Acidification

Long-term exposure to acidic precipitation has devastating effects on the photosynthesis, growth and survival of bryophytes. Forest bryophytes experimentally sprayed with acid water started to show a linear decline in dry weight from pH < 4 (Fig. 9.1). Significant reductions of up to 75% in both frond height and dry weight were recorded at pH 3.5 and lower (Hutchinson & Scott 1988). The decline in growth of bryophytes exposed to experimental acid rains is highly correlated with a significant reduction in chlorophyll content, up to 35% at pH2.5 and 3.0 (Fig. 9.2) (Bakken 1993). The high H+ concentrations may induce the hydrolysis of chlorophyll a by displacing the Mg2+ ions from the chlorophyll molecule and converting it to phaeophytin. Acid rain may also reduce the available magnesium for chlorophyll synthesis by washing off dry depositions, including Mg2+, from the moss surfaces.

While definite positive trends could be observed during the past two decades regarding the emission of SO2, accompanied by a remarkable recol-onization of formerly polluted areas by sensitive species (Bates et al. 1997), the consequences of past acid rains can still be observed today in a wide range of bryophyte communities. The most dramatic vegetation changes attributed to

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pH of spray treatment

Fig. 9.1. Mean dry weight (mg) of fronds of the moss Pleurozium schreberi plotted against pH of a simulated rain treatment, delivered twice-monthly throughout the growing season over 5 yrs. ▲ mean total dry weight; A mean dry weight of the green, photosynthetically active portion of the frond; ■ total frond dry weight, control; □ living frond dry weight, control (reproduced from Hutchinson & Scott 1988 with permission of Canadian Journal of Botany).

pH of spray treatment

Fig. 9.1. Mean dry weight (mg) of fronds of the moss Pleurozium schreberi plotted against pH of a simulated rain treatment, delivered twice-monthly throughout the growing season over 5 yrs. ▲ mean total dry weight; A mean dry weight of the green, photosynthetically active portion of the frond; ■ total frond dry weight, control; □ living frond dry weight, control (reproduced from Hutchinson & Scott 1988 with permission of Canadian Journal of Botany).

Fig. 9.2. Mean chlorophyll content (and SD bars) of the moss Hylocomium splendens after two seasons of exposure to simulated acid rain (25 mm twice a month from May to October) at five pH levels and unsprayed (Un) as controls. Asterisk indicates significantly different mean content from control (reproduced from Bakken 1993 with permission of Lindbergia).

Fig. 9.2. Mean chlorophyll content (and SD bars) of the moss Hylocomium splendens after two seasons of exposure to simulated acid rain (25 mm twice a month from May to October) at five pH levels and unsprayed (Un) as controls. Asterisk indicates significantly different mean content from control (reproduced from Bakken 1993 with permission of Lindbergia).

acidification have taken place in habitats that, due to their low buffering capacity, experienced a rapid and substantial decrease in pH. In epiphytes, for example, communities of rich bark composed of Orthotrichum spp. and Cryphaea heteromalla shifted towards acidophilous communities dominated by Dicranaceae (e.g. Dicranum tauricum and D. montanum), which have drastically increased in abundance since the middle of the twentieth century (Bates et al. 1997). Similarly, in streams, acidification caused a shift in community composition (Stephenson et al. 1995, Thiebaut et al. 1998). A set of species characteristic of neutral waters and sensitive to acidity from the very first stages of their development, such as Platyhypnidium riparioides and Chiloscy-phus polyanthos (Fig. 9.3), progressively disappeared. In contrast, acidophilous species, such as Scapania undulata, are physiologically equipped to face the proton load (Section 8.3.2), hence exhibit greater tolerance to acidic environments (Fig. 9.3), and have therefore spread. Perhaps the most spectacular effects of acidification through SO2 pollution, however, have taken place in ombrotrophic mires. These ecosystems are especially susceptible to atmospheric pollution because they receive the majority of their mineral supply from the atmosphere (Hogg et al. 1995). Although acidification is part of a natural, autogenic evolutionary process in bogs (Section 2.2), succession from mineral-rich fens towards more acid stages dominated by Sphagnum has been occurring on a rapid scale in areas highly exposed to acid deposition. In the most polluted areas, acid rains have changed the course and velocity of succession towards communities, in which Sphagnum itself is absent. For example, the virtual absence of Sphagnum from the southern Pennines, an area of more than 50 000 ha, is a remarkable feature of British vegetation because it remains the only case in which experimental and observational evidence can be combined to demonstrate the ecological importance of SO2 pollution at a large scale (Lee 1998). In the Pennines, the disappearance of Sphagnum in peat stratigraphic profiles indeed correlates with the appearance of soot from industrial origin in the deposits (Tallis 1964). Fumigation experiments in closed chambers further demonstrate that Sphagnum growth is significantly reduced when SO2 concentrations reach 131 mgm~3, a concentration within the range observed during the periods of highest levels of pollution in the twentieth century (Ferguson et al. 1978). Finally, field experiments involving the spraying of dilute concentrations of bisulphite ions onto a Sphagnum bog surface resulted in the removal of the Sphagnum cover within a year (Ferguson & Lee 1980). Collectively, all these experiments demonstrate that the disappearance of Sphagnum from the Pennines can in fact be attributed to the levels of SO2 pollution that the area experienced during the first half of the twentieth century.

Fig. 9.3. Contrasting effects of pH on the development of sporelings in the leafy liverworts Chiloscyphus polyanthos (a) and Scapania undulata (b) and of the protonema of the moss Platyhypnidium riparioides (c) (redrawn from Tremp & Kohler 1993).

Fig. 9.3. Contrasting effects of pH on the development of sporelings in the leafy liverworts Chiloscyphus polyanthos (a) and Scapania undulata (b) and of the protonema of the moss Platyhypnidium riparioides (c) (redrawn from Tremp & Kohler 1993).

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