Forest Ecology and Management 149 (2001) 75±90
Effects of calcium and aluminum chloride additions on foliar
and throughfall chemistry in sugar maples
Torsten W. Bergera,*, Chris Eagarb, Gene E. Likensc,
Gerhard Stingederd
a
Institute of Forest Ecology, Univ. f. Bodenkultur, Peter Jordan-Strasse 82, 1190 Vienna, Austria
b
USDA Forest Service, Northeastern Forest Experiment Station, Durham, NH 03824, USA
c
Institute of Ecosystem Studies, Millbrook, NY 12545, USA
d
Institute of Chemistry, Univ. f. Bodenkultur, 1190 Vienna, Austria
Received 15 December 1999; received in revised form 9 May 2000; accepted 29 May 2000
Abstract
Calcium availability for sugar maple stands at the Hubbard Brook Experimental Forest (New Hampshire, USA) was tested
by experimental addition of CaCl2 and AlCl3. Additions of 10 g Ca m 2 represented the estimated loss from the soil exchange
complex during the last 30 years due to acidic deposition. Four years of data from 12 throughfall collection sites were used to
evaluate the in¯uence of foliar nutrient content, precipitation amount, dry deposition, precipitation acidity and precipitation
solute concentrations on throughfall chemistry. Calcium additions increased Ca foliar contents signi®cantly. Foliar contents
indicated plant uptake of Cl. Leaching of Cl from the canopy increased with elevated Cl content of the green foliage. Leaching
rates for Ca, Mg, and K were not signi®cantly different between the treatments (surprisingly Ca leaching tended to decrease
with increasing foliar Ca content). We suggest that Ca supply to Ca de®cient sugar maple trees protected the foliage from
increased leaching of Ca (and other elements) due to improved integrity of cell membrane and cell wall formation from Ca.
Degradation of the structural material of the foliage (autumnal leaf senescence, damages by ice and hail storms) caused Ca
throughfall ¯uxes in accordance to measured foliar Ca contents. Increasing acidity of precipitation caused increased leaching
of Ca, Mg and K. About half of the cation leaching from these sugar maple canopies is attributable to a cation-exchange
reaction driven almost entirely by H‡ in precipitation. # 2001 Elsevier Science B.V. All rights reserved.
Keywords: Throughfall chemistry; Calcium; Chloride; Acer saccharum
1. Introduction
Calcium (Ca), the ®fth most abundant element in
trees, is an essential component for wood formation
and the maintenance of cell walls (e.g. Lawrence et al.,
1995). Calcium is usually the most abundant of the
*
Corresponding author. Tel.: ‡43-1-47654-4107;
fax: ‡43-1-4797896.
E-mail addresses: [email protected],
[email protected] (T.W. Berger).
alkali and alkaline earth elements on the soil exchange
complex, and is important in the regulation of soil pH
(Bowen, 1979). In contrast to potassium, calcium is
not leached readily from living foliage due to its
relative immobilization in pectates and on membranes. Thus, in forest ecosystems Ca typically cycles
between plants and soil through uptake±litterfall±
mineralization processes (Likens et al., 1998).
Because Ca is a macronutrient for higher
plants, spatial and temporal variations in its supply
are important to the growth and vigor within a forest
0378-1127/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 1 1 2 7 ( 0 0 ) 0 0 5 4 6 - 6
76
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
ecosystem. Soil acidi®cation and accompanying
leaching of Ca from the soil exchange complex is a
gradual natural process. However, during the past 6
decades, concentrations of root available Ca
(exchangeable and acid-extractable forms) in forest¯oor and soils have decreased markedly at some
locations in the northeastern United States (Shortle
and Bondietti, 1992; Johnson et al., 1994; Likens et al.,
1996, 1998). For example, long-term data from the
Hubbard Brook Experimental Forest demonstrate
that before the mid-1950s, annual Ca depletion from
the soil exchange complex was about equal to net
biomass storage, and atmospheric deposition plus
weathering release nearly balanced streamwater loss.
After the mid-1950s, soil depletion of Ca was up to
two times greater than net biomass storage, and
streamwater loss was up to 2.7 times larger than bulk
precipitation plus weathering release (Likens et al.,
1996). Increasing concern about long-term changes
in pools of available Ca for forest ecosystems and
their productivity at many sites in the northeastern
United States has been documented (e.g. Federer
et al., 1989; Lawrence et al., 1995; Bailey et al.,
1996; Likens et al., 1996, 1998).
A re¯ection of the base cation status of forest
ecosystems may be seen in the element ¯uxes in
throughfall. They are measured readily, but it is dif®cult to separate throughfall into its contributions.
Nutrients in throughfall may result from (a) incident
precipitation, passing through the canopy; (b) material deposited as particles, gases, or cloud droplets
prior to the precipitation event being washed off
during the event, and (c) exchange processes within
the canopy (including foliage, woody parts, epiphytes, and microorganisms). Hence, according to
Lovett and Lindberg (1984), net throughfall ¯ux
(NTF) can be de®ned as
NTF ˆ TF
IP ˆ DD ‡ CE
(1)
indicating that the difference between throughfall (TF)
and precipitation inputs (IP) is equal to the sum of dry
deposition (DD) and canopy exchange (CE). Eq. (1)
ignores stemflow (SF) flux, which is often a minor
percentage of throughfall flux. Canopy exchange
includes both leaching (efflux from the canopy) and
uptake or retention (influx to the canopy). Because
neither dry deposition nor canopy exchange is easily
quantified, it is difficult to separate them into their
respective contributions to total chemical deposition
in throughfall. Distinguishing between the two is
important, as dry deposition represents an input to
the ecosystem, while canopy exchange is an intrasystem transfer.
Quantitative separation of these mechanisms is
essential for understanding pollutant effects on canopy
processes, but it is notoriously dif®cult (Lovett, 1994).
Lovett et al. (1996) pointed out that there are several
variables that in¯uence one or both of these processes
(e.g. precipitation amount and rate; source strength
and proximity of dry deposition; precipitation acidity;
precipitation chemical concentrations; composition,
age and nutrient status of the forest; epiphytes; leaf
area and others).
It is hypothesized that foliar nutrient contents will
respond to soil treatments with calcium and that
increased foliar element contents will increase leaching rates of the corresponding element, partly due to
acidic deposition. Since several studies have shown
that H‡ disappearance from bulk deposition can not
account for the total base cation enrichment of precipitation beneath forest canopies, organic anions are
thought to be important counter ions for leached
cations (Eaton et al., 1973; Lovett et al., 1985; Bredemeier, 1987; Sayre and Fahey, 1999). Cronan and
Reiners (1983) concluded that during the growing
season as much as 30±50% of precipitation-borne
strong acidity is neutralized by weak Brùnsted base
leaching and the remaining neutralization was attributable to leaf surface ion exchange in a northern
hardwood canopy. Hence, a further aspect of this work
is the evaluation of the fate of H‡, which requires
determining the magnitude of the cation-exchange
reaction in the canopy, taking acid±base reactions into
account.
Calcium was applied as calcium and aluminum
chloride to northern hardwood forest plots, dominated
by sugar maple, to vary Ca availability manifold (e.g.
increased foliar Ca content due to addition of CaCl2
and decreased Ca content due to Al induced Ca
de®ciency by the AlCl3 treatment). We used 4 years
of data from 12 throughfall collection sites on
untreated, CaCl2 and AlCl3 treated sugar maple stands
to evaluate the in¯uence of foliar nutrient content,
precipitation amount, dry deposition, precipitation
acidity and precipitation solute concentrations on
the NTF of these sites.
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
2. Methods
2.1. Study area
The study was done in an area 300±900 m west of
Watershed 6 (W6) of the Hubbard Brook Experimental Forest (HBEF) (438560 N, 718450 W) in the White
Mountains of New Hampshire. The climate of this
area is humid continental, with 1310 mm average
annual precipitation. The study area spans the elevational range 700±760 m. Watershed 6 is the biogeochemical reference watershed at HBEF and is the site
of long-term vegetation and biogeochemical monitoring (e.g. Likens and Bormann, 1995). The HBEF is
covered mostly with northern hardwood vegetation
growing on Typic Haplorthods that developed in sandy
till. The average pH of the humus layer (Oa horizon) is
3.9, while pH values in the mineral soil increase from
4.2 to 4.7 with depth (Johnson et al., 1991). However,
most soil pro®les of the study area were classi®ed as
Aquic Haplorthods and some as Aquic Haplumbrepts.
Watershed 6 (13 ha) and adjacent land to the west are
covered by mature northern hardwood forest in which
the dominant canopy tree species are sugar maple
(Acer saccharum Marsh.), American beech (Fagus
grandifolia Ehrh.), and yellow birch (Betula alleghaniensis Britt.). Canopy height is generally about 20±
25 m, and epiphyte cover is negligible.
2.2. Study sites and treatments
Within the study area 12 study sites (45 m45 m)
were established, where sugar maple is the dominant
canopy tree species (70±85% of stems). Calcium
availability for these sugar maple stands was varied
threefold: control, CaCl2 treated and AlCl3 treated
plots (in each case four replications). The study was
done during four vegetation periods (compare
Table 1): 1995, no additions (ambient conditions);
1996, part of the additions were done; 1997, before
the growing season the total amount was added; 1998:
no additions (additional time for ecosystem response
to the treatments).
The total amount of 10 g Ca m 2 (500 mmolc
Ca m 2, Table 1) represents the estimated loss from
soil exchange (upper mineral soil) during the last 30
years due to acidic deposition (Federer et al., 1989;
Likens et al., 1996, 1998). The total amounts for Ca,
77
Table 1
Additions of calcium, aluminum and chloride (mmolc m 2) for the
CaCl2 and AlCl3 treated sugar maple standsa
Site
Element
1995
1996
1997
1998
Total
CaCl2
Calcium
Aluminum
Chloride
±
±
±
250
±
250
250
±
250
±
±
±
500
±
500
AlCl3
Calcium
Aluminum
Chloride
±
±
±
±
200
200
±
300
300
±
±
±
±
500
500
a
No additions were performed on the control sites. Five partial
treatments were done as CaCl2 and AlCl3 in fall and spring during
the leafless periods, resulting in the given amounts added before
the growing seasons of 1996 and 1997.
Al and Cl are the same on an equivalent basis, however, at the beginning of the growing season in 1996
20% more CaCl2 was added than AlCl3 (compare
Table 1).
2.3. Foliar analysis
Leaf samples of overstory sugar maple trees (®ve
trees per plot) were collected at the end of August
1996 and 1997 with a shotgun. Individual samples
were analyzed for N (Kjedahl; auto-analyzer) and
cations (plasma emission spectrometer) after digestion
with H2SO4 and H2SeO3 at the USDA forest service.
chloride analyses (amperometric endpoint titration
with coulometric generated silver ions by means of
a aminco chloride titrator; American Instrument Company, 1971) were done after digestion with HNO3 (one
pooled sample out of ®ve replications per plot) at the
Institute of Forest Ecology.
2.4. Atmospheric deposition
Throughfall was collected and analyzed at all sites
during the growing season (1 June to 30 September) of
1995±1998. Samples were collected weekly and the
funnels of the collectors were rinsed with deionized
water. Cleaned (rinsed with deionized water) polyester
plugs were used in the funnels to minimize particulate
inputs and replaced each time of collection with
plastic gloves. Six bulk samplers were placed within
the inner 25 m25 m of the plot (inside the 10 m wide
buffer zone of the treated 45 m45 m area). The
locations of the individual throughfall samplers were
78
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
not changed during the study. These collectors consisted of polyethylene funnels with a 200 mm upper
diameter, placed 60 cm above the forest ¯oor on
wooden sticks. The funnels were connected to onegallon polyethylene reservoirs by black norprene tubings. All samples of the individual collectors at each
site were pooled. Precipitation (bulk deposition) input
was sampled in a clearing at an elevation of 700 m,
350 m east of the study area using the same kind of
sampler (two bulk samplers, 160 cm above ground).
For the regression analysis only single rain events
were used, separated from other rainfall periods by at
least a 12 h dry period. Of the 64 weekly collections
made during the study, 16 represented single precipitation events. Individual rain events were exactly
comparable for all study sites with regard to the length
of the dry period before the event, duration of the rain
event and time of collection after the end of the event.
The volume of throughfall and precipitation was
measured in the ®eld with a graduate cylinder, and a
sample was brought back to the laboratory for analysis. An aliquot was allowed to come to room temperature and pH was measured immediately after
collection with a glass electrode. Another aliquot of
the sample, for chemical analysis of major ions, was
preserved with 100 ml of chloroform per 100 ml of
sample, and a third aliquot, for titrations, was not
preserved. The samples were not ®ltered, but were
stored in clean high-density polyethylene bottles in the
dark at 48C until chemical analysis was completed,
usually within 6 months; titrations were done within
2±3 weeks after collection.
Sulfate, Cl and NO3 were measured by ion chromatography, Ca and Mg by inductively coupled
plasma emission (ICP) spectrophotometry, Na and
K by atomic absorption spectroscopy and NH4 by
auto-analyzer (Institute of Ecosystem Studies). Aluminum analysis (ICP Ð mass spectrometry, Institute
of Chemistry) was performed after monthly volumeadjusted samples were prepared from weekly collections at all four sites of the same treatment. Throughfall ¯uxes were calculated according to measured
solution volumes per area of the collector.
Titrations were performed by hand with a burette
while the sample was stirred (magnetic stirrer) and
purged with N2. Alkalinity was determined by endpoint titration with 0.0005 M HCl to pH 5.0 according
to Richter et al. (1983) and Lovett et al. (1985). Total
acidity was measured by titration with 0.0005 M
NaOH (standardized against HCl) and Gran plot analysis (Lindberg et al., 1984). Weak (undissociated)
acidity was determined by subtracting the free acidity
(measured as pH) from total acidity (measured by
titration, Lovett et al., 1985).
3. Results
3.1. Foliar analysis
Foliar element contents are given in Table 2.
According to the proposed hypothesis Ca additions
(CaCl2) increased Ca foliar contents signi®cantly in
both years. Differences between the control and AlCl3
sites are not signi®cant for Ca but Al additions (AlCl3)
tended to cause lower Ca foliar contents.
Although Mg was not added with the treatment,
foliar Mg contents were signi®cantly higher for the
CaCl2 treated stands than for the control (1996) and
AlCl3 sites (1996 and 1997).
Since Ca and Al were added as chloride to minimize
changes in soil pH, and to accelerate in®ltration of Ca
and Al into deeper soil horizons, the treatments had
distinct effects on foliar Cl content (Table 2, Cl is used
for chloride throughout this paper). In 1997, after the
total amounts were added, foliar Cl contents were
higher for the CaCl2 sites than for the control sites,
indicating plant uptake. Aluminum chloride additions
increased foliar Cl content, however, differences were
not signi®cant. The same trend was monitored in
1996, after two ®fths (compare Table 1) of the total
amount had been added.
Table 2
Foliar element contents (mg g 1) of sugar maple overstory treesa
Year
Site
N
1996
Control 21.8
CaCl2 22.1
AlCl3 20.9
1997
Control 22.1
CaCl2 21.7
AlCl3 20.6
Ca
Mg
K
Cl
Al
4.67 a 0.77 a 8.56
6.03 b 1.09 b 8.59
4.20 a 0.69 a 8.12
0.37
0.54
0.41
0.045
0.039
0.045
5.09 a 0.92 ab 9.17
6.24 b 1.07 b 8.49
4.71 a 0.77 a 8.70
0.40 a 0.039 a
0.85 b 0.039 a
0.59 ab 0.052 b
a
Results of a ScheffeÂ's multiple range test are given only, if
differences were significant (values by the same letter are not
significantly different: Pˆ0.05; number of replications were 20 per
treatment and year, except four for Cl).
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
Aluminum chloride treatments resulted in increased
foliar Al contents of sugar maple in 1997, probably
due to low soil pH, while foliar Al contents were not
different between the control and CaCl2 sites. Foliar
Al contents were one order of magnitude lower than
for the macro nutrients and Al throughfall concentrations were found to be unimportant in the ion balance
of this study.
3.2. Atmospheric deposition
Bulk precipitation and throughfall ¯uxes during the
growing seasons of 1995±1998 are given in Table 3.
During the pre-treatment period (1995) no signi®cant
differences were observed between the control, CaCl2
and AlCl3 sites, enabling comparisons between the
treatments after the additions were done. Signi®cant
differences (Pˆ0.05) were recorded for Ca in 1996
(AlCl3<control), for Na in 1997 (CaCl2, control<AlC3)
79
and for Cl both in 1997 (control<CaCl2<AlCl3) and
1998 (control<CaCl2, AlCl3). Since only these few
comparisons (four out of 52; Table 3) between the
treatments revealed signi®cant differences between
solute ¯uxes in throughfall, four-year averages
(1995±1998) are given in Table 3 as well.
Throughfall ¯uxes of Ca, Cl and Al are shown for
the years 1995±1998 from 1 June to 30 September
(fully developed canopy) in Fig. 1. While Ca ¯uxes
were relatively constant over the years for the control
plots, the additions did change throughfall ¯uxes in
1996, but not signi®cantly in 1997. However, these
changes were in contradiction to the stated hypothesis
(see introduction section). Only in 1998 did Ca
throughfall ¯uxes agree with measured foliar Ca
contents (see Table 2 for 1996 and 1997). Chloride
throughfall ¯uxes indicate increased leaching of Cl,
when foliar levels are elevated. Aluminum throughfall
¯uxes are not as accurate, because samples were
Table 3
Mean fluxes of solutes (mmolc m 2) from 1 June to 30 September in bulk precipitation (open) and throughfall at untreated (control) and
treated (CaCl2. AlCl3) sugar maple stands and precipitation amount (H2O, mm)a
Year
Site
Ca
Mg
K
Na
NH4
NO3
SO4
Cl
H‡
Alkal
WeaAc TotAc
H2 0
1995
Open
Control
CaCl2
AlCl3
1.30
9.33
8.77
8.58
0.47
4.59
4.22
4.43
0.55
14.78
12.77
15.63
0.77
1.00
0.97
1.04
7.38
5.85
6.29
6.04
10.20
10.29
10.28
10.48
21.14
22.65
22.46
24.01
1.64
2.47
2.35
2.62
29.68
10.78
12.33
11.12
0.00
1.84
1.19
1.53
17.59
15.26
13.58
16.91
47.42
24.25
28.75
29.09
444.5
375.7
387.0
398.8
1996
Open
Control
CaCl2
AlCl3
1.11
9.16 b
8.13 ab
7.34 a
0.80
5.09
4.49
4.29
0.50
18.24
16.69
16.38
0.66
0.83
0.73
0.77
5.52
5.18
6.04
4.97
8.88
6.94
7.01
7.89
18.49
21.44
20.11
21.32
1.36
2.35
2.72
2.28
18.22
5.57
6.01
6.45
0.23
3.89
4.41
3.61
14.99
21.16
20.36
20.09
33.21
26.73
26.37
26.54
475.1
413.9
404.5
415.8
1997
Open
Control
CaCl2
AlCl3
1.79
9.62
9.07
9.01
0.89
5.18
4.66
4.99
0.84
16.51
14.21
16.88
1.03
1.17 a
1.15 a
1.67 b
6.40
5.59
5.67
5.30
10.52
10.75
11.23
12.05
21.02
23.61
23.18
24.60
1.64
2.65 a
3.47 b
4.38 c
24.26
11.99
13.99
13.44
0.06
1.02
0.72
1.15
14.88
21.72
22.62
20.92
39.15
33.72
36.61
34.36
441.4
390.4
402.3
400.9
1998
Open
Control
CaCl2
AlCl3
2.18
9.03
9.98
8.22
1.23
4.04
4.00
3.75
1.42
18.18
17.67
18.56
0.94
0.70
0.75
0.81
8.40
2.81
3.61
2.37
10.07
6.68
6.77
6.46
21.46
22.09
21.43
22.44
2.05
2.67 a
3.84 b
4.02 b
24.41
8.83
8.39
9.67
1.75
4.56
5.67
4.17
6.98
18.67
20.17
17.65
29.98
27.50
28.56
27.33
614.3
552.3
542.1
548.1
1995±1998
Open
Control
CaCl2
AlCl3
1.60
9.29
8.99
8.29
0.85
4.73
4.34
4.36
0.83
16.93
15.34
16.86
0.85
0.93
0.90
1.07
6.92
4.86
5.41
4.67
9.92
8.67
8.82
9.22
20.53
22.45
21.80
23.09
1.67
2.53
3.09
3.33
24.14
9.29
10.18
10.17
0.51
2.83
3.00
2.61
13.61
19.20
19.18
18.89
37.44
28.05
30.07
29.33
493.8
433.1
434.0
440.9
a
Weak acidity (weaAc, undissociated) was determined by substracting the free acidity (H‡, measured as pH) from total acidity (totAc,
measured by titration). This difference doesn't match excactly for 95, since pH was measured for all sites, but totAc and alkalinity (alkal) only
for one site out of four replications per treatment. A one way ANOVA (factor treatment) was done for all throughfall fluxes of each year
separately and results of a ScheffeÂ's multiple range test are given only (Backhaus et al., 1994), if differences were significant (values by the
same letter are not significantly different, Pˆ0.05).
80
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
Fig. 1. Mean throughfall fluxes of Ca, Cl and Al from 1 June to 30 September of each year. Standard errors are given for fluxes of Ca and Cl
(four replications per treatment), but not for Al fluxes, since samples were pooled within treatment and month before Al analysis.
pooled before analysis. However, a two way ANOVA
(factors year and treatment; 4 monthly values per
treatment were used as replicates) indicated that the
factor year (1998, 1996<1995) surpassed effects of the
Al treatment (not signi®cant).
Mean solute ¯uxes in NTF are given for the control
sites in Table 4 and differences between the years were
tested by a ScheffeÂ's multiple range test. The use of NTF
instead of TF for comparisons between years seems
more appropriate, because effects of year-to-year
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
81
Table 4
Mean net throughfall fluxes (throughfall flux minus bulk precipitation flux) for solutes (mmolc m 2) from 1 June to 30 September (1995±
1998) at the control sitesa
Year
Ca
1995
1996
1997
1998
8.03
8.05
7.83
6.85
Mg
a
a
a
a
4.12
4.29
4.29
2.81
K
b
b
b
a
14.23
17.74
15.68
16.75
Na
a
b
ab
ab
0.24 b
0.17 b
0.14 b
0.24 a
NH4
1.53
0.33
0.81
5.59
NO3
b
b
b
a
0.09
1.94
0.23
3.38
SO4
b
a
b
a
1.51
2.95
2.60
0.63
H‡
Cl
ab
b
ab
a
0.82
0.99
1.01
0.62
a
a
a
a
18.90
12.65
12.27
15.58
a
c
c
b
a
A massive ice storm struck the study area in January 1998, reducing the average leaf area index (LAI) from approximately 6.5 to 4.7. A
severe hail storm on 24 August 1998 further reduced LAI by 1.0±1.4 units. A ScheffeÂ's multiple range test was performed to test differences
between the years (values with the same letter are not significantly different, Pˆ0.05).
variations in amounts and patterns of precipitation
are minimized.
4. Discussion and conclusions
This study revealed interesting effects of CaCl2 and
AlCl3 treatments on foliar nutrition of sugar maple and
subsequently on the chemistry of throughfall. Additions of 10 g Ca m 2 (500 mmolc Ca m 2) represented the estimated loss from soil exchange (upper
mineral soil) during the last 30 years due to acidic
deposition. The total amounts for Ca, Al and Cl were
the same on an equivalent basis.
4.1. Foliar analysis
Calcium additions increased foliar Ca contents
signi®cantly (Table 2). This fact is not surprising,
since Ca uptake into roots occurs principally by
passive movement in the mass ¯ow of soil water
driven by transpiration (McLaughlin and Wimmer,
1999). Increased Al tended to decrease foliar Ca
content. Aluminum induced Ca de®ciency is well
documented (e.g. Rost-Siebert, 1985) and explained
by the fact that Al interferes with Ca uptake and root
growth. The increase of Ca foliar content at the control
sites from 1996 (4.7 mg g 1) to 1997 (5.1 mg g 1) is
in accordance with observed large year-to-year differences at the HBEF (Likens et al., 1998). These authors
also reported that Ca foliar contents of sugar maple in
the forest west of W6 were higher in 1965
(6.0 mg g 1) than averages for 1992±1995
(5.2 mg g 1) and that foliar Ca content of sugar maple
at high elevations (715 m) was signi®cantly lower
(3.8 mg g 1) than in low and mid-elevations
(5.9 mg g 1). Hence, we conclude that Ca supply of
sugar maple at the high elevation sites of this study is
below the optimum. In addition, Ca de®ciency is
suggested in these trees by the high rates of increase
(29% for 1996) in foliar Ca content resulting from
relatively low amounts of Ca fertilization. The nitrogen/calcium ratio (4.3±4.6; Table 2) for the control
sites is within the `harmonious range' (2±7; Stefan and
FuÈrst, 1998), rejecting the hypothesis of N induced Ca
de®ciency via enhanced atmospheric N deposition
(e.g. Gundersen, 1998) and supporting the hypothesis
of long-term loss of Ca from the ecosystem via natural
or anthropogenic disturbances (e.g. forest harvest,
acid rain) on these base poor soils of the HBEF
(Likens et al., 1996, 1998).
Although Mg was not added with the treatment,
foliar Mg contents were signi®cantly higher for the
CaCl2 treated stands than for the control and AlCl3
sites (Table 2). We don't know, whether this increase
was caused by positive impact of the treatment on the
root system and consequently increased uptake, by
higher Mg soil solution concentrations due to soil
exchange reactions or by elevated soil pH (depression
of Mg uptake by H‡; Marschner, 1986).
Foliar Cl contents re¯ected plant uptake (Table 2).
According to Marschner (1986), chloride is readily
taken up by plants and the mobility of Cl is high, both
in short- and long-distance transport. Since Cl and
bromide have similar physiochemical properties, substitution of Cl by Br is of no practical difference,
except that Cl is much more abundant in forest
ecosystems than Br. Hence, signi®cant uptake of
experimentally added Br by a northern hardwood
forest at the HBEF (Berger et al., 1997; Berger and
82
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
Likens, 1999) is in accordance with the observed Cl
uptake of this study.
4.2. Atmospheric deposition
Average precipitation amounts and bulk precipitation ¯uxes (1995±1998, Table 3) were in the same
range as reported by Lovett et al. (1996) for the same
area (1989±1992) for all major solutes, except SO4
and H‡ ¯uxes which were 30 and 40% lower in this
study. This decrease is undoubtedly due to lower
inputs of SO4 and H‡ in recent years in atmospheric
deposition (Likens and Bormann, 1995). Different
stands and elevations had lower solute throughfall
¯uxes as well (e.g. mean Ca throughfall ¯uxes of this
study were about 50% of ¯uxes measured in a mature
hardwood forest; Lovett et al., 1996).
A massive ice storm struck the study area in January
1998, reducing the average leaf area index (LAI) from
approximately 6.5 to 4.7 and a severe hail storm on 24
August 1998 further reduced LAI by 1.0 to 1.4 units
(Fahey, personal communication). Since the bulk
samplers were not moved during the experimental
period this research represents a case study for evaluating effects of LAI on solute NTF. In fact in 1998,
reduced LAI affected NTF of most solutes signi®cantly (Table 4). Lower NTF for Mg, Na and NH4 in
1998 were probably caused both by the reduced
®ltering capacity of the canopy and lower foliar
leaching rates due to reductions of LAI by up to
50%. Thus, the 1998 data are not taken into consideration for further discussions of differences between
the treatments.
4.3. Factors regulating throughfall flux
Regression analysis on the event-to-event variation
in NTF is used in this study to resolve the dry
deposition (DD) and canopy exchange (CE) components (see Eq. (1)), as proposed by Lovett and Lindberg (1984). This approach is based on the hypothesis
that for single-event collections of throughfall, the dry
deposition component of the NTF is correlated to the
length of the dry period before the event (antecedent
period) and the canopy exchange component is correlated with the amount of precipitation of that event.
Multiple linear regression is performed using the NTF
value for each event as the dependent variable and the
antecedent period and the precipitation amount as
independent variables. Several assumptions and
caveats have been discussed by Lovett and Lindberg
(1984); Lovett et al. (1996), and Berger and Glatzel
(1998).
To be consistent with Lovett et al. (1996), concentrations of H‡ and a particular solute in the incident
precipitation were added, which increased signi®cance of the overall regressions in most cases. Thus,
the regression equations were of the form:
NTFx ˆ a ‡ b1 A ‡ b2 P ‡ b3 CH‡ ‡ b4 Cx
(2)
(see abbreviations and units on Table 5)
The regression results indicated that Eq. (2)
explained a major portion of the variance in NTF
for most sites and solutes (Table 5). Unlike the results
of Lovett and Lindberg (1984) and Berger and Glatzel
(1998), many of the intercept terms in this analysis
were signi®cant. However, these results correspond
with those of Lovett et al. (1996) for comparable sites
at the HBEF. They conclude that the apparent intercept
may be a result of the nonlinearity in the relationship
with independent variables, although this nonlinearity
was not evident in plots of NTF versus the independent
variables in both studies.
4.3.1. Dry deposition
The antecedent period (A) term was highly signi®cant for Ca, Mg and K, indicating that dry deposition
or other processes that cause accumulation of substances on the canopy or in the collectors between rain
events do contribute signi®cantly to the NTF. This
result contrasts to Lovett et al. (1996), who used the
same approach and concluded that dry deposition does
not play a major role in controlling the chemistry of
throughfall at HBEF (compare Likens et al., 1998).
Given the coef®cients of Table 5 for A and P (see
below), measurements of the days of rain-free weather
and amount of rainfall, the relative contributions of
dry deposition and canopy exchange can be estimated.
By doing so only for the single rain events (1996±
1997) of this study, dry deposition of base cations
amounts to 24±45% (Ca 38±45%, Mg 24±37%, K 41±
44%) and the remainder is attributed to foliar leaching
(additional positive cation exchange was caused by
precipitation chemistry). The contribution of dry
deposition to NTF for Ca (in the same range as
measured adjacent to a lime quarry in Austrian oak
Table 5
Number of observations (n), adjusted coefficients of determination (R2), and regression coefficients for single event regressions for untreated (control) and treated (CaCl2, AlCl3)
sugar maple stands during the treatment period (1996±1997) as well as for all sites during the entire experimental period (1995±1998)a
Site
n
Parameter
2
Ca
Mg
***
K
***
Na
***
NH4
**
NO3
**
SO4
H‡
Cl
**
***
Alkal
***
WeaAc
**
TotAc
32
R
a
A
P
CH‡
Cx
0.88
397***
41.6***
22.1***
2.4***
10.1**
0.85
299***
16.3***
12.0***
1.3***
40.2***
0.83
1068***
99.8***
54.4***
5.4***
62.6**
ns
ns
ns
ns
ns
ns
0.33
417***
ns
16.4**
3.3**
8.5*
0.32
ns
38.1**
ns
ns
ns
0.29
ns
ns
ns
ns
ns
0.58
162**
12.0***
7.4**
0.4*
7.1*
0.91
697***
41.1***
33.4***
8.0***
ns
ns
ns
ns
ns
ns
0.41
ns
215.4***
ns
ns
12.2***
0.29**
ns
174.2**
61.0*
ns
12.1**
CaCl2
96±97
32
R2
a
A
P
CH‡
Cx
0.87***
384***
33.8***
21.8***
2.4***
12.8***
0.84***
303***
10.7**
11.8***
1.1***
50.3***
0.84***
1036***
79.2***
44.2***
4.4***
125.7***
ns
ns
ns
ns
ns
ns
0.37**
395***
ns
15.9**
3.8**
8.2*
0.36**
ns
42.2**
ns
ns
ns
0.43**
ns
ns
ns
ns
ns
0.40**
193*
12.2**
10.5**
0.7*
ns
0.85***
590***
46.7***
25.1***
6.8***
ns
ns
ns
ns
ns
ns
0.36**
ns
282.7***
ns
ns
17.6***
0.31**
1626*
236.4**
87.9*
ns
17.6***
AlCl3
96±97
32
R2
a
A
P
CH‡
Cx
0.75***
344***
31.3***
21.8***
2.6***
ns
0.78***
254***
9.0*
12.4***
1.4***
31.3***
0.77***
967***
86.6***
53.9***
5.6***
46.4*
ns
ns
ns
ns
ns
ns
0.53***
455***
15.3*
15.1**
ns
ns
0.33**
ns
33.5**
ns
ns
ns
0.21*
ns
ns
ns
ns
ns
0.29*
389*
ns
19.6**
1.3*
19.2
0.70**
775***
50.8***
35.8***
7.4***
ns
ns
ns
ns
ns
ns
0.49***
ns
228.8***
52.9*
ns
15.1***
0.36**
1557**
175.2**
87.6**
ns
14.9***
Mean
95±98
192
R2
a
A
P
CH‡
Cx
0.86***
284***
35.3***
18.9***
2.5***
3.8**
0.67***
111***
11.2***
8.0***
0.9***
16.5***
0.70***
403***
83.4***
29.9***
2.6***
24.7**
0.21***
8**
0.5*
0.5***
0.1***
0.9**
0.43***
340***
ns
13.1***
2.6***
4.9**
0.20***
ns
15.6***
ns
2.5**
5.6**
0.26***
67*
ns
11.6***
ns
ns
0.30***
56**
9.6***
4.4***
ns
ns
0.85***
493***
39.9***
28.6***
7.2***
0.09**
125***
ns
3.6*
0.7**
ns
0.39***
ns
208.2***
31.9**
ns
11.9***
0.23***
651**
48.3***
53.3***
4.2*
10.3***
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
Control
96±97
a
Regressions are of the form NTFxˆa‡b1A‡b2P‡b3CH‡‡b4Cx; NTFx, net throughfall of element x (mmolc m 2); a, intercept term; A, antecedent period (d); P, precipitation
amount (mm); CH‡ and Cx are the concentrations (mmolc l 1) of H‡ and solute x in precipitation. Units of coefficients are mmolc m 2 per day for A (representing mean dry
deposition rates) and mmolc m 2 per mm for P (representing mean canopy exchange rates) and mmolc m 2 per mmolc l 1 for CH‡ and Cx (representing effects of acid precipitation
on the NTF). Significance of overall regression and individual coefficients. ns, P>0.05, * P<0.5, ** P<0.01, *** P<0.001.
83
84
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
stands by Berger and Glatzel, 1998), Mg and K is
surprisingly high. We believe that dry deposition was
overrated, because samples were collected weekly,
systematically overestimating dry periods between
single rain events used for the performance of this
technique (e.g. the mean dry period between the
seleced single rain events was 6.5 days, while the
mean long-term dry period amounts 2±3 days). Other
ions for which the NTF would be expected to re¯ect
dry deposition, e.g. Na and SO4, have small NTF in
these stands (compare Table 4, NTF from June to
September). Hence, the given coef®cients of Table 5
are not useful for upscaling (e.g. to total growing
season) but provide relative comparisons between
the treatments.
Regression analyses (Table 5) showed high dry
deposition rates for weak (undissociated) acidity,
which was determined by subtracting the free acidity
(measured as pH) from the total acidity (measured by
titration). However, we do not believe that the forest
gained weak acidity via dry deposition, but we consider weak acidity an internal source. One reason, why
this method was not useful for analyzing weak acidity
data, might be that organic anions migrate to the
surface of the leaf in the transpiration stream (exudations) during dry periods (Cronan and Reiners, 1983;
Reiners et al., 1986). Another reason might be that
natural weathering of the cuticle during dry periods
produces organic material on the surface which is
washed off by the following rain event (Likens
et al., 1994). Both processes would show up as dry
deposition in this analysis, although they represent
internal sources. To our knowledge this technique has
not been used before regarding weak acidity NTF.
4.3.2. Canopy leaching
The precipitation amount term (P) was signi®cant
for all base cations except Na, indicating that canopy
leaching is a dominant process controlling event-toevent variation in NTF (Table 5). Leaching rates for
Ca (see Fig. 2), Mg, and K were not signi®cantly
different between the treatments. Leaching coef®cients indicate a trend of reduced leaching for Mg
(despite elevated foliar Mg contents, Table 2) and K on
the CaCl2 sites during 1996 and 1997. This trend is
supported by throughfall ¯uxes for all base cations
(Table 3; 1996±1997). The performance of a CoANOVA for solute NTF with solute 1995 NTF as
Fig. 2. Regression coefficients for P (see Table 5, 1996±1997) for
regressions of Ca and Cl, representing mean canopy exchange rates
(positive values indicate leaching). Dry deposition rates for weak
acidity (coefficients for A) were converted into canopy exchange
rates (adjusted by given significant terms for P of the model),
assuming only internal sources for weak acidity (see text). Error
bars are S.D.
covariate did not change statistical differences
between the treatments given for solute TF
(Table 3), except for 1998, when Ca NTF were signi®cantly higher for the CaCl2 sites than for the
control and AlCl3 sites.
Hence, our stated hypothesis of increased Ca foliar
leaching due to elevated Ca foliar contents must be
rejected for the treatment period (1996±1997), there
even seems to be an opposite trend. Because Ca is
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
important for maintenance of cell walls (Marschner,
1986), we suggest that Ca supply to Ca de®cient sugar
maple trees protects the foliage from increased leaching of Ca, e.g. due to acidic deposition. When Ca is
supplied in excessive amounts, Ca foliar leaching will
occur in accordance to Ca foliar contents, as measured
by Berger and Glatzel (1998) for oak stands on
calcareous soils. The fact that Mg foliar leaching
was slightly reduced despite the coupled increase of
Mg together with Ca in the green foliage supports this
new hypothesis.
Calcium throughfall ¯uxes from 1 to 15 October
1997 (leaf senescence and beginning of litter fall, no
October data were available for 1996 and 1998) were
higher for the CaCl2 treated stands than for the control
and AlCl3 treated stands, despite the opposite trend for
the fully developed canopy from 1 June to 30 September. Hence, we conclude that during leaf senescence, when Ca foliar contents are elevated (Ryan and
Bormann, 1982), Ca is present in a more leachable
form. The deterioration of a broad spectrum of essential physiological processes in Ca-de®cient plants has
led to recognition of the important role of Ca supply in
delaying plant senescence (Poovaiah, 1988). Symptoms of Ca de®ciency that are commonly associated
with senescence include loss of protein, loss of chlorophyll, and reduced integrity of cell membrane and
cell wall (McLaughlin and Wimmer, 1999). The delay
of plant senescence in the CaCl2 treated sugar maple
stands might be an explanation of the fact that
increased Ca foliar levels did not cause higher but
lower Ca leaching of the non senescent foliage. The
same explanation is attributable to the observation that
during the growing season of 1998, Ca foliar leaching
was in accordance to Ca foliar contents. The massive
ice storm in January 1998 (branches of partly damaged
trees died after spring ¯ush) and the severe hail storm
in August 1998 degraded the structural material of the
foliage during the growing season (June Ð September) similar to autumnal leaf senescence. Lovett and
Hubbell (1990) also measured higher (not signi®cant)
leaching of Ca and Mg in damaged sugar maple
branches than in undamaged branches. Interpretations
are dif®cult, however, because we do not know the
fraction of Ca which might be insoluble-stored as Ca
oxalate crystals or Ca pectate as reported for conifer
needles (Fink, 1991). According to DeHayes et al.
(1997, 1999) the dominant, but insoluble, extracellular
85
Ca pool re¯ected in measured total foliar Ca contents
is not a meaningful surrogate for the physiologically
important and labile pool associated with the plasma
membrane-cell wall compartment of red spruce mesophyll cells.
Canopy exchange coef®cients were positive (leaching) for Cl, indicating signi®cant impacts of the Cl
treatments on Cl foliar leaching rates (Table 5, Fig. 2).
As indicated by measured Cl throughfall ¯uxes (compare Fig. 1) the Cl leaching coef®cient was higher for
the AlCl3 treated sites than for the CaCl2 treated sites,
although Cl foliar contents (Table 2) showed the
opposite trend. However, this fact strengthens the
above hypothesis that Ca supply to Ca de®cient sugar
maple trees protects the foliage from increased leaching of Ca and other elements, e.g. Cl, due to Ca
improved integrity of cell membrane, cell wall formation and maybe cuticle formation. Plant uptake of
experimentally added Cl into the foliage of sugar
maple and increased leaching of Cl via throughfall
are especially interesting, since such studies are rare
and contradictory. For example, Ulrich (1983)
assumed that canopy exchange for Cl was zero, however, Kazda (1990) attributed one-third of the Cl in the
stem¯ow of a beech stand to crown leaching.
As discussed above, we do not believe that the
forest gained weak acidity via dry deposition, but
we consider weak acidity an internal source. Dry
deposition rates for weak acidity (coef®cient for A)
were converted into canopy exchange rates (adjusted
by signi®cant terms for P of the model) by multiplying
by the days of rain-free weather and dividing by the
amount of precipitation (only for the selected single
rain events of 1996±1997, Fig. 2). Differences
between the treatments are not signi®cant but leaching
of organic anions tends to increase with increasing
foliar Ca content. Organic anions are frequently
assessed by the anion de®cit of a solution, but this
approach involves large potential errors when the
de®cit is small compared with the cation and anion
totals (Lovett et al., 1985). Anion de®cit throughfall
¯uxes were between 8.7 and 13.4 mmolc m 2 per
growing season (1996±1997) and amounted to 43±
63% of weak acidity throughfall ¯uxes. The anion
de®cit TF was signifcantly lower for the AlCl3 treated
sites than for the control sites (1996: P<0.10; 1997:
P<0.05) supporting the observed trend for roughly
calculated weak acidity leaching rates of Fig. 2. Weak
86
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
acidity represented 38±45% of the total acidity in bulk
deposition, but increased to 61±79% in throughfall
(1996±1997, compare Table 3). These results are in
the same range as reported by Lovett et al. (1985) for
deciduous forest stands in eastern Tennessee (weak
acidity as percent of total acidity was 44% in wet
deposition and 58±66% in throughfall). It may not be
justi®ed to interpret these weak acidity data, since
none of the results are statistically signi®cant. However, when increased leaching of protonated organic
anions reduces H‡ driven cation exchange, the new
hypothesis would be supported, that is, Ca supply
diminishs base cation foliar leaching due to acidic
rain.
4.3.3. Precipitation acidity
All base cations except Na showed signi®cant
positive coef®cients for precipitation H‡ concentrations (Table 5). This ®nding indicated that, in general,
increasing acidity of precipitation caused increased
leaching of Ca, Mg and K. The coef®cients of the CH‡
term (i.e. amount of Ca, Mg or K leached per unit
increase in H‡ concentration in precipitation) did not
differ signi®cantly between the treatments but was
different between the cations in decreasing order:
K>Ca>Mg (Fig. 3). This surprising result was con-
®rmed by regressing Ca NTF against H‡ NTF, using
all rain events during the growing seasons 1996±1997
(nˆ112 per treatment): for all base cations regression
coef®cients were more negative for the control than
for the CaCl2 treated sites (signi®cant coef®cients for
control, CaCl2 and AlCl3 sites: Ca 0.45, 0.38, and
0.36; Mg 0.23, 0.21, and 0.23; K 0.81,
0.73, and 0.79). Again, there seems to be a trend
of reduced acidity effects on cation leaching for the
CaCl2 treated sites. Experimental spraying of arti®cial
rain on plants has shown acid-induced leaching, but in
general effects on leaching were more pronounced for
Ca and Mg than for K or Na (e.g. Lovett and Hubbell,
1990; Sayre and Fahey, 1999). To our knowledge the
only demonstration that foliar leaching of cations
increases in response to increases in precipitation
acidity in naturally occurring rain events was given
by Lovett et al. (1996). However, these authors found
signi®cant acidity effects on leaching of Ca and Mg
only, and not, as we report, for K.
4.3.4. Precipitation ion concentrations
Calcium, Mg and K showed signi®cant ef¯ux from
the canopy with increasing ion concentration in (bulk)
precipitation. While the relative contribution of the Cx
term (see Eq. (2)) to NTF is small for Ca (<9% of
Fig. 3. Regression coefficients for the precipitation acidity term (b3) in Eq. (2), for regressions of Ca, Mg and K. Units for the coefficients are
(mmolc m 2 of Ca, Mg or K) (mmol H‡ l 1) 1. Error bars are S.D.
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
leaching term P) and moderate for Mg (<40% of P),
ef¯ux from the canopy for K was in¯uenced almost in
the same range (up to 90%; CaCl2 treatment 1996±
1997) by K bulk precipitation chemistry (Cx term) as
by the precipitation amount (P term). These high
contributions of ion bulk precipitation chemistry to
NTF is probably caused by the fact that bulk deposition collectors are continuously open, collecting substantial amounts of atmospheric particles, especially
during the long dry periods between the single rain
events (the average antecedent period was 6.5 days for
1996±1997, however, usually it rains 2±3 times a week
at Hubbard Brook; Likens and Bormann, 1995). Particles can be formed on plant surfaces by degradation
of leaf tissue or by migration of salts from the interior
to the exterior of leaves (e.g. Lovett and Lindberg,
1984). These aerosol particles can be transported by
wind and deposited to the bulk deposition collectors in
clearings within the forest. Volume weighted mean
bulk concentration of K was 2.7 times higher than the
mean value of wet only collectors at HBEF (Likens
et al., 1994). They also concluded that some bulk
deposition of K likely represents internal cycling (e.g.
pollen or plant debris from local vegetation) rather
than a true ecosystem input.
In all cases (P<0.001), H‡ showed increased retention (more negative NTF) with increasing concentration of H‡ in precipitation (Table 5). This ®nding
suggests that the retention of H‡ responds to the
concentration gradient between incoming precipitation and absorption sites on or in the plants. As will be
discussed below, H‡ retention is mostly attributable to
passive cation exchange on the leaf surface or interior
(see Lovett et al., 1996).
Increasing NTF for weak acidity with decreasing
solute concentration in precipitation is contradictory
to meachnisms of dry deposition (as falsely given by
the model, see above) but supports the assumption that
weak acidity in NTF represents an internal source.
4.4. Cation exchange and effects of
acidic deposition
Regression analyses of NTF were useful for relative
comparisons between the chemical manipulated study
sites. However, canopy exchange rates of these calculations were not used for estimating total amounts of
canopy exchange, since the selected single rain events
87
for this technique did not properly represent the entire
growing season. Hence, Na was used to calculate
particulate interception deposition (Ulrich, 1983)
and the mean ratio for Na (NTF/bulk precipitation
¯ux; mean of all sites during 1996±1997) was used for
all other elements, assuming the same deposition
velocity for all atmospheric constituents (except that
interception (dry) depostion of alkalinity and weak
acidity was assumed to be zero). Since dry deposition
does not appear to play a major role at the HBEF
(Lovett et al., 1996; compare small NTF for Na and
SO4 in Table 4, which are expected to re¯ect dry
deposition), the error of this assumption is considered
small. According to Ulrich (1983) canopy exchange
for Na and SO4 was assumed zero. This method
provided our best estimate for total amounts of
canopy exchange (Table 6) for the growing seasons
of 1996±1997.
Table 6
Mean canopy exchange (mmolc m 2) per growing season (1 June
to 30 September) for the years 1996 and 1997, estimated according
to Ulrich (1983)a
Control
SO4
NO3
Cl
Alkalinity
Weak acidity
H‡
K
Ca
Mg
Na
NH4
Cation leaching
Anion leaching
Cation exchange (I)
Cation retention
Anion retention
Cation exchange (II)
0.0
3.3
0.6
2.3
6.5
19.8
16.6
7.6
4.1
0.0
2.0
28.2
9.4
18.8
15.4
3.3
12.1
CaCl2
AlCl3
0.0
3.0
1.2
2.4
6.6
19.5
14.6
6.8
3.5
0.0
1.6
24.9
10.2
14.7
14.5
3.0
11.5
0.0
2.1
1.5
2.2
5.6
18.2
15.8
6.4
3.6
0.0
2.3
25.7
9.3
16.5
14.9
2.1
12.8
a
Positive values represent leaching; negative values represent
retention. Dry deposition of alkalinity and weak acidity to the
forest was set at zero, assuming only internal sources for these
compounds. AlkalinityˆHCO3 ‡weak bases. Anion leaching
includes weak acidity, assumed to leach as organic anion. Cation
exchange (I) was calculated as the difference between cation and
anion leaching, cation exchange (II) as the difference between
cation and anion retention. For calculating cation retention, H‡ was
adjusted for protonation of organic anions.
88
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
Charge balance consideration requires that the net
exchange of ions between the canopy and impinging
deposition be electrically balanced according to the
following equation (Lovett et al., 1985):
cation retention
anion retention
ˆ cation leaching
anion leaching
(3)
Taken individually, each side of (3) represents the
magnitude of the cation-exchange reaction in the
canopy. We give in Table 6 all of the terms of (3),
from which we calculate both estimates: cation
exchange (I)ˆcation leaching anion leaching; cation
exchange (II)ˆcation retention anion retention. Calculations were described by Lovett et al. (1985), but
bear repeating here: titrations were done on samples
purged with N2, so no H2CO3 is accounted for in the
weak acidity values. However, any HCO3 left in the
solution would be accounted for in the alkalinity
titrations. Organic anions appear in two forms in
TF: those that were protonated on contact with the
acidic rainfall appear in throughfall as weak acidity,
while those organic anions that remained unprotonated appear as alkalinity. The actual H‡ retention is
equal to the H‡ consumption indicated in Table 6,
minus the H‡ consumed in protonation of organic
anions.
These estimates of cation exchange (both sides of
Eq. (3)) with the total cation leaching (Ca‡Mg‡K)
are plotted in Fig. 4. Cation exchange accounted for
43±67% of the total cation leaching from these canopies, which is in the same range (40±60%) as estimated for deciduous stands in Tennessee by Lovett
et al. (1985). Differences between the treatments were
not signi®cant, but the estimated amount of leached
cations per growing season was lowest for the CaCl2
Fig. 4. Mean canopy exchange from 1 June to 30 September 1996 and 1997. Positive values represent leaching, negative values represent
retention.
T.W. Berger et al. / Forest Ecology and Management 149 (2001) 75±90
89
treated sites (24.9 vs. 28.2 for control and 25.7 for
AlCl3; data in mmolc m 2) supporting the above
®ndings. Retention of deposited H‡, assuming that
all weak acid leaching involved consumption of H‡,
accounted for 87, 90 and 85% of the total cation
retention for the control, CaCl2 and AlCl3 treated
sites, respectively. In general, then, about half of
the cation leaching from these sugar maple canopies
is attributable to a cation-exchange reaction driven
almost entirely by incoming H‡. We hypothesize that
in canopies unimpacted by acidic deposition, leaching
of Ca, Mg and K would be balanced by anion leaching
and any retention of H‡ and NH4 would be balanced
by NO3 (see also, Lovett et al., 1985).
Acknowledgements
4.5. Acidity and alkalinity in the canopy
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