175

8

Experimental Manipulation Studies

The scientific and political value of experimental field studies of acidifica-
tion processes have been well recognized for some time (Wright, 1991).
Other sources of quantitative information, including survey results, moni-
toring, laboratory studies, and modeling are insufficient, on their own, as a
foundation for understanding and predicting acidification and recovery
responses. The results of surveys of water quality in areas impacted by
acidic deposition, as well as areas not impacted by acidic deposition (c.f.,
Sullivan, 1990) have been used for two decades as evidence of acidification
effects. Interpretation of such data is always compromised, however, by dif-
ferences between the impacted and unimpacted areas that are independent
of acidic deposition. Such differences may include aspects of soils, geology,
climate, land use, and hydrology that in some cases can overwhelm the
effects of S or N deposition.
Acidification of aquatic and terrestrial ecosystems operates on time scales
of many years to many decades. There are few time series of monitoring data
available with long enough period of record to confirm the validity of our
understanding of key acidification processes. Furthermore, interpretation of
time series data is often uncertain because a variety of mechanisms can pro-
vide plausible explanations of observed responses. Concurrent changes in
climate, land use, disturbance, or other factors confound the interpretation of
monitoring results.
There has been a large increase during the past decade in the amount of
experimental research being conducted on the environmental effects of atmo-
spheric deposition, especially of N. This research has been initiated mostly in


15

N) tracer studies to
quantify the partitioning of N into various ecosystem pools (i.e., soil, litter,
trees, ground vegetation) and to measure changes in the quantities of stored
N in these pools. Other studies focused on quantifying the rates of important
ecosystem processes, including the N conversion processes of denitrification
and mineralization.
Results of both the broad-scale and detailed studies have been used to
build, test, and validate mathematical models that simulate N processing,
nutrient cycling, and water regulation in coniferous forest ecosystems under
varying depositional and climatic regimes. Ultimately, these models will be
used to predict N saturation, estimate the critical loads of N for European for-
ests, and to specify emission controls needed to protect European forests
from the detrimental effects of excess N deposition.
Such large-scale, controlled whole-ecosystem experiments have become an
increasingly important tool in environmental research regarding the effects
of atmospheric pollutants. It is now realized that all parts of the ecosystem
are involved in the response to an environmental perturbation such as atmo-
spheric N or S input. Key processes must be evaluated in the broader context
of whole-ecosystem structure and function. It is not possible to understand
environmental impacts on the basis of isolated process studies alone. A holis-
tic approach is required. In addition, whole-ecosystem experimental manip-
ulations are needed across gradients of atmospheric deposition, climate, and
other important factors.
It has also become increasingly evident in recent years that it is not pos-
sible to separate research on ecosystem effects attributable to acidic deposi-
tion from the effects of other ecosystem stressors. Climatic fluctuations,
especially precipitation input and its effects on water availability, act syner-

marize some of the key elements of the experimental ecosystem manipula-
tion research.
The European scientists have concluded that it is important to study N
questions as large multidisciplinary, multi-investigator research teams. This
is because of
1. The complexities of the N cycle.
2. The multitude of scientific disciplines involved in its study.
3. The emerging importance of very expensive, large-scale, whole-
system manipulations as a tool for studying N effects.
A high degree of international and inter-institutional cooperation has devel-
oped during the last decade within Europe. This spirit of cooperation has
been evident in several recent international umbrella projects on N effects,
especially NITREX and EXMAN (Wright and van Breemen, 1995; Rasmus-
sen, 1990; Tietema and Beier, 1995).
The NITRogen Saturation EXperiments (NITREX) project was a large,
international, interdisciplinary research program that focused on the impacts
of NO

3
-

and NH

4
+

on forest ecosystems (Wright and Van Breemen, 1995).
NITREX included 11 separate large-scale N addition or removal experiments
at 9 sites that span the European gradient in N deposition, from less than 5
kg N/ha per year in western Norway to greater than 50 kg N/ha per year in

FIGURE 8.1

Location of NITREX research sites as described by Emmett et al. (1998).

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Experimental Manipulation Studies

179
have NO

3
-

leaching (Sogndal, Aber, Gårdsjön, Klosterhede) and those that
receive greater than 200 meq/m

2

per year (approximately 40 kg N/ha per
year) and leach significant amounts of NO

3
-

(Speuld, Solling, Ysselsteyn; Dise
and Wright, 1995). Other aspects of ecosystem acidification also illustrate the
gradient in the NITREX sites. For example, leaching of Ca


S, deposition has been augmented. Typically, the experimental approach
involved acid application during rainfall events by means of sprinkler sys-
tems, using chemically altered water from a nearby lake or spring as a carrier
for the acid or acid precursor addition. In some cases, ammonium sulfate was
applied periodically by helicopter to the experimental watershed. Several of
these studies are highlighted below.

8.1.1 Gårdsjön, Sweden

At the Gårdsjön experimental manipulation catchment included within
NITREX (Catchment G2), about 35 kg N/ha per year was added to the
ambient deposition (12 kg N/ha per year) as NH

4

NO

3

. The sum of the
experimentally added N plus the ambient deposition in this 0.52-ha catch-
ment was in the range of deposition received by damaged forest ecosystems
in central Europe, but much higher than the deposition levels in sensitive
areas of North America. Data have been collected since 1988 and the treat-

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180


3
-

continued during the second year of treatment, including
increased losses during the growing season. However, the watershed retention
of deposited N during year 2 was still quite high (98.9 ± 0.1%). In the untreated
reference catchment, N retention was about 99.9% of the total inorganic N
inputs (Moldan et al., 1995). The monthly mean volume-weighted concentra-
tions of NO

3
-

in runoff increased from near zero during the 2-year pre-treat-
ment period to values typically in the range of 5 to 17

µ

eq/L during year 2.
Moldan et al. (1995) also conducted intensive sampling for a 2-week period
during which three experimental NH

4

NO

3

additions occurred. They found
that NO

ganic N was only about 5% of the incoming N. The cumulative effect of the N
addition was apparent when NO

3
-

concentration was plotted by Moldan and
Wright (1998a) as a function of stream discharge during the autumn periods of
1994, 1995, and 1997. Nitrate concentrations reached higher values at a given
discharge as the experimental acidification proceeded. Discharge rate was the
most important factor influencing NO

3
-

leaching loss. Peak NO

3
-

concentra-
tions in discharge (approximately 20 to 100

µ

eq/L) corresponded temporally
with either times of experimental NH

4


3
-

leaching shows a more dramatic response
at this site.
One of the first biological changes attributed to the experimental treatment
was a change in the ectomycorrhizal fungus flora. Brandrud (1995) reported
that the micorrhizal fruit body production was reduced after 1 1/2 years of
treatment, especially for the dominant genera

Cortinarious

and

Russula

. A
decrease in the amount of fine roots, especially in the upper soil horizons,
was also observed for the

Vaccinium

-dominated portion of the study area
(Clemensson-Lindell and Persson, 1995).

8.1.2 Sogndal, Norway

The Sogndal site in western Norway was part of the Reversing Acidification
in Norway (RAIN) project (Wright et al., 1993). One of the small catchments
(SOG4) received a 1 : 1 mixture of sulfuric and nitric acid (50 meq/m


Empetrum nigrum

, several species of

Vaccin-
ium

, grasses, mosses, and lichens.
Addition of H

2

SO

4

and HNO

3

at SOG4 has caused large changes in runoff
chemistry. During the first 5 years of treatment, NO

3
-

concentrations in runoff
at SOG4 were elevated above concentrations at the control sites (SOG1 and
SOG3) only immediately after acid applications. Since 1989, however, the

tion within the NITREX framework, and is also the only nonforested (alpine)
catchment in the project. The experiment at Sogndal represents the only long-
term study of chronic N addition to an alpine site.
Results of 9 years of N deposition at a level of 9 kg N/ha per year were
summarized by Wright and Tietema (1995). As was found by Moldan et al.
(1995) at Gårdsjön, the general pattern of NO

3
-

concentration in runoff was
one of sharp peaks during and immediately after each acid addition, fol-
lowed by a rapid decline to concentrations near zero. It was only during the
last few years of treatment that the decline in NO

3
-

concentration in runoff
following experimental N additions proceeded more slowly, and runoff

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182

Aquatic Effects of Acidic Deposition

between additions also contained elevated concentrations of NO


3
-

flux during times of high flow and high input concen-
trations and emphasized that the total N deposition at Sogndal was near the
10 kg N/ha per year apparent threshold for N saturation proposed by Gren-
nfelt and Hultberg (1986) and Dise and Wright (1995).

8.1.3 Lake Skjervatjern, Norway

The Humic Lake Acidification Experiment (HUMEX) was initiated by the
Norwegian Institute for Water Research (NIVA) in 1987. The principal goals
of HUMEX were to evaluate the role of humic substances in the acidification
of surface waters and the effects of S and N deposition on the properties of
humic substances in watershed soils and surface waters (Gjessing, 1992).
HUMEX is an investigation of the interaction between acid deposition and
natural organic acids by means of acid addition to the entire catchment of a
pristine humic lake in western Norway. Skjervatjern is a small (2.4 ha), pris-
tine, naturally acidic, humic lake located near Førde, western Norway. The
lake has pH 4.6 with average concentrations of TOC of about 9 mg C/L, non-
marine base cations of about 30

µ

eq/L, and Al

i

of less than 50



SO

4

and NH

4

NO

3

was applied at pH 3.0 to 3.2 weekly, in a volume equivalent to
approximately 10% of ambient precipitation, using water pumped from
nearby Lake Åsvatn. Annual target loadings for SO

4
2-

and total N were 63 to
66 and 17 to 32 kg/ha, respectively. Water chemistries in the treatment and
reference sides of the lake were monitored weekly, for 2 years prior to initia-
tion of the artificial acid additions and for 5 years during which N and S were
applied to one-half of the lake and its respective drainage basin (Gjessing,
1994; Lydersen et al., 1996).
The physical division of Lake Skjervatjern into two basins had some
effects on the water chemistry of the lake, likely due to small differences in
the terrestrial catchments that drain into the two lake halves. Lake water in
the treatment side had equivalent or lower concentrations of all ions and

(-1

µ

eq/L). Lake water
pH was slightly higher on the experimental side (approximately 0.03 pH
units) and TOC was 0.67 mg C/L lower (Gjessing, 1992, 1994).
During the first 2 years of treatment, 8.5 g m

-2

of H

2

SO

4

and 6.7 g m

-2

of
NH

4

NO


applied to the experimental catchment during the first
2 years of treatment should have caused an increase in lake-water SO

4
2-

con-
centration of about 44

µ

eq/L above the premanipulation concentrations,
assuming steady-state conditions and average annual runoff of about 1950
mm. The observed increase in lake-water SO

4
2-

concentration was 15

µ

eq/L
(Gjessing, 1992), suggesting that about two-thirds of the S added during the
first 2 years were retained in the watershed. Over the 5-year treatment period
reported by Lydersen et al. (1996), the mean SO

4
2-


added directly to the lake surface, even without assuming
that some of the NH

4
+

applied to the lake surface would have been converted
to NO

3
-

in the lake water. Total N also increased in the treatment side of the
lake, however, and by an amount considerably greater than the total N
applied to the lake surface (Gjessing, 1994). This suggested that at least some
of the N applied to the terrestrial portion of the catchment also reached the
lake. Nevertheless, about 90% of the added N was retained in the terrestrial
system, lake sediments, and/or biota, and did not contribute to increased
concentrations of NO

3
-

and NH

4
+

in lake water.
Lydersen et al. (1996) used randomized intervention analysis (RIA) to test


i

in Basin A after treatment
compared with the control basin. The average ANC increased in the control
basin during the course of the study, and this was attributed by Lydersen et
al. (1996) to the long-lasting effect of Na

+

leakage after storms having high
inputs of sea salts. During a hurricane in January 1993, the concentration of
Cl

-

in rainfall exceeded 400

µ

eq/L at the nearby weather station. During that
event, the lowest runoff pH (4.25) and ANC (-62

µ

eq/L) values were
recorded in the control basin. ANC remained unchanged in Basin A. Acidifi-
cation of Basin A was observed as a gradual change in the difference in ANC
between the two basins.
Highest concentrations of SO

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Experimental Manipulation Studies

185
The experimental design included three replicate plots that received each
of five treatments in a randomized block design. The treatments began in
1990 and included control, added water, added sodium nitrate (35 and 75 kg
N/ha per year), and ammonium nitrate (35 kg N/ha per year). Emmett et al.
(1995) reported the results after 2 1/2 years of experimental treatment. Soil
water NO

3
-

leaching losses increased in parallel with NO

3
-

additions,
although NH
4
+
additions were virtually completely retained. Little or none of
the applied NO
3
-
N appeared to be taken up by the vegetation, retained in the

, however, at all soil depths. Nitrate leaching accounted for 13%
of the added NO
3
-
, and the forest plot retained 92% of the total N input. Sim-
ilar results were obtained by Aber et al. (1989) at an N-limited pine stand at
Harvard Forest, MA, exposed to a comparable NH
4
NO
3
application rate
(50 kg N/ha per year), where complete N retention was observed after
3 years of experimental N addition.
Soil solution N chemistry changed immediately in response to the N addi-
tions, with NO
3
-
concentration increasing at all depths. The total NO
3
-
leach-
ing increased from less than 0.3 to 4.2 kg N/ha per year by the third year.
Ammonium concentrations increased to 15 cm depth, but NH
4
+
did not leach
out of the system. Changes were not observed in the concentrations of other
ions (Gundersen, 1998).
Based on these results and the results of a nationwide survey of soil water
beneath the rooting zone, Gundersen and Rasmussen (1995) concluded that

eq of SO
4
2-
and NH
4
+
per hectare per year effectively increased total atmo-
spheric loading about 200% for S and 300% for N (Norton et al., 1999). Prior
to the manipulation, stream-water chemistry of both the East and West Bear
Brook catchments showed a volume–weighted annual mean pH of about 5.4,
ANC 0 to 4 µeq/L, base cation concentrations about 184 µeq/L, and SO
4
2-
concentration slightly over 100 µeq/L. DOC values were generally low (less
than 3 mg/L) and NO
3
-
concentrations varied seasonally between about 0
and 30 µeq/L (Norton et al., 1999).
The response of West Bear Brook stream-water chemistry to the experimen-
tal manipulation to date has been summarized by Norton et al. (1994, 1999)
and Kahl et al. (in press). The major responses of the stream-water chemistry
have included increased concentrations of SO
4
2-
, NO
3
-
, base cations, Al
n+

+
concentration of about
15 µeq/L (Norton et al., 1999).
During the first year of treatment, 94% of the added N was retained by
the Bear Brook watershed. Percent retention subsequently decreased to
about 82% for the next 7 years of treatment (Kahl et al., 1993a, in press).
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Experimental Manipulation Studies 187
Both the immediate nature of the N response and the magnitude of the
increase in NO
3
-
flux from the treated West Bear catchment were unex-
pected (Kahl et al., 1993a).
The concentrations of SO
4
2-
in stream water progressively increased
throughout the 7 years of experimental acidification. By 1995, the volume-
weighted mean SO
4
2-
concentration reached 185 µeq/L and the maximum
concentrations during high-flow periods exceeded 220 µeq/L, approaching
the expected steady-state concentration that would be achieved if outputs
equaled inputs (approximately 285 µeq/L). The concentration of SO
4
2-
was

2+
concentrations have declined at high flow
during the period 1993 to 1995. This implies that the base cation supply in the
upper soils is becoming depleted, which will lead to further acidification and
mobilization of Al (Kahl et al., in press).
8.2 Whole-System Nitrogen Exclusion (Roof) Studies
An important tool that has developed in recent years for the study of ecosys-
tem processes and the impacts of atmospheric deposition at the ecosystem
level is the construction of transparent roofs over entire mini catchments or
forested plots. The roof emplacement technique was pioneered at Ris-
dalsheia, Norway, in 1983 in the RAIN project. A number of additional roof
studies were constructed in Europe during the past decade, with roofs rang-
ing in size from about 300 m
2
to the extremely impressive 0.6 ha roof at Gård-
sjön in Sweden. In most cases, the roofs are constructed below the canopy in
well-developed forests. Trees protrude through holes that are often sealed to
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188 Aquatic Effects of Acidic Deposition
prevent throughfall and stemflow from passing through the holes. Runoff
from the roof is collected, chemically altered, and reapplied beneath via
sprinklers. The technique allows simulation of drought and decreased atmo-
spheric deposition to entire terrestrial systems.
8.2.1 Gårdsjön, Sweden
Whole-catchment exclusion of incoming ambient atmospheric pollution has
reached its apex with the construction of the large roof at the Gårdsjön site in
Sweden in 1990. The clear plastic roof intercepts atmospheric deposition at a
height of 2 to 4 m above the ground. Approximately 350 Norway spruce trees
protrude through the roof. The roof experiment excludes about 20 to 30 kg

60%
Al
i
29%
Ca
2+
20%
Mg
2+
28%
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Experimental Manipulation Studies 189
The calculated F factor was about 0.5 after 5 years. Sulfate concentrations in
discharge remained high, however, about 215 µeq/L, in response to desorp-
tion and mineralization of S stored in the watershed soils (Torssander and
Mörth, 1998; Gobran et al., 1998). The pH has changed more slowly than the
other ions. Most of the reduction in acidity has been expressed as declining
Al
i
concentrations (Skeffington and Hultberg, 1998). Although there has been
a steady improvement in the quality of the output water at Gärdsjön in
response to the experimental treatment, the realized reductions in the con-
centrations of H
+
and Al
i
have not been sufficient to mitigate the toxicity of
the water to fish (Hultberg et al., 1998). Further recovery will depend largely
on the rate of release of stored S and the future supply of base cations from

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190 Aquatic Effects of Acidic Deposition
The reduced input of N and S to the “clean” roof plot at Ysselsteyn
resulted in reduced NH
4
+
, NO
3
-
, and SO
4
2-
concentrations in soil solution.
Between 1990 and 1992, the NO
3
-
concentration was reduced 45% and NH
4
+
concentration 80% compared with the roof control plot. The total S flux was
reduced 70%. At Speuld, the N flux was reduced by 80% and the S flux by
20 to 60%, depending on depth in the soil profile. Boxman et al. (1998) sum-
marized the major results for the Scots pine stand at Ysselsteyn after 5 years
of treatment. The mean flux of N in drainage water at 90 cm depth from
1990 to 1995 was reduced to 16 kg N/ha per year under the clean roof, as
compared with 36 kg N/ha per year under the control roof and 69 kg N/ha
per year under the open-air control plot. Vegetation response showed a
more pronounced lag period, although some signs of ecosystem recovery
were evident after 5 years. The concentration of N in needles decreased, but

samples, and annual diameter growth. Despite the variability, however,
higher tree growth rates have been observed in the fertilized and irrigated
plots since 1988.
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Experimental Manipulation Studies 191
8.2.4 Solling, Germany
The Solling research site in central Germany has been under study for about
30 years, and as such, is one of the longest ecosystem studies in the world.
Manipulation experiments began in 1989 when four plots were established to
investigate ecosystem recovery under clean precipitation and the effects of
drought. Solling was included in both the NITREX and EXMAN networks.
Roofs of 300 m
2
area were constructed in the summer of 1991 over three
plots
• The NITREX clean precipitation site.
• Drought manipulation site.
• Roofed control.
The fourth plot served as an open-air control.
The site is located in central Germany at 500 m elevation in a 60-year old
Norway spruce (Picea abies) plantation. The roofs are underneath the canopy
at a height of about 3 m, with the tree trunks passing through preformed
holes in the roofs and fitted with plastic collars. Ambient N deposition is very
high (approximately 38 kg/ha per year). The “clean” rain roof simulates a
90% reduction of wet N input to the soil.
Bredemeier et al. (1995) reported results after 1 1/2 years of treatment.
Nitrogen levels in soil water were reduced dramatically. Within the rooting
zone, NH
4

ment in a temporal cascade of soil solution leading to fine roots leading to
above-ground stand.
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192 Aquatic Effects of Acidic Deposition
8.2.5 Risdalsheia, Norway
The Risdalsheia site in southernmost Norway receives a high loading of
acidic deposition (SO
4
2-
wet + dry loading, 18 kg S/ha per year), and is char-
acterized by exposed granitic bedrock (30 to 50% of surface) and thin,
organic-rich, truncated podzolic soils (Wright et al., 1986). Acid exclusion at
the KIM catchment is accomplished by a 1200 m
2
transparent roof that com-
pletely covers the 860 m
2
catchment. Incoming precipitation is collected from
the roof and pumped through a filter and ion exchange system. Seawater
salts are added back at ambient concentrations and the clean precipitation is
automatically applied beneath the roof by a sprinkler system. During winter,
artificial snow is applied beneath the roof using commercial snow-making
equipment. Controls include a mini catchment with a roof (EGIL) and one
without (ROLF), both of which receive acidic rain and snow. Acid exclusion
at KIM has resulted in substantially lower concentrations of SO
4
2-
and NO
3

Roof manipulation studies, such as those described previously, have
proven valuable for investigating the environmental effects of reduced dep-
osition of S and N and for testing of mathematical models that predict the
effects of abatement strategies. However, a variety of unintended changes
have also been caused by the roof construction and experimental design in
some cases (Beier et al., 1998). These can confound interpretation of the
resulting data. For example, reduced light penetration by 50% to the forest
floor caused a decrease in moss cover at Klosterhede (Gundersen et al., 1995).
Such vegetative changes may, in turn, affect nutrient cycling. Beier et al.
(1998) stressed the importance of selecting roof plates that transmit maxi-
mum light and careful and regular cleaning. In addition, the frequency and
intensity of water sprinkling affects both the hydrology and input of nutri-
ents, which in turn can affect ground flora and microbial communities
(Hansen et al., 1995; Gundersen et al., 1995). The sprinkling system will also
change the spatial variability of water and nutrient delivery to the plot. It is
important that the quantities of nutrients that are removed by filtering are
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Experimental Manipulation Studies 193
calculated and reapplied under the roof, and that the water and nutrient sup-
ply is performed as close to real time as possible (Beier et al., 1998).
8.3 Climatic Interactions
It has become increasingly evident that it is often difficult to separate the
effects of excess N deposition from climatic effects. For example, it has been
proposed that high N loadings result in increased susceptibility of trees to
drought and frost. Elevated deposition of N can affect the water uptake of
trees, via
1. Increased shoot/root ratio, thereby increasing water demand.
2. Shift in root growth from mineral soil to upper organic horizon,
thereby increasing susceptibility to drought.

to quantify the impacts of atmospheric CO
2
enrichment and temperature
increase on ecosystem response, especially the plant–soil–water linkages and
processes. The approach involved whole catchment manipulations of tem-
perature and CO
2
concentration at the Risdalsheia site in southern Norway,
formerly part of the RAIN project.
Jenkins and co-workers measured changes in CO
2
uptake, gas exchange,
and plant phenology; forest growth and nutrient status; ground vegetation;
mineralization of soil organic matter; soil fauna; and biologically mediated
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194 Aquatic Effects of Acidic Deposition
processes, and the quality and quantity of runoff water. Process-oriented
models will be developed to link aquatic and terrestrial processes.
Several of the roof experiments in Europe have examined ecosystem
responses to simulated drought and subsequent rewetting of forest plots. For
example, Bredemeier et al. (1998b) reported the results of simulated intensive
drought periods at Solling. The above-ground parameters describing stand
growth and physiology responded rapidly to the experimental treatment;
height and diameter increment decreased and photosynthetic capacity was
reduced. Fine roots did not show an obvious response to simulated droughts
of 10 to 25 weeks. Soil water chemistry did not show the anticipated acidifi-
cation pulses owing to excess nitrification in the rewetting periods after the
simulated droughts. Consistent patterns of NO
3

analytes. Because of the observed lag time in realizing significant environ-
mental responses to experimental manipulation, it is important to initiate
long-term investigations well in advance of the anticipated need for the
resulting data. Such long-term studies require substantial funding commit-
ments for long periods of time. Current federal funding mechanisms and
funding cycles available in the U.S. are generally not compatible with long-
term, multidisciplinary environmental studies.
An extremely important and novel aspect of the recently initiated Euro-
pean research on N has been the extent of coordination among projects, insti-
tutes, and investigators. Coordination among research teams from different
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Experimental Manipulation Studies 195
countries throughout Europe has been a cornerstone of the large interna-
tional projects such as NITREX and EXMAN. Researchers from across
Europe have shared methodologies, data, and expertise in an unprecedented
fashion. Manipulation studies with somewhat different objectives often
shared data and reference (control) sites. Such an atmosphere of interdiscipli-
nary, interinstitutional research cooperation has not developed to the same
level in the U.S.
The influence of historical forest management on the ability of a given for-
est ecosystem to process N is largely unknown. Nevertheless, forest manage-
ment practices, especially those that have occurred over many generations,
can have important effects on soils (i.e., erosion), nutrient supplies (i.e., har-
vesting), organic material (i.e., litter raking), and thereby many aspects of N
cycling and N effects. European forests have typically been harvested for
many generations, changed in species composition or community type (e.g.,
conversion from heathland to forest), and managed or manipulated in a vari-
ety of ways. The interactions between these activities and atmospheric depo-
sition are unknown.

acidification theory is no longer purely theoretical. Quantitative aspects of
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196 Aquatic Effects of Acidic Deposition
acidification, such as proportional changes in the various ionic constituents
that collectively constitute the acidification response and rates of watershed
retention of acid anions, are now much better understood. And finally,
experimental databases have been provided with which to test, confirm,
and improve mathematical models of acidification dynamics. The author
would argue that the most important advancements in acidification science
of this decade have been direct results of the experimental manipulation
studies. These studies have been expensive, but the gains have far out-
weighed the costs. The scientists who had the foresight and fund-raising
capabilities to initiate this area of research including Wright, Schindler,
Norton, Rasmussen, van Breemen, Hultberg, and many others have had an
enormous impact on acidification science.
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© 2000 by CRC Press LLC