in this way. Acid leaching also deposits metals in the topsoil. These metals—such as aluminum, lead, zinc, copper, and chromium—are toxic for trees, mosses, algae, some bacteria, and fungi.
Acid rain can destroy forest ecosystems. Acidic soil wipes out snail populations, which lowers the calcium intake of songbirds and causes them to produce eggs with thin shells. Birds and mammals that eat calcium-deficient plants may produce young that have weak or stunted bones; mammals may produce less milk.
The forests of the Appalachian Mountains from Maine to Georgia have been vulnerable to acid damage, in part because the soils there are not buffered. Researchers at the Cornell Lab of Ornithology in Ithaca, New York, in a 2002 article in Birdscope pinpointed the decline of a North American songbird, the wood thrush (Hylocichla mustelina), to acid rain. Calcium loss decreased the birds' breeding success, resulting in a population decline of 1.7% per year between 1966 and 1999. Many other bird species in the area are showing similar population declines. The Cornell study was the first to present large-scale evidence linking the decline of a North American bird to acid rain, although these links have been commonly found in Europe. If acid precipitation causes large swaths of trees to die, the life they support may mostly perish.
Germany's forests are in such a bad state that the term Waldsterben— or forest death—is well known in the country. A 1995 survey showed that more than half the trees in the nation's western forests, including the famed Black Forest, were in decline due to acid precipitation, magnesium deficiency, and the effects of other air pollutants. In 2004, nearly three-quarters of the trees were suffering decline from a combination of factors, including acid precipitation, drought, high ozone levels, and high levels of chemicals in the soil. According to a 2004 article in the Guardian, a British newspaper, the decline has been so rapid that Michael Hopf, a spokesman for Greenpeace Germany, said "We have pictures of the same forests taken in 2002 and 2004. You can see the damage very clearly." Germany's agriculture minister, Renate Künast, added, "The state of our forests is alarming. We must seize every possibility to reduce the burden on the forest ecosystem." Forests are suffering in other regions of the world, including Scandinavia, India, Russia, China, and Canada. High acidity can alter forest ecosystems as well as damage trees.
Acid precipitation may also damage crops, since the acid damages leaves and soil. In the developed nations, acid rain damage to agriculture is nullified by fertilizers that replace leached nutrients and lime that neutralizes the acid. However, in countries where farmers cannot afford these remedies, acid precipitation and increased soil acidity have an amplified effect. The World Bank has estimated China's overall annual forest and crop losses due to acid rain at $5 billion.
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