Principles of terrestrial ecosystem ecology.pdf

318 14. Landscape Heterogeneity and Ecosystem Dynamics
Relative units
200
150
100
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0
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Ca:Al and
Mg:N ratios
N mineralization
NPP
Nitrification
cycling, trace gas fluxes, and carbon storage.
Chronic nitrogen deposition initially reduces
nitrogen limitation by increasing nitrogen
cycling rates, foliar nitrogen concentrations,
and NPP. Above some threshold, however, the
ecosystem becomes saturated with nitrogen
(Fig. 14.9) (Aber et al. 1998). As excess nitrate
and sulfate leach from the soil, they carry with
them cations to maintain charge balance, inducing
calcium and magnesium deficiency in
vegetation (Driscoll et al. 2001). In southern
Sweden, for example, over half of the plantavailable
cations have been lost from the upper
70cm of soil in the past half century, probably
due to chronic exposure to acid precipitation
(Hallbacken 1992). The exchange complex
becomes more dominated by aluminum and
hydrogen ions, increasing soil acidity and the
likelihood of aluminum toxicity. Together this
suite of soil changes often enhances frost susceptibility,
impairs root development, and promotes
herbivory, leading to forest decline in
many areas of Europe and the northeastern
United States (Schulze 1989, Aber et al. 1998).
The major surprise, however, has been how
resilient many forests have been to acid rain,
often retaining most of the nitrogen inputs
within the ecosystem for as much as two
decades. The resilience of ecosystems depends
in part on the magnitude of inputs (related to
distance from pollution sources and amount of
precipitation received) and initial soil acidity,
which in turn depends on parent material and
species composition. Many ecosystems, how-
Foliar N
Leaching
1 2 3
Stage of N saturation
Figure 14.9. Changes hypothesized
to occur as forests undergo longterm
nitrogen deposition and nitrogen
saturation. (Redrawn with
permission from BioScience; Aber
et al. 1998.)
ever, now show clear signs of nitrogen saturation,
resulting in forest decline, loss of acidneutralizing
capacity in lakes, and increasing
nitrogen inputs to streams (Aber et al. 1998,
Carpenter et al. 1998, Driscoll et al. 2001).
Nearly all research on the transport, deposition,
and ecosystem consequences of anthropogenic
nitrogen has been conducted in the
temperate zone. Further increases in nitrogen
deposition will, however, likely occur primarily
in the tropics and subtropics (Galloway et al.
1995), where plant and microbial growth are
frequently limited by elements other than
nitrogen. These ecosystems might therefore
show more immediate nitrogen loss in trace
gases or leaching in response to nitrogen deposition
(Matson et al. 1998). On the other hand,
soil properties such as high clay content or
cation exchange capacity may allow tropical
soils to sequester substantial quantities of nitrogen
before they become leaky.
Biomass burning transfers nutrients directly
from terrestrial pools to the atmosphere and
then to down-wind ecosystems. Biomass combustion
releases a suite of gases that reflect the
elemental concentrations in vegetation and fire
intensity. About half of dry biomass consists of
carbon, so the predominant gases released are
carbon compounds in various stages of oxidation,
including carbon dioxide (CO2), methane
(CH4), carbon monoxide (CO), and smaller
quantities of nonmethane hydrocarbons. The
atmospheric role of these gases varies. CO2 and
CH4 are greenhouse gases, whereas carbon