4. Atmospheric and Geologic Constraints on the
Biogeochemistry of North and South American
Temperate Rainforests
Lars o. Hedin and Eugene D. Hetherington
Terrestrial forest ecosystems develop over time within the constraints of
element inputs from atmospheric and geologic sources (e.g., Bormann &
Likens, 1979; Gorham, Vitousek, & Reiners, 1979; Likens et aI. , 1977).
Atmospheric and geologic inputs can be defined as "external sources"! of
elements because they provide long-term supplies of new nutrients to
internal ecosystem pools, such as vegetation, soil organic matter, and soil
cation or anion exchange complexes. Such external nutrient supplies can
generate constraints on the internal pool sizes and processes (e.g., rates
of biomass accretion or hydrologic nutrient loss) of terrestrial ecosystems
and may affect rates of recovery after disturbance events (resilience) .
Here we evaluate the role of external element supplies for temperate
rainforests of North and South America. Our focus is on atmospheric and
1 The question of whether geologic weathering should be considered an external
or internal source of nutrients is a matter of current debate. However, since both
views are equivalent from a practical point of view, the question becomes a
matter of operational definition. Because our aim here is to evaluate how geological sources of elements, together with atmospheric inputs, act to constrain the
development of forest ecosystems, we have, in accordance with Gorham et al.
(1979), defined weathering as an external input. Other analyses of ecosystems,
however, commonly include weathering as an internal process (e.g., Hedin &
Campos, 1991; Likens et al., 1977).
57
Biogeochemistry of North and South American
Temperate Rainforests
Lars o. Hedin and Eugene D. Hetherington
Terrestrial forest ecosystems develop over time within the constraints of
element inputs from atmospheric and geologic sources (e.g., Bormann &
Likens, 1979; Gorham, Vitousek, & Reiners, 1979; Likens et aI. , 1977).
Atmospheric and geologic inputs can be defined as "external sources"! of
elements because they provide long-term supplies of new nutrients to
internal ecosystem pools, such as vegetation, soil organic matter, and soil
cation or anion exchange complexes. Such external nutrient supplies can
generate constraints on the internal pool sizes and processes (e.g., rates
of biomass accretion or hydrologic nutrient loss) of terrestrial ecosystems
and may affect rates of recovery after disturbance events (resilience) .
Here we evaluate the role of external element supplies for temperate
rainforests of North and South America. Our focus is on atmospheric and
1 The question of whether geologic weathering should be considered an external
or internal source of nutrients is a matter of current debate. However, since both
views are equivalent from a practical point of view, the question becomes a
matter of operational definition. Because our aim here is to evaluate how geological sources of elements, together with atmospheric inputs, act to constrain the
development of forest ecosystems, we have, in accordance with Gorham et al.
(1979), defined weathering as an external input. Other analyses of ecosystems,
however, commonly include weathering as an internal process (e.g., Hedin &
Campos, 1991; Likens et al., 1977).
57