Effects of thinning and soil properties (1)
Pores;;;ology
Management
Forest EcologyandManagement77 (1995) l-10
Effects of thinning and soil properties on accumulation
of carbon, nitrogen and phosphorus in the forest floor
of Norway spruce stands
Lars Vesterdal ap* , Mads Dalsgaard a, Claus Felby a, Karsten Raulund-Rasmussen b,
Bruno Bilde Jgrgensen ’
a Unit of Forestry, Royal Veterinary and Agricultural
University,
II H@sholm Kongevej, DK-2970 H#rsholm, Denmark
b Department
of Chemistry, Royal Veterinary and Agricultural
University,
40 Thorvaldsensvej,
DK-1871 Frederiksberg
C, Denmark
’ Danish Forest and Landscape Research Institute, 11 H@rsholm Kongevej, DK-2970 H#rsholm, Denmark
Accepted4 May 1995
Abstract
Area-basedsamplingwas carriedout to investigatethe effect of thinningand soil propertieson accumulationof forest
floor carbon(C), nitrogen(N) and phosphorus
(P) in Norway spruce(Piceu abies (L.) Karst.) standsin Denmark.Four
thinningintensities(unthinned,and about83%, 67% and50% of unthinnedbasalarea)were investigatedat threesitesin
Denmark:a calcareous,
relatively nutrientrich soil with a sandyloam/loam texture andtwo soilswith low to intermediate
nutritionalstatusandsandyloamandloamy sandtextures,respectively.
The effect of thinningon accumulated
carbonandnitrogenwassignificantat two of the investigatedsites.Accumulated
phosphorus
wassignificantly affectedby thinningat oneof thesetwo sitesand at the third site. Accumulatedcarbonand
phosphorus
were negativelylinearly correlatedwith thinningintensity.pH tendedto be highestand C/N and C/P ratios
tendedto be lowestin the heaviestthinnedplots. It is hypothesizedthat the differencesin accumulationmay be dueto a
more favourablemicroclimateand substratefor saprophyticorganismsin the most heavily thinned plots. However, the
differencesbetweensiteswere greaterthan differencesbetweenthinningintensities.The accumulationof carbon,nitrogen
andphosphorus
in the forestfloorswasmuchhigherat the two lessfertile siteswith loamysandandsandyloamthan at the
relatively fertile sitewith sandyloam/loam. Significantdifferencesin pH and in C/N and C/P ratiosat the three sites
indicatethat the amountsof availablenutrientsinfluencethe mineralizationpattern.In addition,at the sitewith the greatest
forest floor root density, competitionfor nutrients and moisturebetweenmycorrhiza-infectedroots and free-living
saprophyticdecomposers
may be co-responsible
for the largeamountsof accumulated
carbon,nitrogenandphosphorus.
Keywords:
Thinningintensity;Soilproperties;
Carbon;Nitrogen;Phosphorus;
Picea
1. Introduction
The forest floor, i.e. the layer of dead organic
matter above the mineral soil, accumulatesnutrients
* Corresponding
author.
abies
in unavailable forms when litter production rates
exceed decomposition rates. Litter decomposition is
thus an important link in the biogeochemicalcycling
of nutrients in forest ecosystems,since organically
bound nutrients are mineralized and rendered available to the vegetation again. Decomposition and
0378-1127/95/$09.50
0 1995ElsevierScience
B.V. All rightsreserved
SSDI 0378-1127(95)03579-6
2
L. Vesterdal
et al. / Forest
Ecology
and Management
release of nutrients from organic matter are influenced by climatic conditions (Meentemeyer and Berg,
1986), soil properties (Perala and Alban, 1982;
Raulund-Rasmussen and Vejre, 1995) and litter quality-for example, in terms of the nutrient content of
the litter (Staaf and Berg, 1982; Bockheim et al.,
1991) and the lignin/nitrogen
ratio (Aber and
Melillo, 1982; Stump and Binkley, 1993). These
factors are held responsible for the variable amount
of accumulated organic matter observed in forest
floors in different forest ecosystems. However, within
a stand the forest floor microclimate also influences
mineralization conditions, and the thinning intensity
is crucial to the forest floor microclimate (Aussenac,
1987). In addition, thinning may reduce possible
competition between tree roots and saprophytic microorganisms for water and nutrients and in that way
enhance mineralization. Therefore, thinning intensity
might be a tool for managing the accumulation of
organic matter and rates of nutrient cycling. Previous
investigations on the effects of thinning practice
have demonstrated a positive effect on mineralization, but have often been carried out as radical
thinning operations in formerly unthinned or weakly
thinned stands (Wollum and Schubert, 1975; Piene
and Van Cleve, 1978; Terlinden and Andre, 1988).
77 (19955) I-10
Only few investigations have taken place in permanent thinning trials (Bomebusch, 1933; Carey et al.,
1982). Further, effects of soil properties on accumulation of nutrients are to our knowledge seldom
considered in connection with effects of thinning.
The aim of the present study was to investigate
the effect of thinning intensity on accumulation of
carbon (Cl, nitrogen (N) and phosphorus (P) in the
forest floor of Norway spruce stands. The forest
floor was chosen as the object of the study because
of its biogeochemical importance and because this
layer was considered to be easily influenced by
thinning intensity and soil properties. The investigation was carried out at three sites and included four
thinning intensities in permanent thinning trials. Accumulation results are discussedin relation to C/N
and C/P ratios, pH, root intensity, and properties of
the mineral soil.
2. Materials and methods
2.1. Sites
Three siteswere investigated: Soenderskov on the
island of Lolland in Southeastern Denmark (54”44’
Table 1
Soil properties
Depth
pH
(cm)
Soenderskou
A
Bt
Ck
Tisted Noerskou
Al
A2
Bt
BC
Store Dyrehaue
A
E
Bhs
Bs
BC
C
0-
Clay
(%)
Silt
(%)
Sand
(%Io)
C
(%)
Total N
(mg g- ‘)
Total P
Ca’+
(mg kg- *) ( cmol,
21
6.3
13.0
23
64
3.0
2.52
502
21- 41
41-120
7.5
7.7
16.0
20.0
16
34
68
46
0.6
-
0.64
0.25
333
400
15
30
3.9
4.1
6.9
6.5
1.5
78
1.5 0.92
18
75
30- 65
65-100
4.2
4.1
10.5
8.5
20
15
69
77
1.3
0.4
0.71
0.27
0.2
2.7
3.2
3.4
4.1
4.3
3.5
1.5
6.5
4.0
4.5
13
12
84
87
11
11
83
85
13
4.3
9.5
6
015-
0291532-
2
9
15
32
64
64-100
kg-‘)
Mgz+
(cmol,
KC
kg-‘)
(cmol,
--kg-‘)
taco,
(%o)
17.50
15.20
0.59
0.15
0.35
0.22
0.14
0.08
0.4
43.1
489
558
268
0.10
0.05
0.03
0.11
0.04
0.04
0.03
0.06
-.
0.18
134
0.28
0.15
0.08
14.6
4.15
122
0.70
0.28
0.12
0.8
2.0
0.27
0.82
21
0.02
0.03
0.06
-
83
1.5 0.64
0.3 0.11
115
149
0.01
0.03
0.03
-
84
0.2
377
0.06
0.08
0.04
0.05
1.09
1.30
0.13
0.12
99
0.08
pH: in CaCl, (1: 2.5). Clay, silt and sand: sedimentation
of soil samples. C: dry combustion.
following
asbing (550°C). Ca *+, Mg’+,
K+: 1 M NH,CH,COO,
pH 7. CaCO,: volumetric.
0.04
0.02
Total N: Kjeldahl
N. Total
P: 6 M H,SO.,
L. Vesterdal
et al. /Forest
Ecology
N, 11’46’ E); Tisted Noerskov in Northeastern Jutland (516~47’ N, 1O”OI’ E); Store Dyrehave on the
island of Zealand in Eastern Denmark (55”54’ N,
12”22’ E). The three soils have all been formed on
Weichsel-glacial till deposits, but marked differences
exist among them. The Soenderskov soil (Mollic
Hapludalf) has a sandy loam/loam
texture (Soil
Survey Staff, 1992) and is developed from calcareous till deposits, whereas the two others (Mollic
Hapludalf at Tisted Noerskov and Typic Dystrochrept at Store Dyrehave) have sandy loam and
loamy sand textures, respectively, and are developed
from non-calcareous and less clayey till deposits.
Results from soil analyses (Table 1) show that the
nutritional status of the Soenderskov soil is relatively
high, while that of the two other soils is relatively
low to intermediate. The sites at Soenderskov (elevation 10 m) and Tisted Noerskov (elevation 55 m) are
almost level and the spatial variation within the
sampling area seems rather small, whereas the topography is locally undulating at Store Dyrehave
(elevation 65 m) and probably causes greater spatial
variation. The climate is approximately identical at
the three sites. At Soenderskov mean annual precipitation is 690 mm and the annual mean temperature is
82°C; at Tisted Noerskov and Store Dyrehave the
mean annual precipitation is 710 mm and 660 mm,
Table 2
Characteristics
for differently
thinned
and Management
77 (1995)
I-10
3
respectively, and. the annual mean temperature is
7.4”C and 7.6”C, respectively (Olesen, 1991).
The planted stands at the three sites consist -of
Norway spruce (Piceu abies (L.) Karst.) of almost
equal age (47-51 years old) and are part of a
permanent thinning trial established by the Danish
Forest and Landscape Research Institute: At each
site, four different thinning treatments have been
performed repeatedly during the last 30 years in
plots of about 0.2 ha. The plots are thinned from
below and only stems are removed. Data on plot
characteristics are given in Table 2. Unfortunately,
there are no replications of the thinning treatments
within the three sites, but the four thinning treatments are located adjacently. The thinning treatments
are related to the basal area of the unthinned plot
(A-thinning), and the B-, C-, and D-thinnings ‘are
kept at basal area levels of about 83%, 67% and
50%, respectively, of the basal area in the A-thinning. All sites were forested before the present stands:
Norway spruce at Tisted Noerskov and Store Dyrehave, and beech (Fagus syluatica L:) at Soenderskov.
2.2. Sampling
During January 1993 are&based sampling of the
forest floor was carried out. Twelve points were
plots at the three sites
(m)
Basal area
(m* ha-‘)
Stem volume
(m3 ha-‘)
Stem number
(ha-‘)
Annual increment
cm3 ha-’ year-‘)
Soenderskov
A
B
C
D
21.6
22.1
23.0
24.5
55.1
47.0
38.9
30.4
619
533
443
353
1870
1313
714
362
22.1
22.6
23.3
22.0
T&ted Noerskou
A
B
c
D
20.7
19.0
22.2
22.6
52.5
44.4
36.8
28.8
565
434
420
333
2008
1646
684
346
21.9
17.8
21.8
20.5
Store Dyrehaue
A
B
C
D
17.9
19.2
21.5
22.4
45.6
38.5
31.6
25.0
418
371
336
276
2706
1260
625
367
16.1
17.6
19.0
18.3
Height (hk)
Status after latest thinning treatment (Soenderskov
Annual increment figures apply to the treatment
at the age of 48, Tisted Noerskov at the age of 49 and Store Dyrehave
at the age of 45).
period of about 30 years. h,, tree height corresponding
to mean basal area.
4
L. Vesterdal
et al. /Forest
Ecology
randomly selected within each thinning treatment at
the three sites. Samples were collected using a frame
covering 25 cm X 25 cm and contamination with
mineral material was avoided as far as possible. The
forest floor was separated into a litter layer fraction
and a humus layer fraction. The litter layer fraction
only consisted of slightly decomposed litter, whereas
the humus layer fraction included the so-called fermentation layer in case such a layer was present.
Herbaceous litter, cones and larger twigs were excluded, whereas roots with a diameter less than 10
mm were collected. The samples were oven dried at
40°C, and roots were sorted out and weighed.
2.3. Analyses
Milled samples of litter and humus layer materials
were analysed: pH in 0.01 M CaCl, (1:lO) by potentiometry; total carbon and nitrogen by dry combustion (Leco CHN 1000 Analyzer); total phosphorus
following dry ashing (550°C) and dissolution in 6 M
H,SO, (70°C) by the molybdenum blue method.
2.4. Statistics
All statistical tests were done using SAS procedures from Statistical Analysis Systems Institute Inc..
Since there was no replication of thinning treatments
at each site, Student’s t-test was used to test the
effect of thinning intensity within each site and the
effect of site within each thinning intensity. This test
also enabled us to take possible heterogeneity of
variance into account. There is no valid way to
separate variance due to thinning intensity from variance due to experimental error in a design without
replications. However, since the four thinning treatments are located adjacently at each site, it is reasonable to believe that experimental error sources are
insignificant. Furthermore, a study on spatial variation in the amount of forest floor carbon has shown
that there is only spatial dependency between sampling points at distances of less than l-4 m (Liski,
1995). In the present study, the sampling points
within thinning treatments were located at larger
mutual distances, and the samples may thus be regarded as independent. The given t-test significances
are based on these assumptions, and the results are
discussed on the basis of this test. Linear regressions
and Management
77 (1995)
l-10
for the three elements were tested using general
linear models procedure (d.f. 11).
3. Results
3.1. Accumulation
of carbon, nitrogen and phospho-
rus
The nature of the organic matter varied considerably between the sites. At Store Dyrehave, the forest
floor was thick and mor-like with a well-developed
humus layer, while at the other extreme, Soenderskov, the forest floor was mull-like and almost lacked
a humified layer (Table 3). Earthworm activity was
only observed in the C- and D-thinnings at Soenderskov (Table 3). Differences in accumulated amounts
were much more substantial between sites than between thinning intensities (Table 4). Significant differences between thinning intensities are given on
the assumption that samples were independent and
differences were not due to random variation between the thinning intensity plots. The highest accumulation was found at Store Dyrehave, the least at
Soenderskov, and the accumulated amounts at Tisted
Noerskov were intermediate. The accumulation in
both litter layer and humus layer was highest in the
unthinned plots (the A-thinning) and lower in the
heavily thinned plots at Soenderskov and Tisted
Noerskov. At Store Dyrehave the effect of thinning
intensity was most marked for the litter layer fraction. Significant differences were most commonly
seen between the heaviest thinning intensity (D) and
the others, or between the unthnmed plots (A> and
the others. A possible effect of thinning intensity on
the total accumulated amounts of carbon and nitrogen at Store Dyrehave was probably obscured by
high variances within thinned plots, since a decline
in accumulation means with increasing thinning intensity is seen when the deviant A-thinning is excluded. Regression analysis show that the total
amounts of carbon and phosphorus was negatively
linearly correlated (P < 0.05) with increasing thinning intensity (or positively correlated with percentage basal area) as shown in Figs. l(a) and l(c), The
accumulated amount of nitrogen was nearly negatively linearly correlated with thinning intensity at a
significant level (P = 0.06) (Fig. l(b)). These results
L. Vesterdal
Table 3
Characterization
et al/Forest
Ecology
and Management
77 (1995)
l-10
5
of the forest floors
Site and treatment
Thickness
Flora
Type of forest floor
Activity of
earthworms
None
None, except sparse Oxalis acetosella
Sparse herb layer, moss,
Ox&is acetosella, Lamium galeobdolon
Vigorous and intense herb layer, moss
Rubus iakeus, Rubus fiuticosus,
Oxalis
Lamium galeobdolon,
Mercurialisperennis
Mull-like
Mull-like
Mor-like
None
None
Sparse
(cm)
Soenderskov
A
B
C
D
Mull
lntense
None
None
None
None
acetosella,
Tisted Noerskov
A
B
C
D
7
5
5
4
None
None
None, except some moss
Moss, Deschampsia flexuosa
Mor
Mor
Mor
Mull-like
Store Dyrehave
A
B
C
D
10
11
11
9
None
None, except sparse moss
None, except some moss
Moss, Deschampsia j7exuosa
Mor
Mor
Mor
Mor
Table 4
Accumulated
amounts
the whole forest floor
of carbon,
nitrogen
and phosphorus
01-horizon
C
Soenderskov
A
B
C
D
Tisted Noerskou
A
B
C
D
Store Dyrehuve
A
B
C
D
(kg ha-‘)
in the litter layer (Ol-horizon)
Oh-horizon
N
P
C
mor
mor
mull
mor
None
None
None
None
and humus layer (Oh-horizon)
O-horizon
N
P
C
N
P
Roots
(kg ha- ’ )
Roots
(mass %)
3698;
3591;
2823;
1245,
133;
129;
95a
8
44,
7.0;
7.4;
5.7;
2.8;
9465;
4230;
3938;
1077;
416;
188;
165;
48;
29.2;
14.3;
10.5;
3.3;
13163;
7822;
6761;
2323;
549”
317
261, f
92;
36.2;
21.7,”
16.2;
6.1;
310a
75l
39z
35
0.51;
0.26ib
0.20;
0.02;
11222a
8697’
8956;
6753,
441;
352ib
361ab
291t
20.4;
16.9Tb
16.8Yb
16.1;
5790,
5974,
5210,
4153,
249,
255,
226,
198,
16.0,
14.2,
13.4,
16.2,
17011;
14671;
14165;
10907;
691;
607ab
587?b
489*E
36.4;
31.1”,”
30.2;
32.3;b
628;
386a
119 2
74;
1.23;
1.07;
0.30;
0.20;
1731b
268;
26gab
115;
8.3ib
11.0;
10.4p
5.2;
42657,
48779,
45962,
37367,
1549,
1818,
1756,
1423,
53.4,
63.1,
58.0,
47.9,
47875,
56002,
53201,
40788,
2785tb
3292;
2685*
b
1739,
2.32,
2.42,
2.11,
1.96,
5218ib
7223;
7239ab
3421;
and in
1722,
2086,
2024,
1538,
61.7,ab
74.1;
68.5tb
53.1;
Root biomass (< 10 mm) and relative root mass are shown for the whole forest floor. Letters indicate significant
differences between
thinning intensities, whereas numbers indicate significant site differences. Values in a column by each site with the same letter (or no letter)
do not differ significantly
at 5% level. Values in a column by each thinning intensity with the same number do not differ significantly
at 5%
level. Thinning treatments: A (unthinned);
B, C and D, with basal areas of about 83%, 67% and 50%, respectively,
of the unthinned plot
basal area.
6
L. Vesterdal
et ul./Forest
Ecology
make it very probable that t-test differences between
treatment plots are due to the thinning treatment and
not just due to random variation. Site differences are
again shown by the fact that the results for all three
elements are described best by a regression model
with individual intercepts for each site (P < 0.001
for carbon and nitrogen, P < 0.05 for phosphorus).
Only for phosphorus, a regression model with sitedependent slopes was significant (P < 0.05).
3.2. Properties
of the forest floor
pH, C/N and C/P ratios in both litter layer and
humus layer exhibited only small differences among
the thinning intensities at the three sites, whereas the
differences among the sites were substantial when
60-
a)
77 (1995)
I-10
the results were analysed using Student’s t-test (Table 5). The C/N and C/P ratios tended to be lowest
in the strongly thinned plots, whereas pH was highest in these plots. Mass-based root density in the
forest floor was significantly higher in the weakly
thinned plots than in the strongly thinned plots at
Soenderskov and Tisted Noerskov (Table 4). The
C/N ratios, and especially the C/P ratios, were
highest at Store Dyrehave, while pH was lowest. The
differences between the other two sites were less
marked for C/N and C/P. but pH was higher in the
forest floor at Soenderskov than at Tisted Noerskov.
C/N ratios were significantly higher in the litter
layer than in the humus layer (C/N,ittJC/NhUmUa
:
1) at all sites (not significant for the B-thinning at
Store Dyrehave), and the same trend applied to C/P
25M)7b)
.
.
and Management
50.
Store
40-
OyTehavs
l
?.2
f
d
3c20-
TistedNoerskov
IO-
c-c
0;
,
50
50
/
60
,
basal
70
area (%
basal
70
area
60
/
60
of unthlnned)
60
(% of wthlnnsd)
,
,
/
so
ICC
SO
100
Fig. 1. Linear regressions (d.f. 11) for the amounts of carbon, nitrogen and phosphorus in the forest floors at the three sites as a function of
thinning intensity (basal area levels are means of the four latest measurements). (a) Carbon. The amount of carbon is positively correlated
with percentage basal area (or negatively linearly correlated with thinning intensity) (P < 0.05) and are best described by a regression model
with the same slope for all three sites. (b) Nitrogen. The amount of nitrogen is nearly positively correlated with percentage br& area
(P = 0.06) when described by a regression model with the same slope for all three sites. (c) PhaspBnoMs.The amoul of
positively correlated with percentage basal area (P < 0.05) and best described by a regression model with site-depende@ %&pes (P < 0.635;
For all three elements, the accumulated amounts are best described by regression models with site&@xtdrmt intercepts (P < 0.001 for
carbon and nitrogen, P < 0.05 for phosphorus).
L. Vesterdal
et al. /Forest
Table 5
pH, C/N and C/P ratios in the litter layer (Ol-horizon)
two forest floor horizons
01-horizon
Ecology
and Management
and humus layer (Oh-horizon)
Oh-horizon
pHx
77 (199s)
l-10
and ratios between
Horizon
7
the C/N
and C/P
ratios in the
differences
C/N
C/P
C/N
C/P
pH ’
Soenderskou
A
B
C
D
C/Nlitror/C/Nhumus
c/pli*ter/c/phumus
28.8ib
28.2!
30.1;
29.9tb
534,
487,
505,
490,
4.26;
4.20;
4.16:
5.09;
22.8,
22.6,
23.5,
22.6,
308;
291;
366;
3194b
4.03;b
3.77;
3.86;
4.53;
1.26
1.25
1.28
1.34
* * *
***
* * *
* * *
1.73
1.67
1.38
1.55
***
* * *
* **
* * *
Tisted Noerskov
A
B
C
D
25.5;
24.4;
24.9zb
23.3;
555;
507;
529ib
409;
4.02;
4.28;
4.03;
3.97;
23.2;
23.2;
22.8;
20.8;
358;
420;
381ap
287;
3.31;
3.38ib
3.30;
3.49;
1.10
1.05
1.09
1.12
*
*
*
*
1.55
1.20
1.39
1.43
***
***
** *
***
Store Dyrehave
A
B
C
D
32.8;
29.0;
29.2;
30.8ib
644;
671;b
698;
661”p
3.60,
3.50,
3.61,
3.57,
27.3;
26.9ib
26.1;
26.2;
778,
763,
774,
749,
2.85,”
2.84;
2.88”,”
2.96;
1.20 ***
1.08
1.12 * *
1.18 * * *
* *
*
* *
* *
0.83 * * *
0.88 *
0.90 *
0.88 *
Letters indicate significant
differences
between thinning intensities,
whereas numbers indicate significant
site differences.
Values in a
column by each site with the same letter (or no letter) do not differ significantly
at 5% level. Values in a column by each thinning intensity
with the same number do not differ significantly
at 5% level. Horizon differences: litter/humus
ratios significantly
different from 1.00 are
P < 0.001, * * P < 0.01 and * P < 0.05). Thinning treatments: A (unthinned);
B, C and D, with basal areas of about
indicated by:
83%, 67% and 50%, respectively,
of the unthinned plot basal area.
’ Arithmetic
mean.
*
*
l
ratios at Soenderskov and Tisted Noerskov (Table 5).
However, the C/P ratios increased significantly as
decomposition
proceeded at Store Dyrehave
< 1) (Table 5). Mass-based root
(‘1
‘litter/
c/phumus
density was much higher at Store Dyrehave than at
Soenderskov and intermediate at Tisted Noerskov
(Table 4).
4. Discussion
4.1. Thinning intensity
The accumulated amounts of carbon, nitrogen and
phosphorus in the forest floor reflect differences
between rates of litter production and rates of decomposition (Olson, 1963). Needle litter production
may, due to comparable figures for annual increment
(Table 21, be assumed to be approximately the same
at the three sites (Miller, 19841, whereas root litter
production in the forest floor must be considerable at
Store Dyrehave but insignificant at the two other
sites judging from root biomass (Vogt et al., 1986).
In detail, however, needle litter production may be
affected by thinning intensity because of decreased
canopy closure (Bray and Gorham, 1964). Thus, the
amounts of accumulated carbon, nitrogen and phosphorus must, apart from differences in decomposition rates, also be caused by different root and needle
litter production rates.
The accumulation of nutrients in the forest floor
decreased with increasing thinning intensity. A thinning operation is followed by a change in forest floor
microclimate towards reduced evapotranspiration and
increased solar and. termal radiation (Aussenac,
1987). These changes provide more favourable moisture and temperature regimes for decomposing microorganisms (Piene and Van Cleve, 1978) and increase the mineralization rate. Furthermore, as suggested by Wollum and Schubert (1975), reduced
competition between the remaining trees might increase the amount of available nutrients per tree
without actually increasing the nutrient capital of the
site and result in a more nutrient-rich and easily
8
L. Vesterdai
et al. /Forest
Ecology
decomposable litter. However, after a few years
canopy closure and root development will suspend
the effects of thinning. Consequently, the effect of
thinning in a long-term perspective must depend on
both frequency and intensity of the individual thinning operations. The thinning operations which have
been repeatedly performed in the investigated plots
may thus be expected to have created different conditions for mineralization over many years.
pH, and C/N and C/P ratios did not seem to be
affected by thinning intensity to the same extent as
the amounts of accumulated carbon, nitrogen and
phosphorus,
Only a tendency towards
more
favourable conditions for mineralization, i.e. higher
pH and lower C/N and C/P ratios, was found in the
strongly thinned plots. This indicates that the accumulation in the four thinning intensities might be
more influenced by the microclimate than by a possible nutritional effect of thinning, since pH and C/N
and C/P ratios were only little affected. Accordingly, Piene and Van Cleve (1978) concluded that a
positive correlation between decomposition rates and
thinning intensity found by use of the litterbag technique was caused by more favourable temperature
and moisture regimes.
Root density in the forest floor is another factor
that has been in focus in relation to mineralization,
since roots may compete with saprophytic microorganisms for nutrients and moisture. Gadgil and
Gadgil (1971, 1975) observed an increase in decomposition rate following exclusion of mycorrhizal roots
from pine litter. These results are supported by other
experiments (Oksbjerg, 1954; Babel, 1977; Parmelee
et al., 1993) and hypothesized by Aber et al. (1983),
but also contradicted by others (Dighton et al., 1987;
Staaf, 1988). Although mass-based root density was
affected by thinning at Soenderskov
and Tisted
Noerskov, it is doubtful whether competition between root-associated
mycorrhizal fungi and free
saprophytic microorganisms was important at these
relatively fertile sites. Rather, the soil fauna and
microflora are stimulated by the litter from the herb
layer, which develops more or less abundantly in the
strongly thinned plots at these sites as a result of
improved light, moisture and temperature conditions.
Mixing of litter from different species may be beneficial, since a positive interaction sometimes exists
between release of nutrients from an easily decom-
and Management
77 (1995)
l-10
posable litter and subsequent usage of the released
nutrients; for example, by microorganisms
decomposing a more nutrient-poor litter type (Chapman et
al., 1988; Blair et al., 1990; Williams and Alexander.
1991). Especially at Soenderskov, litter from the
vigorous herb layer (Table 3) may enhance forest
floor mineralization, and the earthworm activity in
the C- and D-thinnings may be due to the more
versatile litter substrate. An increase in the number
of earthworms with increasing thinning intensity has
been reported from other thinning trials in Norway
spruce (Bomebusch,
1933; Scohy et al,, 1984).
Bornebusch (1933) also found an increase in the
number of other soil fauna species with increasing
thinning intensity, and suggested that this might be
due to development of a herbaceous flora and higher
moisture content.
The regressions for total amounts of carbon and
phosphorus showed a negatively linear correlation
(P < 0.05) between the amounts of these elements
and thinning intensity (Figs. l(a) and l(c)). Wollum
and Schubert (1975) found for ponderosa pine (Pinus
ponderosa Laws.) stands that linear correlations were
valid also when much heavier thinning intensities
were included. The regression lines for carbon and
nitrogen have the same slope for all sites, but it
seems possible that, as for phosphorus, there could
be some degree of site-dependency for these elements too, i.e. a difference in the relative effect of
thinning intensity. According to this, the relative
effect of thinning was greater at Soenderskov than at
Tisted Noerskov. The above-mentioned vigorous herb
layer and earthworm activity developed at Soenderskov may be the factor contributing to a possible
greater relative effect of thinning at this site. Clearly,
site properties must be important for both the degree
of accumulation and the effect of thinning. At Store
Dyrehave, the apparent weak effects of thinning may
be caused by other site-specific factors since this site
appeared to be more variable than the other two sites
with respect to soil properties within and between
the thinning treatments.
4.2. Site properties
The accumulation of carbon, nitrogen and phosphorus, and the properties of the forest floor, were
influenced by site-dependent factors to a much greater
L. Vesterdal
et al. /Forest
Ecology
extent than by thinning intensity. It is interesting that
Raulund-Rasmussen and Vejre (1995) observed the
same pattern in an analogously performed investigation at two sites dealing with effects of four tree
species on nutrient accumulation. In this case, effects
of tree species were exceeded by effects of site
properties.
The morphological characteristics of the forest
floors at the three sites indicate different patterns of
decomposition, which must be related to the soil
properties and the soil fauna. The mull-like forest
floor and the small accumulated amounts at Soenderskov suggest greater activity of macrofauna species.
This may be caused by higher soil pH at this site,
since earthworms are scarcely abundant in acid soils
(pH < 4.5) as found at Tisted Noerskov and Store
Dyrehave (Lee, 1985). In fact, no signs of earthworm activity were observed during sampling at
these two sites. Judging from the contents of nitrogen, phosphorus and exchangeable Ca’+, Mg2+ and
K+ in the mineral soils (Table l), differences in
nutrient content of the litter may be expected (Meyer,
1960; Norden, 1994). The high accumulation at Store
Dyrehave may thus be caused by a more nutrientlimited decomposition pattern. For instance, the total
content of phosphorus is comparatively lower
throughout the soil profile at Store Dyrehave. Differences between Tisted Noerskov and Soenderskov in
pH and C/P and C/N ratios are less marked, and
significant differences in accumulated carbon, nitrogen and phosphorus are smaller too. Of particular
interest is the extremely high C/P ratio at Store
Dyrehave, and especially the increase in C/P ratio
as decomposition proceeds (C/Phumus > C/P,itt,,) at
this site (Table 5). This pattern is contrary to the
generally observed relative increase in the concentration of nutrients as mineralization proceeds (i.e. by
forest floor depth) caused by relative microbial immobilization (Gosz et al., 1973; Staaf and Berg,
1982; McClaugherty et al., 1985). In a review, Harrison (1987) suggests a C/P ratio of 200 as a possible
threshold value for net phosphorus mineralization,
but threshold values vary between sites and the
review refers to other results indicating net mineralization at a C/P ratio of 300. However, it is unlikely
to expect net mineralization of phosphorus at a C/P
ratio about 600-700 as in the litter layer at Store
Dyrehave. Rather, the relative decrease in phosphorus may be caused by selective decomposition or
and Management
77 (1995)
l-10
9
extraction of phosphorus by mycorrhiza-infected
roots as hypothesized for nitrogen by Aber et al.
(1983) and shown for protein nitrogen by Abuzinadah et al. (1986). According to Hfussling and
Marschner (1989), ectomycorrhizas in the upper soil
horizons in Norway spruce stands produce the enzyme acid phosphatase, which is crucial to the mineralization of organic phosphorus. This ability might
enable mycorrhizal roots to compete effectively with
free-living saprophytic microorganisms for phosphorus. As previously discussed, competitive effects of
this kind were proposed by Gadgil and Gadgil (1971,
1975). Root infiltration was very strong in the forest
floor at Store Dyrehave compared with almost none
at Soenderskov and scarce at Tisted Noerskov, and
mycorrhizal fungi are undoubtedly present in the
acid and nutrient poor upper soil layers. Differences
in rooting pattern must be due to differences in the
mineral soils as rooting media. At Store Dyrehave,
root space was narrow due to a pan in 40 cm depth.
Therefore, extreme amounts of accumulated carbon,
nitrogen and phosphorus at this site might be attributed to both additional root litter and suppression
of saprophytic activity.
Accumulated carbon, nitrogen and phosphorus in
the forest floor in even aged Norway spruce stands
may be managed to some extent by means of thinning intensity. However, the effect of site-specific
soil properties are much more pronounced. In this
study, accumulation of carbon, nitrogen and phosphorus in the forest floor has been in focus, but the
organic matter content in the mineral soil may be
affected in the long term too.
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Williams,
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Wollum, II, A.G. and Schubert, G.H., 1975. Effect of thinning on
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Management
Forest EcologyandManagement77 (1995) l-10
Effects of thinning and soil properties on accumulation
of carbon, nitrogen and phosphorus in the forest floor
of Norway spruce stands
Lars Vesterdal ap* , Mads Dalsgaard a, Claus Felby a, Karsten Raulund-Rasmussen b,
Bruno Bilde Jgrgensen ’
a Unit of Forestry, Royal Veterinary and Agricultural
University,
II H@sholm Kongevej, DK-2970 H#rsholm, Denmark
b Department
of Chemistry, Royal Veterinary and Agricultural
University,
40 Thorvaldsensvej,
DK-1871 Frederiksberg
C, Denmark
’ Danish Forest and Landscape Research Institute, 11 H@rsholm Kongevej, DK-2970 H#rsholm, Denmark
Accepted4 May 1995
Abstract
Area-basedsamplingwas carriedout to investigatethe effect of thinningand soil propertieson accumulationof forest
floor carbon(C), nitrogen(N) and phosphorus
(P) in Norway spruce(Piceu abies (L.) Karst.) standsin Denmark.Four
thinningintensities(unthinned,and about83%, 67% and50% of unthinnedbasalarea)were investigatedat threesitesin
Denmark:a calcareous,
relatively nutrientrich soil with a sandyloam/loam texture andtwo soilswith low to intermediate
nutritionalstatusandsandyloamandloamy sandtextures,respectively.
The effect of thinningon accumulated
carbonandnitrogenwassignificantat two of the investigatedsites.Accumulated
phosphorus
wassignificantly affectedby thinningat oneof thesetwo sitesand at the third site. Accumulatedcarbonand
phosphorus
were negativelylinearly correlatedwith thinningintensity.pH tendedto be highestand C/N and C/P ratios
tendedto be lowestin the heaviestthinnedplots. It is hypothesizedthat the differencesin accumulationmay be dueto a
more favourablemicroclimateand substratefor saprophyticorganismsin the most heavily thinned plots. However, the
differencesbetweensiteswere greaterthan differencesbetweenthinningintensities.The accumulationof carbon,nitrogen
andphosphorus
in the forestfloorswasmuchhigherat the two lessfertile siteswith loamysandandsandyloamthan at the
relatively fertile sitewith sandyloam/loam. Significantdifferencesin pH and in C/N and C/P ratiosat the three sites
indicatethat the amountsof availablenutrientsinfluencethe mineralizationpattern.In addition,at the sitewith the greatest
forest floor root density, competitionfor nutrients and moisturebetweenmycorrhiza-infectedroots and free-living
saprophyticdecomposers
may be co-responsible
for the largeamountsof accumulated
carbon,nitrogenandphosphorus.
Keywords:
Thinningintensity;Soilproperties;
Carbon;Nitrogen;Phosphorus;
Picea
1. Introduction
The forest floor, i.e. the layer of dead organic
matter above the mineral soil, accumulatesnutrients
* Corresponding
author.
abies
in unavailable forms when litter production rates
exceed decomposition rates. Litter decomposition is
thus an important link in the biogeochemicalcycling
of nutrients in forest ecosystems,since organically
bound nutrients are mineralized and rendered available to the vegetation again. Decomposition and
0378-1127/95/$09.50
0 1995ElsevierScience
B.V. All rightsreserved
SSDI 0378-1127(95)03579-6
2
L. Vesterdal
et al. / Forest
Ecology
and Management
release of nutrients from organic matter are influenced by climatic conditions (Meentemeyer and Berg,
1986), soil properties (Perala and Alban, 1982;
Raulund-Rasmussen and Vejre, 1995) and litter quality-for example, in terms of the nutrient content of
the litter (Staaf and Berg, 1982; Bockheim et al.,
1991) and the lignin/nitrogen
ratio (Aber and
Melillo, 1982; Stump and Binkley, 1993). These
factors are held responsible for the variable amount
of accumulated organic matter observed in forest
floors in different forest ecosystems. However, within
a stand the forest floor microclimate also influences
mineralization conditions, and the thinning intensity
is crucial to the forest floor microclimate (Aussenac,
1987). In addition, thinning may reduce possible
competition between tree roots and saprophytic microorganisms for water and nutrients and in that way
enhance mineralization. Therefore, thinning intensity
might be a tool for managing the accumulation of
organic matter and rates of nutrient cycling. Previous
investigations on the effects of thinning practice
have demonstrated a positive effect on mineralization, but have often been carried out as radical
thinning operations in formerly unthinned or weakly
thinned stands (Wollum and Schubert, 1975; Piene
and Van Cleve, 1978; Terlinden and Andre, 1988).
77 (19955) I-10
Only few investigations have taken place in permanent thinning trials (Bomebusch, 1933; Carey et al.,
1982). Further, effects of soil properties on accumulation of nutrients are to our knowledge seldom
considered in connection with effects of thinning.
The aim of the present study was to investigate
the effect of thinning intensity on accumulation of
carbon (Cl, nitrogen (N) and phosphorus (P) in the
forest floor of Norway spruce stands. The forest
floor was chosen as the object of the study because
of its biogeochemical importance and because this
layer was considered to be easily influenced by
thinning intensity and soil properties. The investigation was carried out at three sites and included four
thinning intensities in permanent thinning trials. Accumulation results are discussedin relation to C/N
and C/P ratios, pH, root intensity, and properties of
the mineral soil.
2. Materials and methods
2.1. Sites
Three siteswere investigated: Soenderskov on the
island of Lolland in Southeastern Denmark (54”44’
Table 1
Soil properties
Depth
pH
(cm)
Soenderskou
A
Bt
Ck
Tisted Noerskou
Al
A2
Bt
BC
Store Dyrehaue
A
E
Bhs
Bs
BC
C
0-
Clay
(%)
Silt
(%)
Sand
(%Io)
C
(%)
Total N
(mg g- ‘)
Total P
Ca’+
(mg kg- *) ( cmol,
21
6.3
13.0
23
64
3.0
2.52
502
21- 41
41-120
7.5
7.7
16.0
20.0
16
34
68
46
0.6
-
0.64
0.25
333
400
15
30
3.9
4.1
6.9
6.5
1.5
78
1.5 0.92
18
75
30- 65
65-100
4.2
4.1
10.5
8.5
20
15
69
77
1.3
0.4
0.71
0.27
0.2
2.7
3.2
3.4
4.1
4.3
3.5
1.5
6.5
4.0
4.5
13
12
84
87
11
11
83
85
13
4.3
9.5
6
015-
0291532-
2
9
15
32
64
64-100
kg-‘)
Mgz+
(cmol,
KC
kg-‘)
(cmol,
--kg-‘)
taco,
(%o)
17.50
15.20
0.59
0.15
0.35
0.22
0.14
0.08
0.4
43.1
489
558
268
0.10
0.05
0.03
0.11
0.04
0.04
0.03
0.06
-.
0.18
134
0.28
0.15
0.08
14.6
4.15
122
0.70
0.28
0.12
0.8
2.0
0.27
0.82
21
0.02
0.03
0.06
-
83
1.5 0.64
0.3 0.11
115
149
0.01
0.03
0.03
-
84
0.2
377
0.06
0.08
0.04
0.05
1.09
1.30
0.13
0.12
99
0.08
pH: in CaCl, (1: 2.5). Clay, silt and sand: sedimentation
of soil samples. C: dry combustion.
following
asbing (550°C). Ca *+, Mg’+,
K+: 1 M NH,CH,COO,
pH 7. CaCO,: volumetric.
0.04
0.02
Total N: Kjeldahl
N. Total
P: 6 M H,SO.,
L. Vesterdal
et al. /Forest
Ecology
N, 11’46’ E); Tisted Noerskov in Northeastern Jutland (516~47’ N, 1O”OI’ E); Store Dyrehave on the
island of Zealand in Eastern Denmark (55”54’ N,
12”22’ E). The three soils have all been formed on
Weichsel-glacial till deposits, but marked differences
exist among them. The Soenderskov soil (Mollic
Hapludalf) has a sandy loam/loam
texture (Soil
Survey Staff, 1992) and is developed from calcareous till deposits, whereas the two others (Mollic
Hapludalf at Tisted Noerskov and Typic Dystrochrept at Store Dyrehave) have sandy loam and
loamy sand textures, respectively, and are developed
from non-calcareous and less clayey till deposits.
Results from soil analyses (Table 1) show that the
nutritional status of the Soenderskov soil is relatively
high, while that of the two other soils is relatively
low to intermediate. The sites at Soenderskov (elevation 10 m) and Tisted Noerskov (elevation 55 m) are
almost level and the spatial variation within the
sampling area seems rather small, whereas the topography is locally undulating at Store Dyrehave
(elevation 65 m) and probably causes greater spatial
variation. The climate is approximately identical at
the three sites. At Soenderskov mean annual precipitation is 690 mm and the annual mean temperature is
82°C; at Tisted Noerskov and Store Dyrehave the
mean annual precipitation is 710 mm and 660 mm,
Table 2
Characteristics
for differently
thinned
and Management
77 (1995)
I-10
3
respectively, and. the annual mean temperature is
7.4”C and 7.6”C, respectively (Olesen, 1991).
The planted stands at the three sites consist -of
Norway spruce (Piceu abies (L.) Karst.) of almost
equal age (47-51 years old) and are part of a
permanent thinning trial established by the Danish
Forest and Landscape Research Institute: At each
site, four different thinning treatments have been
performed repeatedly during the last 30 years in
plots of about 0.2 ha. The plots are thinned from
below and only stems are removed. Data on plot
characteristics are given in Table 2. Unfortunately,
there are no replications of the thinning treatments
within the three sites, but the four thinning treatments are located adjacently. The thinning treatments
are related to the basal area of the unthinned plot
(A-thinning), and the B-, C-, and D-thinnings ‘are
kept at basal area levels of about 83%, 67% and
50%, respectively, of the basal area in the A-thinning. All sites were forested before the present stands:
Norway spruce at Tisted Noerskov and Store Dyrehave, and beech (Fagus syluatica L:) at Soenderskov.
2.2. Sampling
During January 1993 are&based sampling of the
forest floor was carried out. Twelve points were
plots at the three sites
(m)
Basal area
(m* ha-‘)
Stem volume
(m3 ha-‘)
Stem number
(ha-‘)
Annual increment
cm3 ha-’ year-‘)
Soenderskov
A
B
C
D
21.6
22.1
23.0
24.5
55.1
47.0
38.9
30.4
619
533
443
353
1870
1313
714
362
22.1
22.6
23.3
22.0
T&ted Noerskou
A
B
c
D
20.7
19.0
22.2
22.6
52.5
44.4
36.8
28.8
565
434
420
333
2008
1646
684
346
21.9
17.8
21.8
20.5
Store Dyrehaue
A
B
C
D
17.9
19.2
21.5
22.4
45.6
38.5
31.6
25.0
418
371
336
276
2706
1260
625
367
16.1
17.6
19.0
18.3
Height (hk)
Status after latest thinning treatment (Soenderskov
Annual increment figures apply to the treatment
at the age of 48, Tisted Noerskov at the age of 49 and Store Dyrehave
at the age of 45).
period of about 30 years. h,, tree height corresponding
to mean basal area.
4
L. Vesterdal
et al. /Forest
Ecology
randomly selected within each thinning treatment at
the three sites. Samples were collected using a frame
covering 25 cm X 25 cm and contamination with
mineral material was avoided as far as possible. The
forest floor was separated into a litter layer fraction
and a humus layer fraction. The litter layer fraction
only consisted of slightly decomposed litter, whereas
the humus layer fraction included the so-called fermentation layer in case such a layer was present.
Herbaceous litter, cones and larger twigs were excluded, whereas roots with a diameter less than 10
mm were collected. The samples were oven dried at
40°C, and roots were sorted out and weighed.
2.3. Analyses
Milled samples of litter and humus layer materials
were analysed: pH in 0.01 M CaCl, (1:lO) by potentiometry; total carbon and nitrogen by dry combustion (Leco CHN 1000 Analyzer); total phosphorus
following dry ashing (550°C) and dissolution in 6 M
H,SO, (70°C) by the molybdenum blue method.
2.4. Statistics
All statistical tests were done using SAS procedures from Statistical Analysis Systems Institute Inc..
Since there was no replication of thinning treatments
at each site, Student’s t-test was used to test the
effect of thinning intensity within each site and the
effect of site within each thinning intensity. This test
also enabled us to take possible heterogeneity of
variance into account. There is no valid way to
separate variance due to thinning intensity from variance due to experimental error in a design without
replications. However, since the four thinning treatments are located adjacently at each site, it is reasonable to believe that experimental error sources are
insignificant. Furthermore, a study on spatial variation in the amount of forest floor carbon has shown
that there is only spatial dependency between sampling points at distances of less than l-4 m (Liski,
1995). In the present study, the sampling points
within thinning treatments were located at larger
mutual distances, and the samples may thus be regarded as independent. The given t-test significances
are based on these assumptions, and the results are
discussed on the basis of this test. Linear regressions
and Management
77 (1995)
l-10
for the three elements were tested using general
linear models procedure (d.f. 11).
3. Results
3.1. Accumulation
of carbon, nitrogen and phospho-
rus
The nature of the organic matter varied considerably between the sites. At Store Dyrehave, the forest
floor was thick and mor-like with a well-developed
humus layer, while at the other extreme, Soenderskov, the forest floor was mull-like and almost lacked
a humified layer (Table 3). Earthworm activity was
only observed in the C- and D-thinnings at Soenderskov (Table 3). Differences in accumulated amounts
were much more substantial between sites than between thinning intensities (Table 4). Significant differences between thinning intensities are given on
the assumption that samples were independent and
differences were not due to random variation between the thinning intensity plots. The highest accumulation was found at Store Dyrehave, the least at
Soenderskov, and the accumulated amounts at Tisted
Noerskov were intermediate. The accumulation in
both litter layer and humus layer was highest in the
unthinned plots (the A-thinning) and lower in the
heavily thinned plots at Soenderskov and Tisted
Noerskov. At Store Dyrehave the effect of thinning
intensity was most marked for the litter layer fraction. Significant differences were most commonly
seen between the heaviest thinning intensity (D) and
the others, or between the unthnmed plots (A> and
the others. A possible effect of thinning intensity on
the total accumulated amounts of carbon and nitrogen at Store Dyrehave was probably obscured by
high variances within thinned plots, since a decline
in accumulation means with increasing thinning intensity is seen when the deviant A-thinning is excluded. Regression analysis show that the total
amounts of carbon and phosphorus was negatively
linearly correlated (P < 0.05) with increasing thinning intensity (or positively correlated with percentage basal area) as shown in Figs. l(a) and l(c), The
accumulated amount of nitrogen was nearly negatively linearly correlated with thinning intensity at a
significant level (P = 0.06) (Fig. l(b)). These results
L. Vesterdal
Table 3
Characterization
et al/Forest
Ecology
and Management
77 (1995)
l-10
5
of the forest floors
Site and treatment
Thickness
Flora
Type of forest floor
Activity of
earthworms
None
None, except sparse Oxalis acetosella
Sparse herb layer, moss,
Ox&is acetosella, Lamium galeobdolon
Vigorous and intense herb layer, moss
Rubus iakeus, Rubus fiuticosus,
Oxalis
Lamium galeobdolon,
Mercurialisperennis
Mull-like
Mull-like
Mor-like
None
None
Sparse
(cm)
Soenderskov
A
B
C
D
Mull
lntense
None
None
None
None
acetosella,
Tisted Noerskov
A
B
C
D
7
5
5
4
None
None
None, except some moss
Moss, Deschampsia flexuosa
Mor
Mor
Mor
Mull-like
Store Dyrehave
A
B
C
D
10
11
11
9
None
None, except sparse moss
None, except some moss
Moss, Deschampsia j7exuosa
Mor
Mor
Mor
Mor
Table 4
Accumulated
amounts
the whole forest floor
of carbon,
nitrogen
and phosphorus
01-horizon
C
Soenderskov
A
B
C
D
Tisted Noerskou
A
B
C
D
Store Dyrehuve
A
B
C
D
(kg ha-‘)
in the litter layer (Ol-horizon)
Oh-horizon
N
P
C
mor
mor
mull
mor
None
None
None
None
and humus layer (Oh-horizon)
O-horizon
N
P
C
N
P
Roots
(kg ha- ’ )
Roots
(mass %)
3698;
3591;
2823;
1245,
133;
129;
95a
8
44,
7.0;
7.4;
5.7;
2.8;
9465;
4230;
3938;
1077;
416;
188;
165;
48;
29.2;
14.3;
10.5;
3.3;
13163;
7822;
6761;
2323;
549”
317
261, f
92;
36.2;
21.7,”
16.2;
6.1;
310a
75l
39z
35
0.51;
0.26ib
0.20;
0.02;
11222a
8697’
8956;
6753,
441;
352ib
361ab
291t
20.4;
16.9Tb
16.8Yb
16.1;
5790,
5974,
5210,
4153,
249,
255,
226,
198,
16.0,
14.2,
13.4,
16.2,
17011;
14671;
14165;
10907;
691;
607ab
587?b
489*E
36.4;
31.1”,”
30.2;
32.3;b
628;
386a
119 2
74;
1.23;
1.07;
0.30;
0.20;
1731b
268;
26gab
115;
8.3ib
11.0;
10.4p
5.2;
42657,
48779,
45962,
37367,
1549,
1818,
1756,
1423,
53.4,
63.1,
58.0,
47.9,
47875,
56002,
53201,
40788,
2785tb
3292;
2685*
b
1739,
2.32,
2.42,
2.11,
1.96,
5218ib
7223;
7239ab
3421;
and in
1722,
2086,
2024,
1538,
61.7,ab
74.1;
68.5tb
53.1;
Root biomass (< 10 mm) and relative root mass are shown for the whole forest floor. Letters indicate significant
differences between
thinning intensities, whereas numbers indicate significant site differences. Values in a column by each site with the same letter (or no letter)
do not differ significantly
at 5% level. Values in a column by each thinning intensity with the same number do not differ significantly
at 5%
level. Thinning treatments: A (unthinned);
B, C and D, with basal areas of about 83%, 67% and 50%, respectively,
of the unthinned plot
basal area.
6
L. Vesterdal
et ul./Forest
Ecology
make it very probable that t-test differences between
treatment plots are due to the thinning treatment and
not just due to random variation. Site differences are
again shown by the fact that the results for all three
elements are described best by a regression model
with individual intercepts for each site (P < 0.001
for carbon and nitrogen, P < 0.05 for phosphorus).
Only for phosphorus, a regression model with sitedependent slopes was significant (P < 0.05).
3.2. Properties
of the forest floor
pH, C/N and C/P ratios in both litter layer and
humus layer exhibited only small differences among
the thinning intensities at the three sites, whereas the
differences among the sites were substantial when
60-
a)
77 (1995)
I-10
the results were analysed using Student’s t-test (Table 5). The C/N and C/P ratios tended to be lowest
in the strongly thinned plots, whereas pH was highest in these plots. Mass-based root density in the
forest floor was significantly higher in the weakly
thinned plots than in the strongly thinned plots at
Soenderskov and Tisted Noerskov (Table 4). The
C/N ratios, and especially the C/P ratios, were
highest at Store Dyrehave, while pH was lowest. The
differences between the other two sites were less
marked for C/N and C/P. but pH was higher in the
forest floor at Soenderskov than at Tisted Noerskov.
C/N ratios were significantly higher in the litter
layer than in the humus layer (C/N,ittJC/NhUmUa
:
1) at all sites (not significant for the B-thinning at
Store Dyrehave), and the same trend applied to C/P
25M)7b)
.
.
and Management
50.
Store
40-
OyTehavs
l
?.2
f
d
3c20-
TistedNoerskov
IO-
c-c
0;
,
50
50
/
60
,
basal
70
area (%
basal
70
area
60
/
60
of unthlnned)
60
(% of wthlnnsd)
,
,
/
so
ICC
SO
100
Fig. 1. Linear regressions (d.f. 11) for the amounts of carbon, nitrogen and phosphorus in the forest floors at the three sites as a function of
thinning intensity (basal area levels are means of the four latest measurements). (a) Carbon. The amount of carbon is positively correlated
with percentage basal area (or negatively linearly correlated with thinning intensity) (P < 0.05) and are best described by a regression model
with the same slope for all three sites. (b) Nitrogen. The amount of nitrogen is nearly positively correlated with percentage br& area
(P = 0.06) when described by a regression model with the same slope for all three sites. (c) PhaspBnoMs.The amoul of
positively correlated with percentage basal area (P < 0.05) and best described by a regression model with site-depende@ %&pes (P < 0.635;
For all three elements, the accumulated amounts are best described by regression models with site&@xtdrmt intercepts (P < 0.001 for
carbon and nitrogen, P < 0.05 for phosphorus).
L. Vesterdal
et al. /Forest
Table 5
pH, C/N and C/P ratios in the litter layer (Ol-horizon)
two forest floor horizons
01-horizon
Ecology
and Management
and humus layer (Oh-horizon)
Oh-horizon
pHx
77 (199s)
l-10
and ratios between
Horizon
7
the C/N
and C/P
ratios in the
differences
C/N
C/P
C/N
C/P
pH ’
Soenderskou
A
B
C
D
C/Nlitror/C/Nhumus
c/pli*ter/c/phumus
28.8ib
28.2!
30.1;
29.9tb
534,
487,
505,
490,
4.26;
4.20;
4.16:
5.09;
22.8,
22.6,
23.5,
22.6,
308;
291;
366;
3194b
4.03;b
3.77;
3.86;
4.53;
1.26
1.25
1.28
1.34
* * *
***
* * *
* * *
1.73
1.67
1.38
1.55
***
* * *
* **
* * *
Tisted Noerskov
A
B
C
D
25.5;
24.4;
24.9zb
23.3;
555;
507;
529ib
409;
4.02;
4.28;
4.03;
3.97;
23.2;
23.2;
22.8;
20.8;
358;
420;
381ap
287;
3.31;
3.38ib
3.30;
3.49;
1.10
1.05
1.09
1.12
*
*
*
*
1.55
1.20
1.39
1.43
***
***
** *
***
Store Dyrehave
A
B
C
D
32.8;
29.0;
29.2;
30.8ib
644;
671;b
698;
661”p
3.60,
3.50,
3.61,
3.57,
27.3;
26.9ib
26.1;
26.2;
778,
763,
774,
749,
2.85,”
2.84;
2.88”,”
2.96;
1.20 ***
1.08
1.12 * *
1.18 * * *
* *
*
* *
* *
0.83 * * *
0.88 *
0.90 *
0.88 *
Letters indicate significant
differences
between thinning intensities,
whereas numbers indicate significant
site differences.
Values in a
column by each site with the same letter (or no letter) do not differ significantly
at 5% level. Values in a column by each thinning intensity
with the same number do not differ significantly
at 5% level. Horizon differences: litter/humus
ratios significantly
different from 1.00 are
P < 0.001, * * P < 0.01 and * P < 0.05). Thinning treatments: A (unthinned);
B, C and D, with basal areas of about
indicated by:
83%, 67% and 50%, respectively,
of the unthinned plot basal area.
’ Arithmetic
mean.
*
*
l
ratios at Soenderskov and Tisted Noerskov (Table 5).
However, the C/P ratios increased significantly as
decomposition
proceeded at Store Dyrehave
< 1) (Table 5). Mass-based root
(‘1
‘litter/
c/phumus
density was much higher at Store Dyrehave than at
Soenderskov and intermediate at Tisted Noerskov
(Table 4).
4. Discussion
4.1. Thinning intensity
The accumulated amounts of carbon, nitrogen and
phosphorus in the forest floor reflect differences
between rates of litter production and rates of decomposition (Olson, 1963). Needle litter production
may, due to comparable figures for annual increment
(Table 21, be assumed to be approximately the same
at the three sites (Miller, 19841, whereas root litter
production in the forest floor must be considerable at
Store Dyrehave but insignificant at the two other
sites judging from root biomass (Vogt et al., 1986).
In detail, however, needle litter production may be
affected by thinning intensity because of decreased
canopy closure (Bray and Gorham, 1964). Thus, the
amounts of accumulated carbon, nitrogen and phosphorus must, apart from differences in decomposition rates, also be caused by different root and needle
litter production rates.
The accumulation of nutrients in the forest floor
decreased with increasing thinning intensity. A thinning operation is followed by a change in forest floor
microclimate towards reduced evapotranspiration and
increased solar and. termal radiation (Aussenac,
1987). These changes provide more favourable moisture and temperature regimes for decomposing microorganisms (Piene and Van Cleve, 1978) and increase the mineralization rate. Furthermore, as suggested by Wollum and Schubert (1975), reduced
competition between the remaining trees might increase the amount of available nutrients per tree
without actually increasing the nutrient capital of the
site and result in a more nutrient-rich and easily
8
L. Vesterdai
et al. /Forest
Ecology
decomposable litter. However, after a few years
canopy closure and root development will suspend
the effects of thinning. Consequently, the effect of
thinning in a long-term perspective must depend on
both frequency and intensity of the individual thinning operations. The thinning operations which have
been repeatedly performed in the investigated plots
may thus be expected to have created different conditions for mineralization over many years.
pH, and C/N and C/P ratios did not seem to be
affected by thinning intensity to the same extent as
the amounts of accumulated carbon, nitrogen and
phosphorus,
Only a tendency towards
more
favourable conditions for mineralization, i.e. higher
pH and lower C/N and C/P ratios, was found in the
strongly thinned plots. This indicates that the accumulation in the four thinning intensities might be
more influenced by the microclimate than by a possible nutritional effect of thinning, since pH and C/N
and C/P ratios were only little affected. Accordingly, Piene and Van Cleve (1978) concluded that a
positive correlation between decomposition rates and
thinning intensity found by use of the litterbag technique was caused by more favourable temperature
and moisture regimes.
Root density in the forest floor is another factor
that has been in focus in relation to mineralization,
since roots may compete with saprophytic microorganisms for nutrients and moisture. Gadgil and
Gadgil (1971, 1975) observed an increase in decomposition rate following exclusion of mycorrhizal roots
from pine litter. These results are supported by other
experiments (Oksbjerg, 1954; Babel, 1977; Parmelee
et al., 1993) and hypothesized by Aber et al. (1983),
but also contradicted by others (Dighton et al., 1987;
Staaf, 1988). Although mass-based root density was
affected by thinning at Soenderskov
and Tisted
Noerskov, it is doubtful whether competition between root-associated
mycorrhizal fungi and free
saprophytic microorganisms was important at these
relatively fertile sites. Rather, the soil fauna and
microflora are stimulated by the litter from the herb
layer, which develops more or less abundantly in the
strongly thinned plots at these sites as a result of
improved light, moisture and temperature conditions.
Mixing of litter from different species may be beneficial, since a positive interaction sometimes exists
between release of nutrients from an easily decom-
and Management
77 (1995)
l-10
posable litter and subsequent usage of the released
nutrients; for example, by microorganisms
decomposing a more nutrient-poor litter type (Chapman et
al., 1988; Blair et al., 1990; Williams and Alexander.
1991). Especially at Soenderskov, litter from the
vigorous herb layer (Table 3) may enhance forest
floor mineralization, and the earthworm activity in
the C- and D-thinnings may be due to the more
versatile litter substrate. An increase in the number
of earthworms with increasing thinning intensity has
been reported from other thinning trials in Norway
spruce (Bomebusch,
1933; Scohy et al,, 1984).
Bornebusch (1933) also found an increase in the
number of other soil fauna species with increasing
thinning intensity, and suggested that this might be
due to development of a herbaceous flora and higher
moisture content.
The regressions for total amounts of carbon and
phosphorus showed a negatively linear correlation
(P < 0.05) between the amounts of these elements
and thinning intensity (Figs. l(a) and l(c)). Wollum
and Schubert (1975) found for ponderosa pine (Pinus
ponderosa Laws.) stands that linear correlations were
valid also when much heavier thinning intensities
were included. The regression lines for carbon and
nitrogen have the same slope for all sites, but it
seems possible that, as for phosphorus, there could
be some degree of site-dependency for these elements too, i.e. a difference in the relative effect of
thinning intensity. According to this, the relative
effect of thinning was greater at Soenderskov than at
Tisted Noerskov. The above-mentioned vigorous herb
layer and earthworm activity developed at Soenderskov may be the factor contributing to a possible
greater relative effect of thinning at this site. Clearly,
site properties must be important for both the degree
of accumulation and the effect of thinning. At Store
Dyrehave, the apparent weak effects of thinning may
be caused by other site-specific factors since this site
appeared to be more variable than the other two sites
with respect to soil properties within and between
the thinning treatments.
4.2. Site properties
The accumulation of carbon, nitrogen and phosphorus, and the properties of the forest floor, were
influenced by site-dependent factors to a much greater
L. Vesterdal
et al. /Forest
Ecology
extent than by thinning intensity. It is interesting that
Raulund-Rasmussen and Vejre (1995) observed the
same pattern in an analogously performed investigation at two sites dealing with effects of four tree
species on nutrient accumulation. In this case, effects
of tree species were exceeded by effects of site
properties.
The morphological characteristics of the forest
floors at the three sites indicate different patterns of
decomposition, which must be related to the soil
properties and the soil fauna. The mull-like forest
floor and the small accumulated amounts at Soenderskov suggest greater activity of macrofauna species.
This may be caused by higher soil pH at this site,
since earthworms are scarcely abundant in acid soils
(pH < 4.5) as found at Tisted Noerskov and Store
Dyrehave (Lee, 1985). In fact, no signs of earthworm activity were observed during sampling at
these two sites. Judging from the contents of nitrogen, phosphorus and exchangeable Ca’+, Mg2+ and
K+ in the mineral soils (Table l), differences in
nutrient content of the litter may be expected (Meyer,
1960; Norden, 1994). The high accumulation at Store
Dyrehave may thus be caused by a more nutrientlimited decomposition pattern. For instance, the total
content of phosphorus is comparatively lower
throughout the soil profile at Store Dyrehave. Differences between Tisted Noerskov and Soenderskov in
pH and C/P and C/N ratios are less marked, and
significant differences in accumulated carbon, nitrogen and phosphorus are smaller too. Of particular
interest is the extremely high C/P ratio at Store
Dyrehave, and especially the increase in C/P ratio
as decomposition proceeds (C/Phumus > C/P,itt,,) at
this site (Table 5). This pattern is contrary to the
generally observed relative increase in the concentration of nutrients as mineralization proceeds (i.e. by
forest floor depth) caused by relative microbial immobilization (Gosz et al., 1973; Staaf and Berg,
1982; McClaugherty et al., 1985). In a review, Harrison (1987) suggests a C/P ratio of 200 as a possible
threshold value for net phosphorus mineralization,
but threshold values vary between sites and the
review refers to other results indicating net mineralization at a C/P ratio of 300. However, it is unlikely
to expect net mineralization of phosphorus at a C/P
ratio about 600-700 as in the litter layer at Store
Dyrehave. Rather, the relative decrease in phosphorus may be caused by selective decomposition or
and Management
77 (1995)
l-10
9
extraction of phosphorus by mycorrhiza-infected
roots as hypothesized for nitrogen by Aber et al.
(1983) and shown for protein nitrogen by Abuzinadah et al. (1986). According to Hfussling and
Marschner (1989), ectomycorrhizas in the upper soil
horizons in Norway spruce stands produce the enzyme acid phosphatase, which is crucial to the mineralization of organic phosphorus. This ability might
enable mycorrhizal roots to compete effectively with
free-living saprophytic microorganisms for phosphorus. As previously discussed, competitive effects of
this kind were proposed by Gadgil and Gadgil (1971,
1975). Root infiltration was very strong in the forest
floor at Store Dyrehave compared with almost none
at Soenderskov and scarce at Tisted Noerskov, and
mycorrhizal fungi are undoubtedly present in the
acid and nutrient poor upper soil layers. Differences
in rooting pattern must be due to differences in the
mineral soils as rooting media. At Store Dyrehave,
root space was narrow due to a pan in 40 cm depth.
Therefore, extreme amounts of accumulated carbon,
nitrogen and phosphorus at this site might be attributed to both additional root litter and suppression
of saprophytic activity.
Accumulated carbon, nitrogen and phosphorus in
the forest floor in even aged Norway spruce stands
may be managed to some extent by means of thinning intensity. However, the effect of site-specific
soil properties are much more pronounced. In this
study, accumulation of carbon, nitrogen and phosphorus in the forest floor has been in focus, but the
organic matter content in the mineral soil may be
affected in the long term too.
References
Aber, J.D. and Melillo, J.M., 1982. Nitrogen
immobilization
in
decaying hardwood leaf litter as a function of initial nitrogen
and lignin content. Can. J. Bot., 60: 2263-2269.
Aber, J.D., Melillo,
J.M., McClaugherty,
C.A. and Eshleman,
K.N., 1983. Potential sinks for mineralized
nitrogen following
disturbance
in forest ecosystems.
In: R. Hallberg
(Editor),
Environmental
Biogeochemistry.
Ecol. Bull. Stockh., 35: 179192.
Abuzinadah,
R.A., Finlay, R.D. and Read, D.J., 1986. The role of
proteins in the nitrogen nutrition of ectomycorrhizal
plants. II.
Utilization
of protein by mycorrhiial
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