Application to Environmental Risks from Substances2

4.1 Global Biogeochemical Cycles Are Influenced by Human Activity

Carbon, nitrogen, and sulfur are essential to the life of animals, plants, and microbes. Interactions between these elements link the internal biogeochemistry of terrestrial ecosystems. Naturally, the availability of these substances is limited in terrestrial ecosystems and this has led to various adaptations of the biota. Nowadays, high anthropogenic emissions of various compounds of carbon, nitrogen, and sulfur have created a new situation for terrestrial ecosystems: The surplus (regional) of these three limited elements can affect terrestrial ecosystems in multiple ways and on different time scales (see Table 8).

4.2 Risk Classification of Environmental Risks from Substances

Despite a quite good kowledge of many determining processes, uncertainty remains about the expected geographical dispersion of the potential damages, the time when they will occur, and the extent of the damages. Not only anthropogenic influences but also natural disturbances cause multiple stress to forest ecosystems and make the determination of the risk potentials and the extent of damage even more difficult. In the face of the underlying complex processes and the possibly high latency between initial events and response of the ecosystems, the risk perception is even lower than for risks of direct impact. Although persistency is rather high and reversibility of the potential damage is low, the potential for mobilization is generally low. This leads to its characterization as the risk type Cassandra.

4.3 Forest Ecosystems Are Influenced by the Changing Biogeochemical Cycles

The anthropogenic influence on global biogeochemical cycles lead to a new situation for forest ecosystems: Increasingly, multiple compounds of nitrogen, sulfur, and carbon are simultaneously available in large quantities (regionally even in surplus). In the preindustrial era, the mean global atmospheric N input was in the range of 1-5 kg N ha-1 year-1 (Kimmins, 1987; Flaig and Mohr, 1996). Therefore, input of nitrogen was the limiting factor for plant growth in most forest ecosystems until the beginning of the industrial revolution (Kimmins, 1987). During the past decades

2 This part was written by Gerald Busch, Friedrich Beese, and Gerhard Lammel.

TABLE 8 Overview of Possible Impacts and Risk Potential of Anthropogenic Changes in Global Biogeochemical Cycles ( j" , Increase; j, , Decrease, -, No Change).*

Substances

Possible Reactions and Effects

Associated Risk Potential

N-input in

| N-contents, f Mineralization,

| Nitrate leaching to groundwater, j, Frost-, drought-, or

ecosystems

j" N-tumover, j NPP, \ N

pest resistance, changes in species and vegetation,

(Eutrophication)

surplus, | Mycorrhiza, j" / J,

degradation of N-limiteded ecosystems, f loss of

Humus layer

biodiversity, f loss of ecosystem functions

N and SO;-

| Soil acidification, f Al toxicity,

| Nitrate leaching to groundwater, f acidification of

deposition

| damage of fine roots and

freshwaters, j drought resistance, f nutrient imbalances,

mycorrhiza, \ cation leaching

| forest desease and forest decline, f loss of biodiversity,

| loss of ecosystem functions

| [CO,] (lowN-availibility)

-/1 NPP, f / j. C/N ratio, N accumulation/leaching ,

Reactions are very site specific and indépendant of

t / i Mineralization, f / j, root/shoot ratio, -/ f

species, e.g., -/ f NPP of vegetation, changes in stocks

Water use efficiency. Nutrient use efficiency

and site composition (C,-, C4-plants), species-related

changes in population of herbivores

f [CO,] and f N inputs

| C and N accumulation because of f NPP,

Changes in site composition, loss of biodiversity,

j" / j, Humus accumulation

sudden emission/loss of accumulated N and C because

of external disturbances (land-use change, fire, climate change),

Climate change, f [CO,] and

Shift of vegetation and water

Highly uncertain: f climate change (positive feedback

f Nand f S inputs

budget, global: | Mineralization, f NPP,

of vegetation, e.g., f CO,-emissions), shift of

j. C-sequestration

vegetation, | invasion of alien species, desertification

Examples ot only multiple reactions and associated possible risks can be shown in this table.

Sources: Mooney etill., 1998; Walker etal., 1998; Arnone III and Hirschel, 1997; Foster et a I., 1997; Hungate et ill., 1997; Kinney et ill., 1997; Vitousek et a I., 1997; Drake el al., 1997; Fiaig and Möhr, 1996; IPCC, 1996; Körner and Bazzaz, 1996; Koch and Mooney, 1996; Walker and Steffen, 1996; Amthor, 1995, Dixon and Wisniewski, 1995; Ileywood and Watson, 1995; Woodwell and Mackenzie, 1995; Möhr and Müntz, 1994; Vitousek, 1994; Schulze et al, 1989.

Examples ot only multiple reactions and associated possible risks can be shown in this table.

Sources: Mooney etill., 1998; Walker etal., 1998; Arnone III and Hirschel, 1997; Foster et a I., 1997; Hungate et ill., 1997; Kinney et ill., 1997; Vitousek et a I., 1997; Drake el al., 1997; Fiaig and Möhr, 1996; IPCC, 1996; Körner and Bazzaz, 1996; Koch and Mooney, 1996; Walker and Steffen, 1996; Amthor, 1995, Dixon and Wisniewski, 1995; Ileywood and Watson, 1995; Woodwell and Mackenzie, 1995; Möhr and Müntz, 1994; Vitousek, 1994; Schulze et al, 1989.

forests in Europe and Northeastern America have been in transition from nitrogen-deficient to nitrogen-saturated systems due to increasing nitrogen deposition. The impact of nitrogen deposition on plants and soil is through both fertilizing and toxic effects, eu-trophication and acidification (see Aber et al., 1998; Gundersen et al, 1998; Boxman etal, 1998).

In the last decades, anthropogenic sulfur and nitrogen emissions have been discussed in the context of acid rain (see Ulrich and Sumner, 1991; van Breemen et al, 1983; Reuss and Johnson, 1986) and this point of view has influenced policy in Europe and North America (e.g., LRTAP UN-ECE Second Sulphur Protocol, Clean Air Act).

As a consequence of population growth and rapid economic development, increasing loads of nitrogen and sulfur on the terrestrial ecosystems might well not remain limited to the known "hotspots" in Europe and North America but could expand and become critical as well for tropical and subtropical regions.

4.4 Changing Patterns of Nitrogen and Sulfur Deposition

In the following section we delineate the disposition of global forest in context of the dynamics of changes in deposition patterns. For quantification we use, besides other data sets, present-day and — under a scenario — future acid and nitrogen deposition data as produced by global-scale models to show the regional distribution of the increasing bias between acidification of forest soils and nitrogen fertilization.

In a first step, nutrient-depleted soils with low buffering capacity are identified to assess regions with potential for destabiliza-tion of forest ecosystems by acid deposition. Because the "acidity neutralization capacity" of soils (ANC) cannot be accurately determined from the global data, a simple approach based on the "Soil Map of the World" (FAO, 1995) is carried out. To evaluate the buffering capacity of the topsoils, the CEC data (cation exchange capacity) and the base saturation data (Na, K, Mg, and Ca) are combined with a map of the global distribution of forests (WCMC, 1997) to obtain the measures for forest soils with low buffering capacity.

To evaluate the buffering capacity of the forest soils, the actual (1980-1990) acidic input and nitrogen turnover are applied to the identified regions. SO/ = S02 + sulfate) and NOr ( = N02 + HN03 + nitrate) deposition fields and the related acidic inputs are taken from a general-circulation model of the atmosphere, ECHAM4 (Roeckner et al, 1996). NHV ( = ammonia + ammonium) deposition fields are taken from a run of the global tracertransport model MOGUNTIA (Zimmermann, 1988), the only model so far that describes reduced nitrogen compounds. By GIS-analysis those regions were identified in which the buffering capacity is depleted in 25-100 years by corresponding acid loads. Under the assumptions of a future scenario (IS92a-IPCC, 1996) the same assessment is carried out for the years 2040-2050. In a second step, nitrogen deposition in forest ecosystems is analyzed for the same time horizons. This assessment focuses on nitrogen deposition that exceeds natural input; the threshold was set to 5 kg N ha-1 year- 1 (Bobbink et al, 1992; UN-ECE, 1996).

4.5 Saturation of Forest Soils Buffering Capacity

In relation to acid input from 1980 to 1990, the buffering capacity of 1.8 Mio km2 or 15% of the acid-sensitive forest soils tends to become saturated in the next 25-100 years. Under the assumptions of the IS92a scenario, this share more than doubles and increases to 4.0 Mio km2 or 34% between 2040 and 2050. For 1980-1990, the mean buffering capacity of these sols based on our methodology is supposed to last for 65 years more. Under changed inputs this period tends to decrease for 2040-2050 to

50 years, which is less than half the lifetime of most of the managed tree species. For 1980-1990, four regions are mainly affected by acid deposition: the Eastern part of Northern America, Europe, Scandinavia with the Northwestern Russian Federation, and Southern China. The situation for 2040-2050 changes in such a way that the "old hotspots" are still present but the area of saturation increases only moderately with the main increase taking place in the tropical and subtropical regions of South America and South and Southeast Asia (see Fig. 3; see also color insert).

FIGURE 3 Distribution of exceeded forest soils buffering capacity, (a) today (1980-1990) and (b) (2040-2050). Red areas show forest soils with an exceeded buffering capacity while the green areas show the not-affected areas of acid sensitive and nutrient deficient forest soils. See also color insert.

FIGURE 3 Distribution of exceeded forest soils buffering capacity, (a) today (1980-1990) and (b) (2040-2050). Red areas show forest soils with an exceeded buffering capacity while the green areas show the not-affected areas of acid sensitive and nutrient deficient forest soils. See also color insert.

4.6. Nitrogen Deposition in Nutrient-Deficient and Acid-Sensitive Soils

For 2040-2050, nearly 54% of the forest ecosystems on acidified soils are projected to receive a nitrogen load greater 5 kg N ha" 1 year- '. Because of better soil conditions the affected area is smaller in India, Eastern North America, and Europe. In absolute numbers, the forests in the Eastern North America will be affected most, followed by those in Southeast Asia and China (see Fig. 4).

Greatest changes in aerial distribution and increase of concentration will occur in the Asian region (see Fig. 4). Regions with acid-sensitive soils and high N-depositions are concentrated to China and Southeast Asia, Western and Central Europe, and

Eastern North America. Again, in absolute numbers Eastern North America shows the largest distribution of forest areas with an exceeded soil buffering capacity and high nitrogen deposition, followed by Southeast Asia, China, and Europe.

4.7 Conclusion

It has been shown that under the assumptions of the IPCC IS92a scenario, the contrast between unbalanced nutrient input and acidification or nutrient depletion will increase. Greatest changes are most likely to occur in subtropical and tropical regions of Asia but the well-known hotspots of Europe and Eastern North America will remain so. Both forest areas with both depletion of o <

China AmeNca2 Russia

ASEAN Canada

China

South

Russia

ASEAN Canada

America

FIGURE 4 Regional distribution of acid input into forest ecosystems on acidified soils with minimum, maximum, and mean values in selected regions or countries, (a) today (1980-1990) and (b) (2040-2050). 'Brazil, Ecuador, Bolivia, Columbia, Paraguay, Peru, and Venezuela; Myanmar, Thailand, Laos, Vietnam, Brunei, Philippines, Singapore, Indonesia, and Malaysia; dashed line: threshold of natural nitrogen input.

soil's buffering capacity and increasing nitrogen deposition will expand in several regions. The forest areas likely to meet these two risks are still a minor fraction of the global forest ecosystems.

Soils in forest ecosystems provide the transformation function for nutrient flows and material flows in general, besides other functions. Soils' nutrient reservoirs and buffering capacities get depleted in increasingly large areas. On the other hand, growth induced by increasing N availability triggers additional nutrient demand, which in most cases cannot be satisfied. We note that nutrient and acidification status of those soils that are subject to increasingly high inputs will necessarily change in foreseeable periods.

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