70
L.G . Hedin and E.D. Hetherington
reduction) effects on SO/- in the North American coastal forests (Figure
4.3).
It should be noted that residual element concentrations after sea-salt
correction may not exclusively be due to geologic inputs, but may reflect
atmospheric sources other than sea-salt aerosols or element losses associated with internal ecosystem processes (e.g. , net biomass depreciation ;
Bormann & Likens, 1979; Vitousek & Reiners , 1975), or element loss
from soil exchange sites. For example , precipitation chemistry in southwestern British Columbia and the northwestern United States is subject to
atmospheric pollution that causes increased levels of sulfur and nitrogen
in wet and dry deposition (Feller , 1987; Nikleva, 1986). The limited effect
of sea-salt corrections on SO/- levels in North American watersheds
(Figure 4.3) could be due in part to such air-pollution inputs (Feller,
1987). Weathering of sulfur-rich minerals could also supply SO/- to
watershed streams. While recent studies have indicated a link between
human activities and the atmospheric deposition of base cations (Hedin et
aI., 1994), this effect is likely limited to relatively polluted and arid, noncoastal areas.
For base cations, the strong correlations between residual (non-seasalt) Mg z+ , Ca?" , and K+ versus SiO z (see A and B in Figure 4.5)
indicate that silicate weathering processes contribute to variations in
supplies of base cation to the coastal rainforests considered here. Feller
and Kimmins (1979) found that while watershed budgets showed strong
net losses for most elements from a coastal rainforest in British Columbia
(UBCRC) , losses were especially strong for SiO z , Ca2+, and Mg z+. Feller
and Kimmins interpreted this to indicate a particularly strong supply of
these elements from weathering. It is, however, difficult to evaluate
Feller and Kimmins' budgets , since atmospheric inputs from cloud water
and dry deposition, while potentially significant in such forests (e.g. ,
Borman, Tarrant, McClellan, & Savage, 1989), were not considered. We
conclude that external supplies of major elements to coastal rainforests in
North America and CPE occur primarily from weathering (Ca?" , Mg z+ ,
and SO/-) or as a balance between weathering and atmospheric inputs
(Na+, 0 -, and K+). Thus, while marine air masses provide important
inputs of several elements , these forests do not appear to be linked as
exclusively to marine processes as CP watersheds (Figure 4.5).
General Model
Our results indicate that coastal temperate rainforests in North and South
America have developed over time within the biogeochemical constraints
of inputs of dilute atmospheric sea-salt aerosols coupled with low rates of
weathering . Atmospheric inputs of sea salts constitute the major external
L.G . Hedin and E.D. Hetherington
reduction) effects on SO/- in the North American coastal forests (Figure
4.3).
It should be noted that residual element concentrations after sea-salt
correction may not exclusively be due to geologic inputs, but may reflect
atmospheric sources other than sea-salt aerosols or element losses associated with internal ecosystem processes (e.g. , net biomass depreciation ;
Bormann & Likens, 1979; Vitousek & Reiners , 1975), or element loss
from soil exchange sites. For example , precipitation chemistry in southwestern British Columbia and the northwestern United States is subject to
atmospheric pollution that causes increased levels of sulfur and nitrogen
in wet and dry deposition (Feller , 1987; Nikleva, 1986). The limited effect
of sea-salt corrections on SO/- levels in North American watersheds
(Figure 4.3) could be due in part to such air-pollution inputs (Feller,
1987). Weathering of sulfur-rich minerals could also supply SO/- to
watershed streams. While recent studies have indicated a link between
human activities and the atmospheric deposition of base cations (Hedin et
aI., 1994), this effect is likely limited to relatively polluted and arid, noncoastal areas.
For base cations, the strong correlations between residual (non-seasalt) Mg z+ , Ca?" , and K+ versus SiO z (see A and B in Figure 4.5)
indicate that silicate weathering processes contribute to variations in
supplies of base cation to the coastal rainforests considered here. Feller
and Kimmins (1979) found that while watershed budgets showed strong
net losses for most elements from a coastal rainforest in British Columbia
(UBCRC) , losses were especially strong for SiO z , Ca2+, and Mg z+. Feller
and Kimmins interpreted this to indicate a particularly strong supply of
these elements from weathering. It is, however, difficult to evaluate
Feller and Kimmins' budgets , since atmospheric inputs from cloud water
and dry deposition, while potentially significant in such forests (e.g. ,
Borman, Tarrant, McClellan, & Savage, 1989), were not considered. We
conclude that external supplies of major elements to coastal rainforests in
North America and CPE occur primarily from weathering (Ca?" , Mg z+ ,
and SO/-) or as a balance between weathering and atmospheric inputs
(Na+, 0 -, and K+). Thus, while marine air masses provide important
inputs of several elements , these forests do not appear to be linked as
exclusively to marine processes as CP watersheds (Figure 4.5).
General Model
Our results indicate that coastal temperate rainforests in North and South
America have developed over time within the biogeochemical constraints
of inputs of dilute atmospheric sea-salt aerosols coupled with low rates of
weathering . Atmospheric inputs of sea salts constitute the major external