266
oceans (Duce and Tindale 1991). Analogously,
Cornell et al. (1995) proposed that atmospheric
deposition of dissolved organic N (DON) might be
a dominant source of new N in certain ocean areas.
These examples illustrate the fundamental importance of atmospheric deposition for understanding how ecosystems function in the world, and for
understanding how human activities are impacting
natural ecosystems. However, the examples also
illustrate that our view of atmosphere-ecosystem
interactions is still in a formative stage. Despite
considerable progress, much remains to be understood about the link between ecosystems and the
atmosphere.
Vectors of Delivery
Atmospheric nutrients and pollutants enter ecosystems via three main vectors of deposition, each subject to its own set of controlling factors and methods of measurement: (1) Wet deposition consists of
chemical elements that enter via precipitation,
mainly as dissolved solutes but also as particles entrained in raindrops; (2) cloud deposition consists
of elements dissolved in cloud or fog droplets that
impact onto terrestrial surfaces, such as leaves or
branches; and (3) Dry deposition consists of aerosols and/or gases that are deposited directly onto
surfaces of vegetation, soils, or water.
The relative contributions of these vectors can
vary substantially among ecosystems and with
time, due to factors that act along the path of transport from emission sources to deposition in ecosystems (e.g., Ollinger et al. 1993), and due to factors that are intrinsic to receiving ecosystems (e.g.,
leaf area, stomatal conductance, etc.). At the scale
of individual ecosystems, the contributions by the
different vectors depend on the exact chemical
form (and thus reactivity) of nutrients or pollutants
in the atmosphere and on local-scale meteorological and geographical conditions, such as topographic location and exposure (Miller et al. 1993;
Lovett et al. 1997). Wet and dry deposition are
typically the dominant deposition vectors with dry
deposition becoming progressively more important
in arid regions (Gilette et al. 1992). However,
cloud deposition can be important in cloud-rich
locations such as high-elevation forests (Lovett
Lars O. Hedin
and Kinsman 1990; Johnson et al. 1991; Friedland
et al. 1991; Schemenauer and Cereceda 1992a;
Weathers and Likens, 1997), particularly since
even small amounts of cloud water can contain
high concentrations of pollutants and nutrients
(Weathers, et al., 1988); Lin et al. 1995). A summary of factors that control the different vectors
of atmospheric deposition is beyond the scope of
this chapter; the reader is referred to a number of
excellent texts and reviews (e.g., Finlayson-Pitts
and Pitts 1986; Fowler et al. 1991; Graedel and
Crutzen 1993; Duyzer and Fowler 1994; Lovett
1994; Taylor and Johnson 1994; Winner 1994;
Moncrieff et al. 1997; Lovett et al. 1997; Sutton
et al. 1998).
Many ecosystem-level questions demand an understanding of the total deposition of a nutrient or
pollutant to an ecosystem. It is in these cases insufficient to measure only a single vector of deposition, and investigators must carefully consider
each vector that is likely to be significant. However
many studies report only wet deposition, primarily
because this vector is less laborious and less expensive to estimate than cloud or dry deposition.
Nevertheless, it is important to note that wet deposition will to varying degrees underestimate total
nutrient deposition to an ecosystem. For example,
Butler and Likens (1995) have found dry deposition
to contribute as much as 34% of total Sand 32%
of total N deposition to forests in Ithaca, NY (also
see Lovett 1994). This means that it is often difficult to gain a comprehensive understanding of atmospheric deposition to ecosystems. This difficulty
has spurred the development of alternative approaches for inferring atmospheric deposition, including mass-balance flux techniques and natural
abundance stable isotopic techniques. These alternative approaches are discussed at the end of this
chapter.
Scales of Inquiry
It is often difficult to know how measures of atmospheric deposition at single locations scale to
larger areas. Deposition rates depend on several
local-scale factors, including forest edge effects
(Weathers et al. 1992, 1995), orographic effects
along elevational gradients (Fowler et al. 1991),
canopy structure and topographic location in land-
oceans (Duce and Tindale 1991). Analogously,
Cornell et al. (1995) proposed that atmospheric
deposition of dissolved organic N (DON) might be
a dominant source of new N in certain ocean areas.
These examples illustrate the fundamental importance of atmospheric deposition for understanding how ecosystems function in the world, and for
understanding how human activities are impacting
natural ecosystems. However, the examples also
illustrate that our view of atmosphere-ecosystem
interactions is still in a formative stage. Despite
considerable progress, much remains to be understood about the link between ecosystems and the
atmosphere.
Vectors of Delivery
Atmospheric nutrients and pollutants enter ecosystems via three main vectors of deposition, each subject to its own set of controlling factors and methods of measurement: (1) Wet deposition consists of
chemical elements that enter via precipitation,
mainly as dissolved solutes but also as particles entrained in raindrops; (2) cloud deposition consists
of elements dissolved in cloud or fog droplets that
impact onto terrestrial surfaces, such as leaves or
branches; and (3) Dry deposition consists of aerosols and/or gases that are deposited directly onto
surfaces of vegetation, soils, or water.
The relative contributions of these vectors can
vary substantially among ecosystems and with
time, due to factors that act along the path of transport from emission sources to deposition in ecosystems (e.g., Ollinger et al. 1993), and due to factors that are intrinsic to receiving ecosystems (e.g.,
leaf area, stomatal conductance, etc.). At the scale
of individual ecosystems, the contributions by the
different vectors depend on the exact chemical
form (and thus reactivity) of nutrients or pollutants
in the atmosphere and on local-scale meteorological and geographical conditions, such as topographic location and exposure (Miller et al. 1993;
Lovett et al. 1997). Wet and dry deposition are
typically the dominant deposition vectors with dry
deposition becoming progressively more important
in arid regions (Gilette et al. 1992). However,
cloud deposition can be important in cloud-rich
locations such as high-elevation forests (Lovett
Lars O. Hedin
and Kinsman 1990; Johnson et al. 1991; Friedland
et al. 1991; Schemenauer and Cereceda 1992a;
Weathers and Likens, 1997), particularly since
even small amounts of cloud water can contain
high concentrations of pollutants and nutrients
(Weathers, et al., 1988); Lin et al. 1995). A summary of factors that control the different vectors
of atmospheric deposition is beyond the scope of
this chapter; the reader is referred to a number of
excellent texts and reviews (e.g., Finlayson-Pitts
and Pitts 1986; Fowler et al. 1991; Graedel and
Crutzen 1993; Duyzer and Fowler 1994; Lovett
1994; Taylor and Johnson 1994; Winner 1994;
Moncrieff et al. 1997; Lovett et al. 1997; Sutton
et al. 1998).
Many ecosystem-level questions demand an understanding of the total deposition of a nutrient or
pollutant to an ecosystem. It is in these cases insufficient to measure only a single vector of deposition, and investigators must carefully consider
each vector that is likely to be significant. However
many studies report only wet deposition, primarily
because this vector is less laborious and less expensive to estimate than cloud or dry deposition.
Nevertheless, it is important to note that wet deposition will to varying degrees underestimate total
nutrient deposition to an ecosystem. For example,
Butler and Likens (1995) have found dry deposition
to contribute as much as 34% of total Sand 32%
of total N deposition to forests in Ithaca, NY (also
see Lovett 1994). This means that it is often difficult to gain a comprehensive understanding of atmospheric deposition to ecosystems. This difficulty
has spurred the development of alternative approaches for inferring atmospheric deposition, including mass-balance flux techniques and natural
abundance stable isotopic techniques. These alternative approaches are discussed at the end of this
chapter.
Scales of Inquiry
It is often difficult to know how measures of atmospheric deposition at single locations scale to
larger areas. Deposition rates depend on several
local-scale factors, including forest edge effects
(Weathers et al. 1992, 1995), orographic effects
along elevational gradients (Fowler et al. 1991),
canopy structure and topographic location in land-