198
A. Escarrc!, A. Carratala, A. Avila, J. Bellot, J. Pinol and M. Millan
Acidic precipitation events (pH < 4.5) occurred both at Montseny and
Prades. They accounted, respectively, for 20 and 14% of the number of samples. Alkaline rains were more frequent, with 34% (Montseny) and 48%
(Prades) of the samples having a pH> 5.6. The pH of individual events (or
weekly samples) ranged from 4 to 8 at both sites. Such co-occurrence of
acidic and alkaline rains within a site precludes the use of the conventional
method of computing mean precipitation pH from VWM H+ concentrations
because H+ is not conservative under these conditions. Instead, alkalinity is
the conservative property to average (Liljestrand 1985; Young et al. 1988).
The resultant VWM alkalinities (Table 14.1) would correspond to an airequilibrated pH of 6.4 at Montseny and 6.7 at Prades, assuming that bicarbonate accounts for most of the alkalinity. Despite the occurrence of some
acidic events, the overall picture that emerges from these data is that acidic
precipitation has not been a relevant environmental stress in either holm oak
ecosystem during the last 20 years.
14.3 Geographical Patterns
Precipitation chemistry at our two holm oak sites in NE Spain can be compared with that at seven other localities spread along the Mediterranean seaboard of Spain (Table 14.2). These localities span a gradient of increasing regional aridity from the Catalan Pyrenees in NE Spain to Filabres (Almeria, SE
Spain), though the respective annual rainfalls reported in Table 14.2 partially
mask this regional trend due to local orographic effects. Precipitation at all
sites has positive VWM alkalinity and high Ca2+ concentrations (Table 14.2).
Ion concentrations are generally lower in the wettest Pyrenees site and in our
two holm oak forest sites (Table 14.1) than at the other sites, which are regionally drier and are surrounded by less vegetated or more agricultural
landscapes.
The alkaline character of precipitation all along eastern Spain results from
the incorporation of carbonate-rich aerosols into the rain, of local or remote
origin. Local sources are minor in forested areas, such as our experimental
sites at Prades and Montseny, but they increase with increasing aridity.
Nonetheless, a major remote source in the circum-Mediterranean area is the
long-range transport of dust particles from North Africa (Loye-Pilot et al.
1986; Glavas 1988; Caboi et al. 1992; Avila et al. 1996, 1997; Carratala et al.
1996). This northward transport produces episodes of dust-laden rains
(known as red rains), which have high alkalinity and high Ca2+ concentrations (Loye-Pilot et al. 1986; Roda et al. 1993). The important neutralizing
role of African red rains at our sites is revealed by the positive relationship
between the percentage of annual precipitation occurring as red rains and
the annual mean pH (Fig. 14.1). Note that years without important contributions of red rains present acidic mean pHs.
A. Escarrc!, A. Carratala, A. Avila, J. Bellot, J. Pinol and M. Millan
Acidic precipitation events (pH < 4.5) occurred both at Montseny and
Prades. They accounted, respectively, for 20 and 14% of the number of samples. Alkaline rains were more frequent, with 34% (Montseny) and 48%
(Prades) of the samples having a pH> 5.6. The pH of individual events (or
weekly samples) ranged from 4 to 8 at both sites. Such co-occurrence of
acidic and alkaline rains within a site precludes the use of the conventional
method of computing mean precipitation pH from VWM H+ concentrations
because H+ is not conservative under these conditions. Instead, alkalinity is
the conservative property to average (Liljestrand 1985; Young et al. 1988).
The resultant VWM alkalinities (Table 14.1) would correspond to an airequilibrated pH of 6.4 at Montseny and 6.7 at Prades, assuming that bicarbonate accounts for most of the alkalinity. Despite the occurrence of some
acidic events, the overall picture that emerges from these data is that acidic
precipitation has not been a relevant environmental stress in either holm oak
ecosystem during the last 20 years.
14.3 Geographical Patterns
Precipitation chemistry at our two holm oak sites in NE Spain can be compared with that at seven other localities spread along the Mediterranean seaboard of Spain (Table 14.2). These localities span a gradient of increasing regional aridity from the Catalan Pyrenees in NE Spain to Filabres (Almeria, SE
Spain), though the respective annual rainfalls reported in Table 14.2 partially
mask this regional trend due to local orographic effects. Precipitation at all
sites has positive VWM alkalinity and high Ca2+ concentrations (Table 14.2).
Ion concentrations are generally lower in the wettest Pyrenees site and in our
two holm oak forest sites (Table 14.1) than at the other sites, which are regionally drier and are surrounded by less vegetated or more agricultural
landscapes.
The alkaline character of precipitation all along eastern Spain results from
the incorporation of carbonate-rich aerosols into the rain, of local or remote
origin. Local sources are minor in forested areas, such as our experimental
sites at Prades and Montseny, but they increase with increasing aridity.
Nonetheless, a major remote source in the circum-Mediterranean area is the
long-range transport of dust particles from North Africa (Loye-Pilot et al.
1986; Glavas 1988; Caboi et al. 1992; Avila et al. 1996, 1997; Carratala et al.
1996). This northward transport produces episodes of dust-laden rains
(known as red rains), which have high alkalinity and high Ca2+ concentrations (Loye-Pilot et al. 1986; Roda et al. 1993). The important neutralizing
role of African red rains at our sites is revealed by the positive relationship
between the percentage of annual precipitation occurring as red rains and
the annual mean pH (Fig. 14.1). Note that years without important contributions of red rains present acidic mean pHs.