Forest Ecology and Management 172 (2003) 315±325
Dissolved organic carbon in precipitation, throughfall, stem¯ow,
soil solution, and stream water at the
Guandaushi subtropical forest in Taiwan
Chiung Pin Liua, Bor Hung Sheub,*
a
Division of Watershed Management, Taiwan Forest Research Institute, 53 Nanhai Rd., Taipei 100, Taiwan
b
Department of Forestry, National Chung Hsing University, 250 Kuokwang Rd., Taichung 40227, Taiwan
Received 25 April 2000; received in revised form 17 October 2001; accepted 17 October 2001
Abstract
The concentration and ¯ux of dissolved organic carbon (DOC) were measured in precipitation, throughfall, stem¯ow, soil
solution, and stream water for three types of subtropical forest stands, a Chinese ®r (Cunninghamia lanceolata) plantation, a
secondary hardwood, and a natural hardwood stand in Guandaushi forest in central Taiwan from January 1998 to December 1998.
The mean DOC concentration in precipitation was 4.7 mg l 1. However, in the rain passing through the tree canopies and barks as
throughfall and stem¯ow, the mean concentrations were 7.0 and 30.8, 9.9 and 10.0, and 8.3 and 7.2 mg l 1 in the Chinese ®r
plantation, the secondary hardwood, and the natural hardwood, respectively. Mean DOC concentrations in soil solution were
lower in the Chinese ®r plantation than both hardwoods, and decreased with depth of soil pro®les. Stem¯ow DOC ¯ux
(132.4 kg ha 1) in the Chinese ®r plantation was much higher than the other hardwood stands (15.3 and 6.7 kg ha 1 in secondary
and natural hardwood, respectively). The monthly variations of DOC concentrations were very similar in throughfall and stem¯ow
at the three stands, showing an increase in the beginning of the growing season in April. No clear monthly variations in soil
solution DOC concentrations (mean from 3.2 to 21.3 mg l 1 in different stands and for different depths) were found in our study.
DOC concentrations (mean 2.7 mg l 1) in the stream draining the watershed were higher in spring and in winter.
# 2002 Elsevier Science B.V. All rights reserved.
Keywords: Dissolved organic carbon; Throughfall; Stem¯ow; Soil solution; Stream water; Subtropical forest
1. Introduction
DOC (dissolved organic carbon) is an important
parameter of water quality. In soil and in stream water,
DOC can reach concentrations of 50 mg l 1 or more
and may in¯uence water acidity, mobility and toxicity
of metals, and nutrient availability (Dalva and Moore,
1991). However, most of the research concerning
*
Corresponding author. Tel.: ‡886-4-22850134;
fax: ‡886-4-22873628.
E-mail address: [email protected] (B.H. Sheu).
DOC in terrestrial ecosystems has focused on solutions within the soil (Evans et al., 1988; Kaiser and
Zech, 1998; Nambu and Yonebayashi, 1999).
Recently, the concentrations and ¯uxes of DOC
measurement in throughfall have indicated that the
¯ows of carbon compounds through the canopies of
trees varies greatly among trees in the same plot and
within trees (Seiler and Matzner, 1995), among different species of trees (Currie et al., 1996; Inagaki et al.,
1995), and seasonally (McDowell and Likens, 1988).
In most cases, leaching, leaf washing, and volumes of
rain events are considered to be the primary factors
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 7 9 3 - 9
316
C.P. Liu, B.H. Sheu / Forest Ecology and Management 172 (2003) 315±325
in¯uencing variability in throughfall carbon concentration. Although, DOC in precipitation, throughfall,
stem¯ow, soil solution, and stream water have been
measured in several temperate and tropical forests all
over the world (Currie et al., 1996; Inagaki et al., 1995;
McDowell and Likens, 1988), little is known about
DOC ¯ux in subtropical forest ecosystems.
The monitoring survey for acid precipitation passage through canopies of subtropical forests in Taiwan
has been carried out (King and Shiue, 1992; Liu and
Sheu, 1996, 1997, 1999). From the data of these past
surveys, they found that the properties of the throughfall and stem¯ow were different among tree species. It
was also found that stem¯ow of Chinese ®r, which is
the popular conifer species planted in medium to low
elevation Taiwan arti®cial forest, had very low pH
(Liu and Sheu, 1999). The major factor controlling the
decrease in the pH of stem¯ow in coniferous stands
was thought to be organic matter leaching from tree
canopies and barks (Cronan and Reiners, 1983; Johnson and Lindberg, 1992). This DOC is not only
important in element cycling, but a large portion of
DOC is composed of organic acids ranges from simple
to complex humic and fulvic acids, which are important in mediating cation leaching, metal dissolution,
mineral weathering, and absorption±desorption of
acidic anions (Liechty et al., 1995). The objective
of the present study was to determine the concentration and monthly changes of DOC in one conifer and
two natural hardwood stands of a subtropical forest in
central Taiwan and to compare the data with other
temperate and tropical forests.
2. Methods
This study was carried out in a 47 ha watershed in
the Guandaushi forest, central Taiwan (Fig. 1). Altitude ranges from 1100 to 1700 m. The annual rainfall
ranges from 2300 to 2700 mm, with distinct rainy and
dry seasons. The mean maximum annual temperature
was 22.4 8C and the minimum was 9.8 8C during the
year of the study (1998).
Soil texture of Chinese ®r plantation is heavy clay,
secondary hardwood is light loam, and natural hardwood is light loam. Ion concentrations and cation
exchange capacity are higher in soil under natural
hardwood compared to the secondary hardwood or
Fig. 1. Location of the study site at Guandaushi Experimental
Forest in central Taiwan.
the Chinese ®r plantation. The ion concentrations
decrease and pH increases with increasing soil depth.
Soil pH is lower in the Chinese ®r plantation than in
secondary hardwood or natural hardwood (Sheu and
Guo, 1999).
Typhoons occur occasionally between June and
September bringing a high intensity of precipitation
and disturbance to the site. The site is a typical
subtropical mixed-hardwood forest in central Taiwan,
which is characterized by steep topographies, abundant riparian ferns, virgin hardwood forests, and
abundant epiphytes. The forests on the ridges have
been cut and planted with Chinese ®r.
Three adjacent stands of Chinese ®r (Cunninghamia
lanceolata) plantation, secondary hardwood, and natural hardwood in the same 47 ha watershed were
investigated. Both the hardwoods are the typical
Lauro-Fagaceae association of Taiwan. Lauracea
(15 species) and Fagaceae (14 species) are the major
families in this study area and they occupy 4.60 and
4.29% of the total forest composition, respectively.
From the results of matrix cluster analysis, the vegetation in the study area can be divided into seven forest
types. These are Helicia formosana, Litsea acuminata,
Chamaecyparis obtusa, C. lanceolata, Engelhardtria
roxburghiana±Cinnamomum randaience, Rhododendron formosanum and Pinus morrisonicola forest
types (Lu and Ou, 1996).
C.P. Liu, B.H. Sheu / Forest Ecology and Management 172 (2003) 315±325
Samples of precipitation were collected on a rain
event bases from January 1998 to December 1998
using three bulk precipitation collectors mounted on
the top of a 24 m tower. Each collector had two 19 cm
diameter polyethylene funnels connected to a 30 l
sampling bottle with black polypropylene tubes. All
sample bottles were rinsed with distilled water immediately after each collection, then 500 ml samples
were transported in a cooler to the analytical laboratory and stored at 4 8C until chemical analyses, usually
within 24 h of sampling.
Each throughfall collector consisted of three 19 cm
diameter polyethylene funnels mounted about 1 m
above the ground and arranged in a triangular shape;
the funnels were connected to a 30 l sampling bottle
with black polypropylene tubes. In order to keep out
leaves, small branches, and insects, 3 mm mesh plastic
screening was used to cover the funnels. Six such
throughfall collectors were installed randomly in each
stand. Samples of throughfall were collected at the
same time and in the same manner as samples of
precipitation.
The stem¯ow collector consisted of 4 and 1 cm o.d.
tygon tubing with a length of about 100 cm. The
tubing was split longitudinally, wrapped on a downward spiral around the tree bole, fastened with stainless steel staples and sealed to the bark with acrylic
caulk. On species with rough bark, the bark was
shaved to improve the seal while taking care not to
damage the cambium layer. The lower, unsplit end of
the tubing was inserted into a hole in the lid of a 30 l
plastic sampling bottle. Six trees of a stand were
chosen to collect stem¯ow in three stands at the same
time and manner as samples of precipitation and
throughfall collecting.
Samples in stream water were taken from above the
weir of the watershed on the same day as water
samples of precipitation, throughfall, and stem¯ow
collection. Lysimeters (porous cup ceramic tension
lysimeters) were installed at depths of 15, 30, and
60 cm at three locations in each stand to collect soil
solution. Prior to installation, lysimeters were acidwashed (10% HCl), rinsed copiously, and soaked in
deionized water for 3 days, the ®rst litter collected
from each lysimeter was discarded to allow the porous
cup to reach equilibrium with the soil solution (Debyle
et al., 1988). A vacuum (0.7 MPa) was applied to each
lysimeter following sample collection and typically
317
some vacuum was maintained until the next sample
collection. Each lysimeter typically collected soil
solution bi-weekly; only rarely (several times during
the study) were soil conditions dry enough that no
water was collected in one or more lysimeters.
All water samples were pre®ltered (Gelmanscience
GN-6 grid 0.45 mm sterilized ®lter paper) before
analyzing DOC and, thereafter, stored in the dark at
4 8C during the analysis of DOC. The concentrations
of DOC were determined as CO2 by catalytic combustion (High Temperature TOC, Elementar Analysensysteme GmbH, Germany). Samples were acidi®ed
with 2 N HCl and sparged with ultra zero grade CO2
free air to remove all inorganic carbon. Sparged
samples were combusted at 950 8C and CO2 detected
using a nondispersive infrared (NDIR) detector.
In the three stands of the watershed, Liu and Sheu
(1999) reported that the total throughfall was 77, 76,
and 74% of the precipitation in Chinese ®r plantation,
secondary hardwood, and natural hardwood, respectively. The total stem¯ow was 14, 5 and 3% of
precipitation, respectively. Based on their study and
the annual precipitation of 1998 (3107 mm), throughfall and stem¯ow are estimated to average approximately 2394 and 435, 2363 mm and 155, 2301
and 93 mm in Chinese ®r plantation, secondary hardwood, and natural hardwood, respectively. We used
these values to calculate the ¯ux of DOC in precipitation, throughfall, and stem¯ow by multiplying concentration and volume on an event basis from January
to October 1998. Using the same method, DOC output in 1998 was calculated using DOC concentration
in the stream and discharge of the weir. Duncan's
multiple range test was used for mean comparison if
the results of the F-test were signi®cant at the 5%
level.
3. Results
3.1. Concentrations and ¯uxes of DOC
DOC concentrations in precipitation averaged
4.7 mg l 1 and increased as precipitation passed
through forest canopies, with means of 7.0, 9.9, and
8.3 mg l 1 in a Chinese ®r plantation, a secondary hardwood, and a natural hardwood, respectively (Table 1).
The ¯uxes of DOC in precipitation, throughfall of
318
C.P. Liu, B.H. Sheu / Forest Ecology and Management 172 (2003) 315±325
Table 1
Concentrations and ¯uxes of DOC in precipitation, throughfall,
stem¯ow, soil solution, and stream water at the Guandaushi
subtropical forest during January±December 1998
Bulk precipitation
DOC (mg l 1)b
DOC
(kg ha
4.7 2.9 ef
142.8
Chinese-fir plantation
Throughfall
Stemflow
Soil-15a
Soil-30
Soil-60
7.0
30.8
8.8
7.7
3.2
2.5 def
17.3 a
14.5 cd
16.7 cd
2.9 ef
166.5
132.4
±
±
±
Secondary hardwood
Throughfall
Stemflow
Soil-15
Soil-30
Soil-60
9.9
10.0
15.5
13.5
8.5
4.0 c
4.7 c
11.6 b
7.7 b
6.2 cd
231.3
15.3
±
±
±
Natural hardwood
Throughfall
Stemflow
Soil-15
Soil-30
Soil-60
8.3
7.2
21.3
11.0
10.1
3.4 cd
4.1 cd
11.9 b
7.1 c
5.9 c
188.8
6.7
±
±
±
Stream water
2.7 1.9 f
1
per year)
25.0
a
Soil-15, -30, and -60: water sampled at depth in cm.
b
Means for each measurement at DOC concentration followed
by the same letter are not signi®cantly different at the 5% level
using Duncan±Waller multiple range test.
Chinese ®r plantation, secondary hardwood, and natural hardwood were 142.8, 166.5, 231.3, and 188.8
kg ha 1 per year, respectively.
DOC concentrations of stem¯ow were 30.8, 10.0,
and 7.2 mg l 1 in Chinese ®r plantation, secondary
hardwood, and natural hardwood, respectively, as
shown in Table 1. In Chinese ®r plantation, not only
the concentration but also the DOC ¯ux (132.4 kg ha 1
per year) in stem¯ow was much higher than the other
hardwoods (15.3 and 6.7 in secondary and natural
hardwood, respectively).
Mean values of DOC concentrations in soil water
collected from 15 cm deep soil of Chinese ®r plantation, secondary hardwood, and natural hardwood were
8.8, 15.5, and 21.3 mg l 1, respectively. All of these
values were higher than collections from 30 and 60 cm
depths with lower DOC concentration in Chinese ®r
plantation. The lowest DOC concentration was in
stream water. The DOC output in the stream water
of this watershed in 1998 was 25.0 kg ha 1.
3.2. Monthly changes of DOC concentration
during the sampling periods
Monthly changes were observed in the concentrations of DOC in precipitation, throughfall, stem¯ow,
and soil solutions (15, 30, and 60 cm) of Chinese ®r
plantation, secondary hardwood, natural hardwood,
and stream water in this watershed (Figs. 2±5). Concentrations were generally higher in throughfall, stem¯ow, and soil solution than in precipitation throughout
the year. The monthly variations of DOC concentrations were very similar in throughfall and stem¯ow,
showing an increase at the beginning of growing
season in April. Although, there was no clear monthly
variation to soil solution DOC concentrations in our
study, DOC concentrations tended to be higher in
spring (February and March). DOC concentrations
in the stream draining the watershed were higher in
the spring and winter.
4. Discussion
4.1. Concentrations and ¯uxes of DOC
In this study, the concentration of DOC entering the
ecosystems by precipitation was much higher relative
to the tropical forest of Luquillo Mountains of Puerto
Rico (1.0 mg l 1) and many temperate forests
(Table 2). Precipitation is a signi®cant source of
DOC for subtropical forest ecology, which may possess relatively high proportions of low molecular mass
organics (Likens et al., 1983). The ¯ux of DOC in
precipitation was also much higher than for other
forests. Because of the high proportion of simple
organic compounds and the possible inputs of a large
variety of organic compounds of anthropogenic origin
(Hoffman et al., 1980), DOC of atmospheric origin
may be ecologically much more signi®cant in the
subtropical forests, as in the Hubbard Brook landscape
(McDowell and Likens, 1988), than quantitative analysis alone would imply.
DOC concentrations increased as precipitation
passed through forest canopies. When compared with
other studies (Table 2), values of DOC in throughfall
C.P. Liu, B.H. Sheu / Forest Ecology and Management 172 (2003) 315±325
Fig. 2. Monthly precipitation and throughfall DOC concentrations and precipitation depth in three stands.
Fig. 3. Monthly stem¯ow DOC concentrations and precipitation depth in three stands.
319
320
C.P. Liu, B.H. Sheu / Forest Ecology and Management 172 (2003) 315±325
Fig. 4. Monthly DOC concentrations in soil solution and precipitation depth in three stands.
were similar or higher or lower to both of temperate
and tropical forests.
The ¯uxes of DOC in throughfall were higher in the
secondary hardwood stand than the other two stands.
Probably due to leaf leaching (Tukey, 1970) and leaf
wash (McDowell and Likens, 1988), DOC ¯uxes in
throughfall were higher than in precipitation.
The higher DOC ¯uxes of throughfall in these three
stands con®rms that throughfall may be an important
energy sources for microorganisms growing in the
forest ¯oor and may play an important role in allelochemical interactions between microorganisms and
trees, such as found from studies of carbon production
or retention in the forest canopy (Muller et al., 1968).
C.P. Liu, B.H. Sheu / Forest Ecology and Management 172 (2003) 315±325
321
Fig. 5. Monthly stream water DOC concentrations and discharge precipitation depth in three stands.
Additionally, recent investigations dealing with the
spatial or temporal variability in soil solution chemistry and ion ¯uxes in forest ecosystems have largely
attributed the origin of variation in soil solutions to
throughfall ¯uxes (Manderscheid and Matzner, 1995).
DOC concentration was the highest in stem¯ow of
Chinese ®r plantation. DOC concentration in stem¯ow
can be regulated by retention time when precipitation
was retained in bark. It implies that the DOC concentration from the stem¯ow is affected by different
bark morphology (Inagaki et al., 1995). Chinese ®r has
a multi-layered rough ®brous bark which could retain
precipitation longer than single-layered bark thus
leaching more DOC. Fan et al. (1999), reporting the
remarkable acidi®cation and nutrient enrichment of
rainwater ¯owing down the indented bark surfaces of
Chinese ®r, revealed that a larger part of the chemicals
added to intercepted precipitation was derived from
dry deposition rather than from leaching metabolites.
Therefore, dry deposition was the other factor for
in¯uencing the DOC concentration in the stem¯ow
of the Chinese ®r plantation. On the other hand, the
higher DOC ¯ux (142.8 kg ha 1 per year) of bulk (dry
and wet) precipitation recorded for this site area would
imply that organic carbon content in dry deposition is
very signi®cant. There is also the possibility that
higher DOC ¯ux in stem¯ow of Chinese ®r plantation
may be in¯uenced by insect herbivores such as aphids,
which excrete copious amount of honeydew. Gaze and
Clout (1983) showed that beech honeydew could
contain between 20 and 80 g of sugar per 100 g of
solution and, thus, in¯uence carbon availability (Stadler and Michalzik, 1998). The fourth possibility is
that the volume of precipitation entering the watershed
as stem¯ow in Chinese ®r plantation (14%) is much
higher than the other two hardwood stands (5 and 3%
in secondary and natural hardwood stand, respectively) (Liu and Sheu, 1999). This is the reason
why the DOC ¯ux of stem¯ow in Chinese ®r stand
is much higher than the other hardwoods.
Mean values of DOC concentrations in soil water
collected from depths of 15 cm were higher than
collecting those from 30 and 60 cm depths. High
DOC concentrations are a common feature in the
organic pro®le of forest soils (Dalva and Moore,
1991), resulting not only from the input of DOC from
the forest canopies and barks but also from the release
of DOC decayed from soil organic matter and litter.
Although, the total DOC ¯ux in throughfall and stem¯ow were higher in Chinese ®r plantation than in
secondary and natural hardwood, the DOC concentrations in soil solution of Chinese ®r plantation were
Table 2
Concentrations and ¯ux of DOC in precipitation, throughfall, stem¯ow, soil water, and stream water collected at some forested sites
Site
Vegetation
Type
Adirondack Mountains,
New York, USA
Hardwood
Conifer
TFa
TF
Hubbard Brook, New
Hampshire, USA
Hardwood
Pb
TF
SWc, 15 cm
SW, 30 cm
Stream
1.1
33.9
5.9
3.0
3.1
Mont St, Hilaire, Quebec
Hardwood
P
TF
SW, A
SW, B
2.0
12.3
47.6
18.0
Olympic National Park,
Washington, USA
Conifer
P
TF
SFd
TF
SF
SW, 0 cm
SW, 15 cm
SW, 40 cm
Stream
1.5
10.5
25.5
7.3
26.3
10.8
9.0
2.9
1.0
Hardwood
DOC
(mg l 1)
4.8
9.6
16.3
47.3
54.5
23.0
19.5
1.1
17.9
9.3
5.5
16.3
1.1
4.6
1.1
1.0
P
TF
SW, 40 cm
SW, 80 cm
Stream
1.0
6.2
5.3
2.4
1.9
Lower Wisconsin River
Valley, Wisconsin, USA
Conifer
P
TF
SW, 25 cm
SW, 60 cm
SW, 140 cm
2.9
11.8
27.5
13.3
7.3
0.2
0.9
1.4
1.0
0.6
Westland, New Zealand
Hardwood
P
TF
SF
SW, 10±15 cm
SW, 30±40 cm
Stream
1.4
16.0
35.6
55.7
11.8
4.5
0.7
12.3
686.1
37.6
6.7
1.7
Kyushu Research Center and
Tatsuda-yama Experimental
Forest, Kumamoto, Japan
Conifer
P
TF
SF
TF
SF
TF
SF
1.0
4.3
11.5
2.9
12.3
3.1
7.1
P
TF
TF
1.8 0.7
29 6.9
24.7 3.2
Harvard Forest,
Massachusetts, USA
a
Hardwood
Conifer
Throughfall.
Precipitation.
c
Soil water sampled at depth shown in cm.
d
Stem¯ow.
b
per year)
Source
McDowell and Likens (1988)
Dalva and Moore (1991)
Hardwood
Hardwood
1
David and Driscoll (1984)
Luquillo Mountains,
Puerto Rico
Conifer
DOC
(kg ha
Edmonds et al. (1995)
McDowell (1998)
Quideau and Bockheim (1997)
34
280
128
836
177
68
Moore and Jackson (1989)
Inagaki et al. (1995)
13.8
117
139
Currie et al. (1996)
C.P. Liu, B.H. Sheu / Forest Ecology and Management 172 (2003) 315±325
lower. According to soil properties of the same site
(Tan et al., 1998), organic matter content of soil was
the lowest in Chinese ®r plantation among the three
stands. Dalva and Moore's (1991) study showed that
although the mechanisms for adsorption of DOC may
be related to extractable iron and aluminum, the
organic carbon content of the soil might play an
important role in determining equilibrium DOC concentrations, particularly in the upper subsoil layers.
Therefore, the lower DOC concentrations of soil
solution were due to the lower organic matter content
in Chinese ®r plantation. On the other hand, laboratory
studies have indicated that the levels of DOC in O
layers or surface soil solutions decrease with increased
acidic inputs because neutralization of acidic inputs by
O layer material can occur when active functional
groups associated with organic matter consumed protons (James and Riha, 1986). For example, Duffy and
Schreiber (1990) and Stroo and Alexander (1986) both
reported a signi®cant decrease in DOC levels when
simulated rain pH was reduced from 5.25 to 4.15 and
5.6 to 3.5, respectively. In our experiments, the pH of
throughfall and stem¯ow of Chinese ®r plantation was
very low (4.4 and 3.6, respectively). Therefore, it may
be some other factors that decrease DOC concentrations in Chinese ®r soil solution.
The deeper the pro®le of soil, the lower is the DOC
concentrations found in all three stands, the lowest
concentration was in stream water as shown in Table 1.
The pronounced decrease in DOC concentration as
water passes through subsoil layers and into the stream
is a common feature of many temperate and tropical
forests (Table 2). Subsoils, especially those containing
low concentrations of organic carbon and high concentrations of extractable iron and aluminum, exhibit
the capacity to adsorb DOC as water percolates down
through the soil pro®les resulting in lower DOC
concentrations (Dalva and Moore, 1991). The higher
DOC input (142.8 kg ha 1 per year) and the lower ¯ux
output (25.0 kg ha 1 per year) can be related to the
higher capacity of this Gudaushi subtropic forest soils
to adsorb DOC.
The differences in the proportion of throughfall
and stem¯ow passing through the forest ¯oor and
collected by the lysimeters re¯ect variations in precipitation intensity, differences in temporal distribution of precipitation events, the spatial variation in
micro relief or forest ¯oor amounts at the three sites,
323
and/or differences in collector ef®ciency. The causes
of the variations in these concentrations are poorly
understood and the release of DOC from different
litter types warrants further study.
4.2. Monthly changes of DOC concentration
during the sampling periods
The monthly variations of DOC concentrations
were very similar in throughfall and stem¯ow, showing an increase of the beginning of the growing season
in April. In most cases, leaching, leaf and stem
washing, biological activities in canopy and bark,
and volumes of rain events are considered to be the
primary factors in¯uencing variability of DOC concentrations in throughfall and stem¯ow (Currie et al.,
1996; Edmonds et al., 1995; Grieve, 1990; Hoffman
et al., 1980; Inagaki et al., 1995; Moore, 1989; Stadler
and Michalzik, 1998). Recently, herbivores have been
shown to be an important source of DOC (Stadler and
Michalzik, 1998). Based on limited data available
(only 1 year), we cannot determine the extent to which
the DOC concentrations of throughfall and stem¯ow
re¯ect that of the primary factors. Data projections of
longer-term results, coinciding with biological ecology, would provide more realistic estimates of ®eld
conditions.
Although, there was no clear seasonality in soil
solution DOC concentrations in our study, there tended
to be higher DOC concentrations in spring (February
and March). The major controls on DOC solubilization in soil or forest ¯oor are temperature and precipitation frequency, primarily affecting microbial
activities and leaching patterns. Carbon is soluble
and available for leaching by the next precipitation
(GoÈdde et al., 1996), therefore, the higher DOC concentrations in spring were due to high precipitation
but not higher temperatures, which is low in spring.
DOC concentrations in the stream draining the
watershed were higher in the spring and winter. This
is different from other studies (Grieve, 1984, 1990;
Moore, 1987; Visser, 1984), where the maximum
DOC in the summer/autumn, coinciding with temperature maxima, has been demonstrated for stream
water and for soil solutions. These short-term and
monthly variations make the assessment of discharge
and temperature effects on DOC dif®cult, thus, indicating the need for an intensive sampling program.
324
C.P. Liu, B.H. Sheu / Forest Ecology and Management 172 (2003) 315±325
Acknowledgements
The authors wish to thank the National Science
Council for ®nancial support under grant (NSC882621-B-005-004-A10). Contribution No. 174 of Taiwan Forestry Research Institute.
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