Pollution acid rain and forest decline

Acid rain became a problem when combustion of fossil fuels increased after the industrial revolution, releasing increasing quantities of sulphur dioxide (SO2), nitrogen oxides (NOx - see next section for a definition) and other acidifying particles into the atmosphere to join what is naturally produced. The sulphur dioxide and nitrous oxides dissolve in atmospheric moisture to form sulphuric (H2SO4) and nitric (HNO3) acids which are brought to ground as acid precipitation. Unpolluted rain has a pH of 5.6 (on the acidic side of chemical neutrality due to CO2 dissolving in it to form weak carbonic acid) while acidic rain usually has a pH between 4.0-4.5 with extreme cases of pH 2.0 (more acidic than lemon juice) occasionally reported. Acid rain effects became apparent in the 1970s, and by the 1980s acid rain was widespread around the globe and particularly bad in eastern North America, north-west Europe, eastern Asia and parts of the southern hemisphere such as Brazil. Although the acidic components are generated mainly by industrial centres, long-distance transport has resulted in widespread problems.

Sulphur dioxide emissions, the main cause of acid rain, are reducing in many countries due to legislative initiatives and the improvement of technology in industry. Nitrous oxide emissions, however, are on the whole less regulated and are increasing globally. They are thus likely to make up the main component of acid rain in the future, supplanting sulphuric acid (see Likens (2004) for a discussion on legislative changes on emissions in the USA). In the early days of taking acid rain seriously, it was hotly debated whether sulphur dioxide was ultimately responsible for acid rain. The long-term studies at the Hubbard Brook Experimental Forest (HBEF - see Box 8.1) have shown categorically that concentrations of sulphur dioxide in precipitation and stream water are correlated with emissions of sulphur dioxide upwind of HBEF (Likens et al, 2002). Similarly, atmospheric depositions of nitrate (NO3) at HBEF are correlated with nitrous oxide emissions.

Acid rain acidifies those terrestrial and aquatic ecosystems that are not sufficiently buffered causing loss of species and accelerated leaching of nutrients particularly calcium and magnesium, but also potassium and sodium. At HBEF between 1940 and 1955 precipitation supplied 29% of the calcium required to balance that taken up in biomass and lost in streamflow (the rest was made up from weathering and from the soil storage complex). By 1976-1993 precipitation was supplying only 12%. Around a half of the pool of available calcium in the soils at HBEF has been depleted during the past 50 years by acid rain. This loss leads to a lack of buffering capacity to absorb further acidity, so forest and aquatic ecosystems become more sensitive to further damage by acid rain. The effects of acid rain on such things as fish and trees are well-known but the effects are more widespread. For example, acidification of soil leads to a loss of soil fauna with a consequent reduction in the rates of organic matter decomposition. Larger animals are not immune: calcium shortage can limit reproduction of passerine birds (small perching birds living near the ground). Tilgar et al. (2002) found that great tits Parus major in coniferous and hardwood stands in Estonia laid more eggs and raised more and bigger fledglings when calcium was abundant.

By the 1980s there were widespread fears that acid rain was leading to wholesale forest decline, partly driven by the severe die-back of forests seen in parts of

Germany (termed 'Waldsterben', meaning forest death). By the late 1980s most European countries had more than 15% of trees with severe defoliation (defined as trees with crown densities less than 25% that of healthy trees). The causes of this decline, while so apparent to the media, were in reality not so straightforward. There are certainly direct effects of the acidity leading to calcium and magnesium loss and aluminium toxicity. In addition, the abundant sulphur and nitrogen causes imbalance (stoichiometric problems) that results in chlorosis and potentially death. The problem may also be exacerbated by high nitrogen input (discussed below) which increases growth, puts greater demands on magnesium and produces softer growth more at risk from insects, pathogens, frost and drought. High ozone levels are also known to directly damage leaf structure. Finally, the hot dry summers and cold winters of the 1970s and 1980s may also have taken their toll. The frustrating part is that direct cause and effect are difficult to pinpoint on already acidic soils.

Fortunately the effects of acid rain, in terrestrial ecosystems at least, have been less serious than originally feared, and by the 1990s recovery seemed evident in many forests as sulphur emissions declined. Bear in mind that at the height of concern, severe decline covered 8000 km2 of Europe which amounts to less than 0.5% of the forest area. Acid rain sceptics also point out that some forests were showing improved growth during this period. Between 1971 and 1990 European forests increased their growing stock (total volume of timber) and growth rates (wood produced per year) by 30% (Kauppi et al., 1992). Other forests are not faring as well; forest biomass increase at HBEF has declined to a small rate since 1987 (Likens et al., 1996). Calcium is undoubtedly being lost with potentially dire consequences (Fig. 11.6) but there is evidence that young hardwood stands and conifer stands are able in some way to access calcium from pools not traditionally considered available (Hamburg et al., 2003; Jandl et al., 2004), so the potential for acid rain to cause depletion problems may be less than feared. Levels of soil phosphorus are often a limiting factor in natural ecosystems, as discussed in Section 8.4.3. Unlike nitrogen, this element cannot be supplied by fixation from the atmosphere, and thus limited supply is a quite fundamental feature of many woodlands, particularly as soil phosphate in ancient woodland soils is normally less than that in secondary woodland.

Despite the fact that acid rain has ceased to be front page news, it is still a long-term concern in as much that it is one more pressure on forests. Moreover, much of the emphasis has been on reduced productivity and mortality of the trees. Acid rain and other pollution such as nitrogen enrichment (see below) at sublethal levels may have more far-reaching effects on species composition leading to large changes in native vegetation.

Acid Rain Pollution Facts

Figure 11.6 Pathways by which depletion of calcium (Ca) could lead to declines of forest ecosystem health. A reduction in Ca in plants (through low uptake or excessive loss) could lead to reduced ability to detect or react to environmental stress. Low calcium levels in animals may impede reproduction. The net result could make forest ecosystems prone to other stresses, threatening long-term health, function and stability of the ecosystem. Concern about soil phosphate levels (see Section 8.4.3) must also be remembered. (Modified from Schaberg et al., 2001.Ecosystem Health 7, Blackwell Publishing.)

Figure 11.6 Pathways by which depletion of calcium (Ca) could lead to declines of forest ecosystem health. A reduction in Ca in plants (through low uptake or excessive loss) could lead to reduced ability to detect or react to environmental stress. Low calcium levels in animals may impede reproduction. The net result could make forest ecosystems prone to other stresses, threatening long-term health, function and stability of the ecosystem. Concern about soil phosphate levels (see Section 8.4.3) must also be remembered. (Modified from Schaberg et al., 2001.Ecosystem Health 7, Blackwell Publishing.)

11.4.3 Nitrogen pollution

As discussed in Chapter 8, nitrogen is in short supply in many forests -particularly in temperate areas - due to it being locked up in various unavailable forms in the soil and due to ready leaching of any temporary excess into groundwater and streams. It might then be expected that a little extra nitrogen in the form of pollution would be a good thing, and in small quantities this is true. The current problems, particularly in temperate areas, of long-term nitrogen enrichment stem from too much of a good thing (see Nosengo, 2003 for a short, readable review). Although the atmosphere is 78% nitrogen this is in a form unusable by most organisms (see Section 8.4.1); nitrogen pollution comes from excess quantities of ammonia (NH3) and nitrogen oxides (NOx). (In this context, NOx refers primarily to nitric oxide (NO), nitrogen dioxide (NO2) and nitrous oxide (N2O).) The largest source of ammonia is from agriculture; from crop fertilizers (especially those based on urea) and emissions from livestock and their manure (from the breakdown of urea). In the UK, 80% of the total annual atmospheric ammonia production of 320 000 tonnes comes from agriculture: 47% from cattle, 14% from poultry, 9% from pigs and 11% from fertilizers. Non-agricultural sources of nitrogen include sewage sludge spread on land, landfill sites and vehicles with catalytic converters. Nitrogen oxides are primarily from industrial and vehicular combustion of fossil fuels. Anthropogenic production of nitrogen has increased from around 15 million tonnes (Mt) year -1 in 1860 to around 165 Mt year-1 in 2000. Background deposition of nitrogen is rarely more than 1 kg ha-1 year-1 but since the 1980s, nitrogen enrichment has increased this by 10-40 times or even higher. As with acid rain, this nitrogen enrichment is mainly a northern hemisphere problem, especially in Europe, North America and Japan, a reflection of population density and intensity of land use.

Part of the problem with nitrogen is that the same atom of nitrogen can have many effects in the atmosphere, terrestrial, freshwater and marine ecosystems, and on human health, one after the other. Nitrogen oxides, for example, while in the atmosphere can reduce atmospheric visibility, are involved in harmful levels of low-level ozone production and contribute to acid rain. Once on the ground they cause problems for forest vegetation as discussed below, and then are flushed into streams and the oceans causing eutrophication. Galloway et al. (2003) refer to this extending list as the nitrogen cascade.

Box 11.1 (page 458) shows that forests can become nitrogen saturated, i.e. have an availability of ammonium and nitrate in excess of the total combined demand of plants and microbes (Aber et al., 1998). Figure 11.7 gives four hypothetical stages that a temperate forest goes through as nitrogen enrichment increases. Stage 0 represents a highly nitrogen-limited forest unaffected by external nitrogen. In stage 1 there is increased nitrogen deposition which relieves some of the nitrogen limitation and nitrogen mineralization is increased; the plants contain more nitrogen (leaf N) and growth as measured by net primary productivity (NPP) goes up: the extra nitrogen is beneficial. By

-Leaf N Ca:Al and Mg:N ratios

N mineralization ---Nitrification

-Leaf N Ca:Al and Mg:N ratios

N mineralization ---Nitrification

Stage

Figure 11.7 Hypothetical responses of a temperate forest to long-term chronic nitrogen additions at various stages through time. (Redrawn from Aber et al., 1998. Bioscience 48.)

Stage

Figure 11.7 Hypothetical responses of a temperate forest to long-term chronic nitrogen additions at various stages through time. (Redrawn from Aber et al., 1998. Bioscience 48.)

stage 2, nitrification (the conversion of ammonium to nitrate) starts and since nitrate is readily leached, some nitrogen starts being washed out of the soil. Growth may not be as high as expected since much of the extra nitrogen is either leached away or locked up in the soil. Beyond this, in stage 3, the amount of nitrogen is so high that it is no longer limiting to growth but growth (NPP) will decline and may lead to death. A variety of reasons for the decline in growth can be put forward, mostly covered under 'forest decline' above, but an important issue is the increasing imbalance of nutrients and the pressure put on dwindling magnesium and calcium supplies due to acidic conditions (nitrogen compounds can bind with these two cations forming compounds that can be readily leached). In this final stage the leaching of nitrate is also high, increasing water pollution - part of the nitrogen cascade.

The key point in this hypothesis is that growth is stimulated at low enrichment but decreases once a critical threshold is passed. The pine plots at Harvard Forest discussed in Box 11.1 are clearly entering stage 3 while the hardwood plots are nearer stage 1. This shows that different forests move between stages at different rates, have different starting points (degrees of nitrogen-limitation), and different critical thresholds. Certainly not all forests are in decline from nitrogen enrichment. In Sweden nitrogen deposition is fairly modest even at its highest (27 kg ha-1 y-1). Binkley and Hogberg (1997) observed that in forests around Sweden the amount of nitrogen being leached is less than is arriving, suggesting that the forest is not nitrogen saturated. Adding other nutrients gave no extra growth so nitrogen still seems to be limiting. In fact Swedish forests increased in growth by 30% (measured as an increase in basal area) between 1953 and 1992. The sensitivity of forests to nitrogen enrichment depends to some degree on the soil nutrient status and the buffering capacity of soils to extra nitrogen (the ability to absorb and lock-up nitrogen), sensitivity to increased acidity of soil (as described above, nitrogen compounds contribute to acid rain) and the rate and duration of nitrogen deposition.

As with acid rain, stand decline and death are not the only concerns. At fairly low enrichment levels species diversity may increase as more nitrophilic species (nitrogen loving) and acid-resistant species invade, but diversity will decline in the long-term as native species are progressively lost (see Bobbink et al., 1998 for more details). They are lost not just because of unsuitable nutrient conditions but due to changes in competitive relations. Secondary stresses from pathogens, frost and drought may also contribute to species loss and decline of vigour.

Fortunately, the effects of nitrogen-enrichment look to be reversible. The NITREX project in Europe (see the end of Box 11.1) have investigated the effects of removing nitrogen by using transparent roofs above plots watered with a mixture approximating pre-nitrogen-enrichment rain. Boxman et al. (1998) used these in a Scots pine stand in the Netherlands. Within their plots, the reduction in nitrogen led almost immediately to reduced leaching of nitrate and nitrogen levels in the soil remained high. Nevertheless within 3-4 years the nitrogen content of the pine needles had declined and magnesium and potassium levels had increased, helping to correct the previous nutrient imbalance. This was matched by an increase in diameter growth of the dominant trees. There was also evidence that the understorey vegetation was recovering, with a decline in nitrophilous species such as brambles Rubus spp. and the broad-buckler fern Dryopteris dilatata and an increase in the fruiting bodies of mycorrhizal fungi. Boxman and colleagues suggest that the effects of reducing nitrogen follow a reversed nitrogen cascade, with a reaction first of all in the soil water, then the soil and then the vegetation.

Nitrogen pollution is also having an impact in indirect ways. Volatile organic compounds (VOCs) produced by such things as solvent-rich paints and varnishes (anthropogenic VOCs), have declined due to legislation but in eastern USA, particularly the south (and undoubtedly elsewhere in the world), VOCs produced by forest trees (biogenic VOCs) have increased (Purves et al., 2004). During heat-waves (when more VOCs are emitted, particularly iso-prenes and monoterpenes) biogenic VOCs may greatly outweigh anthropogenic VOCs. Woody plants are the main source and pioneer species such as aspen, poplars and sweetgum Liquidambar styraciflua emit high levels of

VOCs while late successional species such as beech and maples tend to emit none (no VOCs have been detected from any native USA maple). Moreover, common plantation species such as poplars, pines and eucalypts are high emitters. The increase in biotic VOCs in the USA is primarily due to changes in land use, both the doubling in forest cover in north-eastern USA over the last 100 years due to farm abandonment (involving pioneer species), and an increase in plantations. Where does nitrogen come into this? Volatile organic compounds interact with chemicals such as carbon monoxide and methane, potent greenhouse gases, with consequent impacts on climate change. Photochemical oxidation of VOCs in the presence of nitrogen oxides also produces ground-surface ozone (O3), harmful to human health and agriculture. Damaging ozone levels (defined as > 60 ppb - parts per billion) occur over 24% of the world's forest area and are predicted to affect 50% of forest area by 2100 (Fowler et al, 1999).

Nitrous oxide (N2O) is a potent greenhouse gas that has been increasing in the atmosphere over recent decades with about one third of the increase directly due to human causes. A large source of this increase is from tropical soils due to forest clearing, agricultural fertilization and forest regrowth dominated by legumes. As such, any disturbance is likely to increase nitrous oxide production. Hurricane Georges crossing Puerto Rico in September 1998 caused extensive defoliation, and emissions of nitrous oxide were five times above normal over the next 7 months and still twice normal after 2 years (Erickson and Ayala, 2004). Assuming hurricanes come through once every 50 years on average, they calculated that each hurricane would contribute only 18% of the production of nitrous oxides over a 50-year period but this would obviously be more significant if hurricane frequency increased.

11.4.4 Heavy metal pollution

Acid rain and nitrogen enrichment are frequently accompanied by heavy metal pollution. This can come from the mobilization of heavy metals already in soils due to increasing acidity, possibly supplemented by pollution associated with the industrial processes causing acid rain. Bedrock can contain a suite of up to 38 heavy metals (defined as those with a density greater than 5 g cm-3) which are potentially toxic to life, usually by damaging proteins and enzymes. The commonest found in polluted soils are: cadmium, arsenic, chromium, mercury (very toxic); lead, nickel (moderately toxic); boron, copper and zinc (least toxic). Some of these are, of course, essential micronutrients in low concentrations.

Heavy metals can cause direct toxicity to roots and they can disrupt nutrient uptake causing deficiencies. A large number of trees are sensitive to metal contamination; in Europe these include silver birch Betula pendula, ash Fraxinus excelsior, rowan Sorbus aucuparia, small-leaved lime Tilia cordata and apple Malus domestica (Sawidis et al., 1995). Others are more tolerant and take up some metals without apparent ill-effect. These are useful as biomonitors of metal contamination. European examples include white willow Salix alba, silver lime Tilia tomentosa, elder Sambucus nigra, pedunculate oak Quercus robur, beech Fagus sylvatica and Italian poplar Populus nigra ssp. italica. Trees can also act as biomonitors by intercepting and holding particu-late pollution on the outside of the leaf. In Greece, Sawidis et al. (1995) studied a selection of tree species as biomonitors of zinc and copper. The investigators discovered that the strongest metal accumulators had rougher leaf surfaces which more effectively trapped and held the particles.

It is well known that some herbaceous plants, including grasses have metal-tolerant ecotypes, varieties within the species that can tolerate high metal contamination, although the tolerance is almost always metal-specific. These appear to be less common in trees, although there are examples of zinc-tolerant birch and clones of the North American aspen Populus tremuloides that tolerate metal-rich air pollution. However, care is needed since Turner and Dickinson (1993) found that sycamore Acer pseudoplatanus growing on contaminated sites in the UK could grow abundant roots in uncontaminated pockets of soil and so just look tolerant. Moreover, seedlings on completely contaminated sites could grow for at least 3 years even though they were doomed to die. In some species, a careful pre-exposure to non-toxic levels of heavy metals can lead to more tolerance of what are normally toxic levels. Punshon and Dickinson (1997) exposed various clones of willow to subtoxic concentrations of single metals (0.15mg of copper l-1, 0.15mg cadmium l-1 or 2.5 mg zinc l-1) with concentrations gradually raised 10-fold over 128 days, and found reduced phytotoxicity and increased resistance, most notably to cadmium. But increased tolerance is not found in all species. Wisniewski and Dickinson (2003) found pre-exposure of pedunculate oak Quercus robur to copper did not produce any acclimation. They concluded that survival of seedlings on copper-rich soil was due to soil or rhizosphere processes alleviating metal toxicity. Mycorrhizas also play a role: various pines such as jack pine Pinus banksiana and Scots pine Pinus sylvestris inoculated with fungi of the Suillus genus can tolerate increasing concentrations of lead, nickel and zinc until the metals damage the fungus itself. The fungus helps by sequestering the metals in the fungal mycelium so that tree roots experience lower concentrations of the metals.

Trees that will grow on heavy metal-enriched soil can be used for phyto-remediation, the use of green plants to clean-up contaminated soils or sediments (phyto - Greek for plant, remedium - Latin word meaning to correct or remove an evil). Suresh and Ravishankar (2004) highlight that this is brought about in two possible ways. The first is that the tree can facilitate microbial degradation or fixing of the metals in the rhizosphere (or even further out into the soil) - see Section 5.5.1. Secondly, trees can extract and store metals in their woody skeleton and leaves. This works especially well for metals like nickel, zinc, copper, lead, chromium and cadmium. Poplars have proved useful in that they can accumulate relatively high levels of metals, especially cadmium, zinc and aluminium. Laureysens et al. (2004) found lowest concentrations of the metals in the wood and highest concentrations usually in the leaves being shed in the autumn. Thus the wood could be used as biomass for energy production without putting too many pollutants back into circulation, and the fallen leaves could be collected to effectively remove the metals from the site.

Phytoremediation is a simple and inexpensive means of extracting contaminants from subsurface soils and water that is 2-5 times less expensive than traditional capping, sealing in the contamination with an impervious layer. The main limitations are that the contaminants below the rooting depth cannot be extracted by the plant root system and plants will not grow on the most polluted sites.

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Responses

  • Campbell
    When nitrogen causes forest decline?
    6 months ago
  • benjamin koehler
    How do mycelium cause loss of vigour and decline?
    5 months ago
  • mirabella smallburrow
    How does acid rain cause problems in both forests and in cities?
    4 months ago

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