Effects of Acidic Deposition on Forest Ecosystems

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In acid-sensitive regions, acidic deposition alters soils, stresses forest vegetation, acidifies lakes and streams, and harms fish and other aquatic life. These effects can

Sulfate ion wet deposition, 2005

Sulfate ion wet deposition, 2005

Nitrate ion wet deposition, 2005

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Figure 1 Map of wet sulfate and nitrate deposition for the US for 2005. Data are from the National Atmospheric Deposition Program (NADP) network.

Nitrate as NO3 (kg ha-1) < 4 4-6 6-8 8-10 10-12 12-14 14 - 16 16 - 18 18 - 20 > 20

Figure 1 Map of wet sulfate and nitrate deposition for the US for 2005. Data are from the National Atmospheric Deposition Program (NADP) network.

Table 1 The links between sulfur dioxide and nitrogen oxide emissions, acidic deposition, and other important environmental issues Problem Linkage to acid deposition

Coastal eutrophication Atmospheric deposition adds nitrogen to coastal waters.

Mercury Surface water acidification increases mercury accumulation in fish.

Visibility Sulfate aerosols diminish visibility and views.

Tropospheric ozone Emissions of nitrogen oxides contribute to the formation of ozone.

interfere with important ecosystem functions and services such as forest diversity and productivity and water quality. Years of acidic deposition have also made many ecosystems more sensitive to continuing pollution. Moreover, the same pollutants that cause acidic deposition contribute to a wide array of other important environmental issues at local, regional, and global scales (see Table 1).

Effects of Acidic Deposition on Forest Soils

Research has shown that acidic deposition has chemically altered soils with serious consequences for acid-sensitive ecosystems. Soils compromised by acidic deposition lose their ability to neutralize continuing inputs of strong acids, provide poorer growing conditions for plants, and extend the time needed for ecosystems to recover from acidic deposition.

Acidic deposition has altered and continues to alter soils in sensitive regions in three important ways. Acidic deposition depletes available calcium and other nutrient cations from exchange sites in soil, facilitates the mobilization of dissolved inorganic aluminum into soil water, and increases the accumulation of sulfur and nitrogen in soil.

Loss of calcium and other nutrient cations

The cycling of calcium and other nutrient cations in forest ecosystems involves the inputs and losses of these materials (Figure 2). For most forest ecosystems the supply of calcium and other nutrient cations largely occurs by weathering (i.e., the breakdown of rocks and minerals in soil). Calcium and other nutrient cations may also enter forests by atmospheric deposition, although this pathway is generally much smaller than weathering. Losses largely occur by vegetation uptake and drainage waters. An important pool of ecosystem calcium and nutrient cations is the soil available pool or the soil cation exchange complex. Plants are generally able to utilize this source of nutrients. Forest ecosystems that are naturally sensitive to acidic deposition are generally characterized by low rates of weathering and generally low quantities of available nutrient cations. Under conditions of elevated inputs of acidic deposition and subsequent transport of sulfate and nitrate in drainage waters, nutrient cations will be displaced from available pools and leached from soil.

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Weathering Deposition

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Figure 2 Conceptual diagram illustrating the inputs and losses of calcium and other nutrient cations for forest ecosystems. Ecosystems with low weathering rates have low supplies of calcium and other nutrient cations available to soil and trees. High inputs of acidic deposition increase the leaching of calcium and other nutrient cations from soil. In acid-sensitive regions with low weathering rates this can deplete soil pools of available calcium and other nutrient cations and limit quantities available for tree growth.

This condition is not problematic for areas with high weathering rates and high pools of available nutrient cations. However, in acid-sensitive areas with shallow soil and which contain minerals that are resistant to weathering, the enhanced loss of calcium and other nutrient cations can result in a depletion of soil available pools.

Over the last century, acidic deposition has accelerated the loss of large amounts of available calcium from acid-sensitive soil in acid-sensitive areas. This conclusion is based on more than 20 studies conducted throughout the world. Depletion occurs when nutrient cations are displaced from the soil by acidic deposition at a rate faster than they can be replenished by the slow breakdown of rocks or the deposition of nutrient cations from the atmosphere. This depletion ofnutrient cations fundamentally alters soil processes, compromises the nutrition of some trees, and hinders the capacity for sensitive soils to recover. For example, more than half of the available calcium has been lost from soil at the Hubbard Brook Experimental Forest, New Hampshire, over the past 60 years. Note that while acidic deposition to acid-sensitive areas is decreasing and there is some associated recovery of the acid-neutralizing capacity (ANC) of surface waters (see below), it appears that forest soils continue to exhibit depletion of exchangeable nutrient cations.

Mobilization of aluminum

Aluminum is often released from soil to soil water, lakes, and streams in forested regions with high acidic deposition, low stores of available calcium, and high soil acidity. One of the most significant ecological effects of acidic deposition is the mobilization of aluminum from soil and a shift in the form of aluminum in water from nontoxic organic forms to highly toxic inorganic forms.

Concentrations of aluminum increase markedly with decreases in pH, particularly the toxic inorganic forms of aluminum. It is evident that concentrations of aluminum increase exponentially when surface water pH decreases below 6. Aluminum concentrations are thought to be ecologically significant when they increase to values above 2 mmol l-1. This condition clearly occurs below pH 6.0.

High concentrations of dissolved inorganic aluminum can be toxic to plants, fish, and other organisms. Concentrations of dissolved inorganic aluminum in streams in eastern North America and areas of Europe are often above levels considered toxic to fish and much greater than concentrations observed in forest watersheds that receive low inputs of acidic deposition.

Effects of Acidic Deposition on Trees

Acidic deposition has contributed to the decline of red spruce and sugar maple trees in the eastern US (Figure 3). Symptoms of tree decline include poor condition of the canopy, reduced growth, and unusually high levels of mortality. Declines of red spruce and sugar maple in the northeastern US have occurred during the past four decades. Factors associated with declines of both species have been studied and include important links to acidic deposition.

Red spruce

Since the 1960s, more than half of large canopy trees in the Adirondack Mountains of New York and the Green Mountains of Vermont and approximately one-quarter of large canopy trees in the White Mountains of New Hampshire have died. Significant growth declines and winter injury to red spruce have been observed throughout its range. Acidic deposition is the major cause of red spruce decline at high elevations in the northeast.

Red spruce decline occurs by both direct and indirect effects of acidic deposition. Direct effects include the leaching of calcium from leaves and needles of trees (i.e., foliage), whereas indirect effects refer to acidification of the underlying soil chemistry.

The decline of red spruce is linked to the leaching of calcium from cell membranes in spruce needles by acid

Acid deposition effects on trees

Red spruce

Sugar maple

Calcium leached from needle membranes

Decreased cold tolerance

Increased freezing injury

Calcium and magnesium leached from soil

Aluminum mobilized and taken up by tree

Calcium and magnesium leached from soil

Aluminum mobilized and taken up by tree

Aluminum

Calcium and magnesium

Figure 3 Conceptual diagram illustrating the mechanisms by which acidic deposition impacts red spruce and sugar maple. Acidic deposition impacts red spruce through loss of membrane calcium due to direct leaching from foliage or reduced uptake of calcium from soil. The loss of membrane calcium makes red spruce more susceptible to winter injury. Acidic deposition results show loss of soil available calcium and magnesium and less uptake by sugar maple. This condition may make sugar maple more susceptible to insect or drought stress.

Aluminum

Calcium and magnesium

Figure 3 Conceptual diagram illustrating the mechanisms by which acidic deposition impacts red spruce and sugar maple. Acidic deposition impacts red spruce through loss of membrane calcium due to direct leaching from foliage or reduced uptake of calcium from soil. The loss of membrane calcium makes red spruce more susceptible to winter injury. Acidic deposition results show loss of soil available calcium and magnesium and less uptake by sugar maple. This condition may make sugar maple more susceptible to insect or drought stress.

mist or fog. The loss of calcium renders the needles more susceptible to freezing damage, thereby reducing the tolerance of trees to low temperatures and increasing the occurrence of winter injury and subsequent tree damage or death. In addition, elevated aluminum concentrations in the soil may limit the ability of red spruce to take up water and nutrients through its roots. Water and nutrient deficiencies can lower the tolerance of trees to other environmental stresses and cause decline.

Sugar maple

The decline of sugar maple has been studied in the eastern US since the 1950s. Extensive mortality among sugar maples appears to have resulted from deficiencies of nutrient cations, coupled with other stresses such as insect defoliation or drought. The probability of decreases in the vigor ofthe sugar maple canopy or incidence oftree death increased on sites where the supply of calcium and magnesium to soil and foliage are lowest and stress from insect defoliation and/or drought is high. Low levels of nutrient cations can cause a nutrient imbalance and reduce the ability of a tree to respond to stresses such as insect infestation and drought.

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