4.1 Effects of pollutants
The largest air pollution problems in industrial barrens are episodic, with high ambient concentrations lasting a few hours to two days (Sivertsen et al. 1994). Near the smelters in the Kola Peninsula these episodes in 1980s occurred during 3-5% of days in winter and 1-2% in summer (Baklanov and Sivertsen 1994). During episodes, hourly concentrations of SO2 reached 1200 ig m-3 near Monchegorsk (Baklanov and Rodyushkina 1993), 2500 ig m-3 near Nikel (Sivertsen et al. 1994), and 14,800 ig m-3 near Norilsk (Savchenko 1998). During the summer of 1994, monthly average concentrations of SO2 in industrial barrens near Monchegorsk were 150-270 ig m-3 (Zvereva and Kozlov 2005, and unpublished). These values should be regarded intolerable for local forests, because the proposed SO2 critical levels estimated by different methods range 5-15 ig m-3 as a growing season mean (Manninen and Huttunen 1997).
Although acute damage by sulphur dioxide, the main phytotoxic component of smelter fumes, definitely contributed to vegetation decline (e.g. Haywood 1910; Euler 1939; Makhnev et al. 1990), the importance of this damage for the development of industrial barrens remains unclear. Extreme levels of sulphur dioxide, especially near the roastbeds, killed adjacent forests during relatively short time (Hedgcock 1912; Dean and Swain 1944; Gordon and Gorham 1963; Kryuch-kov 1993; Hutchinson and Symington 1997). However, absence of industrial barrens around power plants and aluminium smelters hints that development of this kind of landscape is impossible without severe soil contamination by heavy metals (see Sect. 5.1). In particular, near Anaconda smelter, percent bare ground positively correlated with both concentrations of hazardous substances in soil and soil phytotoxicity revealed by laboratory experiments, thus suggesting the leading role of soil contamination in loss of vegetation in the field (Galbraith et al. 1995).
Smelting industry was usually preceded by or accompanied with other human-induced disturbances, which share the responsibility for the development of industrial barrens. In many historical smelter sites (e.g. Kellogg, Sudbury, Copper Basin, Queenstown, Ashio) deforestation was primarily due to harvesting for mine timber and fuel (Usui and Suzuki 1973; Hansen and Mitchell 1978; Winterhalder 2000). Shortage of fuel for smelters in the Copper Basin was noticed as early as in 1861; by 1878, about 130 km2 had been stripped of vegetation (Anonymous 2005b). Some 3,262,000 m3 of wood were consumed for roasting in Sudbury between 1890 and 1930 (Allum and Dreisinger 1987). Similarly, over 3 million tonnes of timber were cut down around Queenstown between 1896 and 1923 (Anonymous 2005a).
When commercial forests started to die due to pollution impact, the very first reaction of the foresters was to cut down the damaged stands in order to prevent losses of timber. This practice was widely applied in Central and Northern Europe (e.g. Gilbert 1975) until at least the mid-1970s, when it became obvious that the cost of harvested timber is minor in relation to the costs of rehabilitation measures required after clear-cutting. In spite of this knowledge, some local regulations (existing, for example, in Russia) still require immediate felling of pollution-damaged forest in order to make use of the timber (Kozlov 2004). Alternatively, dead trees can be removed to make the landscape more 'attractive' visually; in spite of advice of the local scientists, this selective logging was conducted in spring of 2006 near the Monchegorsk smelter (pers. obs.). However, the polluted forest never become completely dead, and even the dead trees maintain some climatic and biotic stability in the contaminated habitats, in particular by ameliorating microclimate (Wolk 1977) and preventing soil erosion. The old clearcuts under severe pollution impact near both Monchegorsk and Nikel have been rapidly transformed to industrial barrens, while some vegetation in the adjacent uncut areas is still alive (Kozlov 2004).
Industries build in 1920-1940s did not require so much of timber as a fuel. Still forests were cut for building purposes, e.g. around Monchegorsk. Initial deforestation around Norilsk, Siberia, was caused by quite peculiar reasons: since prisoners were used extensively for the construction of the Norilsk industry, a buffer of 3-4 km in width was cut around each of prison camps (Kharuk 2000). Importantly, logging causes soil disturbance that may facilitate erosion.
Forests with unusually open canopies, like one observed near Palmerton smelter in 1970s (Jordan 1975), or forest with stunted trees and dead or nearly dead ground layer vegetation, recently existing e.g. near Krompachy, Harjavalta, Ykspihlaja, Revda and Karabash smelters (pers. obs.), seem to represent a transitional stage between forests and industrial barrens. For example, field layer vegetation cover near Harjavalta is ca. 5%, while cover of Scots pine remains as high as 44% (Salemaa et al. 2001). This kind of forest may gradually turn to industrial barrens; however, it is likely that most, if not all, industrial barrens evolved following a fire that destroyed remnants of forest or shrubby vegetation. In semi-barren and barren sites near both Monchegorsk and Nikel we have observed several highly localised fires that consumed woody debris and destroyed most of remaining vegetation, leading to the extension of the barren area. We suppose that dying forests around the smelters listed above will immediately turn into industrial barrens following a fire, because a natural post-fire regeneration is hampered by soil toxicity and altered microclimate (Jordan 1975; Hansen and Mitchell 1978; Vajda and Venalainen 2005). The detrimental role of occasional forest fires was considered the primary reason of forest deterioration around Bell Bay aluminium smelter in Australia, mostly due to disruption of nutrient cycles and alteration of soil moisture regime (Mitchell 1982).
Both logging and forest damage by fumes from smelting and roasting increased the amount of woody debris in the impacted areas, making them especially vulnerable to occasional fires. Unusually large amount of woody debris is clearly seen on photographs taken in the vicinity of several smelters (Figs. 1-3). In Sudbury, not only sparks from wood-burning locomotives of the Canadian Pacific Railway started the fires, but also prospectors often burned the remaining vegetation and duff to reveal the bedrock below (Winterhalder et al. 2001). In Palmerton, hillside vegetation behind zinc plant was originally destroyed by fire (Pommerening 1977). In the Copper Basin, in times of dry weather, almost daily fires swept the earth bare of vegetation (Teale 1951). The degradation of the Copper Basin landscape may have been further exacer bated by lowland farmers, who trucked cattle into the Basin for free grazing, and regularly burned the land to encourage growth of a sparse pasture (Clay 1983).
All catastrophic events that destroy vegetation, like fires or avalanches, are always followed by natural recovery that, sooner or later, results in restoration of about the same vegetation community. The extant plants growing in industrial barrens may sustain extreme pollution loads and produce viable seeds, sometimes even in larger amounts than in unpolluted sites (Zvereva and Kozlov 2001, 2005; Kozlov and Zvereva 2004). These seeds preserved in the soil seedbank remain viable for decades (Komulainen et al. 1994); still natural regeneration is absent or nearly absent in industrial barrens (Jordan 1975; Kozlov and Haukioja 1999; Rigina and Kozlov 2000). This is most likely due to high concentrations of heavy metals in uppermost soil layers (Table 2) that stunted radicle growth of many plant species (Stavrova 1990; Zeid 2001). Although seeds of several native plant species were capable of germinating in soils of industrial barrens at both Sudbury and Monchegorsk, root growth was so inhibited that seedlings quickly dried off and died completely (Winterhalder et al. 2001; Kozlov 2005). Persistence of some plants in industrial barrens may be transient, being explained not only by higher resistance of the survivors, but also by past phenotypic acclimatisation of mature plants to gradual increase in pollution (Kozlov 2005), and the extent of industrial barrens increases as plants age and die (Zverev and Kozlov, unpublished).
Additional problems for vegetation recovery may be imposed by slow decomposition of litter. This was in particular noticed around the Palmerton smelter, where a considerable portion of the area is covered with a layer of undecom-posed tree bark (Sopper 1989) or by thick (6-16 cm) litter (Jordan 1975). Similarly, leaf litter is the major problem of semi-barren communities in Sudbury, because it hinders the establishment of understorey species by seeds (Winterhalder 2000).
Table 2 Chemistry of humus layer or uppermost soil horizon in industrial barrens
Location Extreme Maximum concentrations of contaminants, ng g 1
Minimum level of nutrients, mg g_1
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