58
L.O. Hedin and E.D. Hetherington
geologic sources of elements, which are of direct importance in defining
the biogeochemical environment within which plant communities have
developed and evolved.
Atmospheric inputs of elements occur as wet, dry and cloud deposition
or as active biological fixation (e .g., of N 2 or CO 2 ) , and can supply
important amounts of nitrogen (N), phosphorus (P), sulfur (S), and base
cations (sum of Ca
2+,
Mg2+, Na2+, and K+) to terrestrial ecosystems
over time (Gorham, 1961; Hedin & Campos, 1991; Hedin et aI., 1994).
Geologic inputs of elements derive from chemical weathering of primary
and secondary minerals, and variations in weathering rates can exert
considerable control on soil fertility and biotic processes within terrestrial
ecosystems (e.g., Kronberg & Melfi, 1987). Internal sources of elements
in forest ecosystems derive from inorganic nutrients that are produced
during re-mineralization of soil organic matter and from elements that are
stored in soil cation or anion exchange pools.
The relative contribution of internal versus external supplies of nutrients
is thought to change predictably during ecosystem succession. In early
primary succession, plant communities are thought to depend mainly
on external nutrient inputs from atmospheric and weathering sources
(Bormann & Likens, 1979; Gorham et aI., 1979; Vitousek & Reiners,
1975). External nutrient supplies are also thought to be significant after
disturbance events that are severe enough to cause significant loss of
internal nutrient pools in soil organic matter or on exchange surfaces.
Such disturbances may include landslides, deforestation, and especially
hot fires. External sources consistently dominate for elements, such as
CI-, which do not participate significantly in internal ecosystem cycles. In
contrast internal supplies of nutrients generally increase as soil organic
matter accumulates over successional time (Bormann & Likens, 1979;
Gorham et aI., 1979; Riley and Vitousek, 1995; Vitousek & Reiners,
1975). Thus, old-growth forest ecosystems are generally thought to be
characterized by high internal nutrient supply rates, with external inputs
playing a less important role in the supply of nutrients to vegetation and
heterotrophs (Robertson & Vitousek, 1981; Tilman, 1986; Vitousek,
Matson, & Van Cleve, 1989).
Despite the increasing role of internal nutrient supplies during ecosystem
succession, it is not always appreciated that the long-term biogeochemical
state of a forest ecosystem continues to be constrained by external element
supplies from atmospheric and geologic sources. Barring any significant
changes in plant nutrient-use efficiencies, long-term net accumulation of
nutrients in internal ecosystem pools can occur only as a function of
external inputs of new nutrients to the ecosystem. Thus, external nutrient
inputs constrain the long-term accumulation of nutrients and biomass
within forest ecosystems. External inputs ultimately also constrain patterns
of element loss from watersheds. For example, increased inputs of N
from air pollution can, over time, result in increased soil N pools, in-
L.O. Hedin and E.D. Hetherington
geologic sources of elements, which are of direct importance in defining
the biogeochemical environment within which plant communities have
developed and evolved.
Atmospheric inputs of elements occur as wet, dry and cloud deposition
or as active biological fixation (e .g., of N 2 or CO 2 ) , and can supply
important amounts of nitrogen (N), phosphorus (P), sulfur (S), and base
cations (sum of Ca
2+,
Mg2+, Na2+, and K+) to terrestrial ecosystems
over time (Gorham, 1961; Hedin & Campos, 1991; Hedin et aI., 1994).
Geologic inputs of elements derive from chemical weathering of primary
and secondary minerals, and variations in weathering rates can exert
considerable control on soil fertility and biotic processes within terrestrial
ecosystems (e.g., Kronberg & Melfi, 1987). Internal sources of elements
in forest ecosystems derive from inorganic nutrients that are produced
during re-mineralization of soil organic matter and from elements that are
stored in soil cation or anion exchange pools.
The relative contribution of internal versus external supplies of nutrients
is thought to change predictably during ecosystem succession. In early
primary succession, plant communities are thought to depend mainly
on external nutrient inputs from atmospheric and weathering sources
(Bormann & Likens, 1979; Gorham et aI., 1979; Vitousek & Reiners,
1975). External nutrient supplies are also thought to be significant after
disturbance events that are severe enough to cause significant loss of
internal nutrient pools in soil organic matter or on exchange surfaces.
Such disturbances may include landslides, deforestation, and especially
hot fires. External sources consistently dominate for elements, such as
CI-, which do not participate significantly in internal ecosystem cycles. In
contrast internal supplies of nutrients generally increase as soil organic
matter accumulates over successional time (Bormann & Likens, 1979;
Gorham et aI., 1979; Riley and Vitousek, 1995; Vitousek & Reiners,
1975). Thus, old-growth forest ecosystems are generally thought to be
characterized by high internal nutrient supply rates, with external inputs
playing a less important role in the supply of nutrients to vegetation and
heterotrophs (Robertson & Vitousek, 1981; Tilman, 1986; Vitousek,
Matson, & Van Cleve, 1989).
Despite the increasing role of internal nutrient supplies during ecosystem
succession, it is not always appreciated that the long-term biogeochemical
state of a forest ecosystem continues to be constrained by external element
supplies from atmospheric and geologic sources. Barring any significant
changes in plant nutrient-use efficiencies, long-term net accumulation of
nutrients in internal ecosystem pools can occur only as a function of
external inputs of new nutrients to the ecosystem. Thus, external nutrient
inputs constrain the long-term accumulation of nutrients and biomass
within forest ecosystems. External inputs ultimately also constrain patterns
of element loss from watersheds. For example, increased inputs of N
from air pollution can, over time, result in increased soil N pools, in-