602 (4) (cr/k) 25/6/99 1:04 pm Page 207
The long-term decomposition of
Sitka spruce needles in brash
B.D. TITUS* AND D.C. MALCOLM
School of Forestry, Institute of Ecology and Resource Management, University of Edinburgh, Darwin Building,
King’s Buildings, Mayfield Road, Edinburgh EH9 3JU, Scotland
Summary
A chronosequence approach was used to estimate Sitka spruce brash needle decomposition rates
over 7 years following clearfelling, by collecting brash needles from forest floors and incubating
them in litterbags over a 2-year period on three plots of age 0, 2 and 5 years from the time of
harvesting. The data were sequentially fitted to produce a 7-year mass loss curve consisting of four
exponential phases: (1) a rapid mass loss phase for labile material over the first 105 days (k = 0.51
a–1), (2) a slower second mass loss phase for the first plot up to 2 years (k = 0.33 a–1), (3) a third yet
slower exponential mass loss phase for the second plot between years 3 and 4 (k = 0.28 a–1), and (4)
a final rate of k = 0.10 a–1 for 6–7 years following clearfelling on the third plot. A new approach to
analysis of litterbag data was used to demonstrate that if experimental designs use individual
collection stations then decomposition rates can be determined for individual microsites as well as
for sites as a whole. Microsite decomposition rates varied by up to 300 per cent on each of the three
sites examined, but high R2 values (ranging from 0.71 to 0.99) indicate that, despite this large
variation between microsites, decomposition within a given microsite proceeds in a consistent
–
manner with time. Further, the decomposition rate for a plot or site can be expressed as k, or the
mean of individual microsite decomposition rates. A review of long-term decomposition studies
demonstrates that comparing k for individual years is more meaningful than comparing k for longer
time periods. Nutritionally, N was retained in the Sitka spruce needles. Phosphorus and Ca content
decreased at the same rate as needle mass loss but more labile nutrients such as K, Na and Mg were
rapidly lost from the needles. This has implications for long-term nutrient cycling processes on these
sites.
Introduction
In upland Sitka spruce (Picea sitchensis (Bong.)
Carr.) plantations in the UK, organized harvesting techniques are used to fell trees in a distinct
pattern so that the resultant bands of brash (or
slash) can be used as a road bed for extraction
equipment. The rate of decomposition of these
brash swathes is of interest because the highly
patterned distribution of brash represents the
*Present location, and address to which correspondence should be sent: Natural Resources Canada, Canadian Forest
Service, Pacific Forestry Centre, 506 West Burnside Rd., Victoria, BC, Canada V8Z 1M5.
© Institute of Chartered Foresters, 1999
Forestry, Vol. 72, No. 3, 1999
602 (4) (cr/k) 25/6/99 1:04 pm Page 208
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F O R E S T RY
aggregation of nutrient capital over only a
portion of a site. The needles in the brash make
up a sizeable fraction of this capital (Titus and
Malcolm, 1991) and because needles decompose
more quickly than twigs and branches (Berg and
Staaf, 1983) it is this component of the brash that
may supply an initial pulse of nutrients to second
rotation transplants, in addition to those nutrients mineralized in the old LFH horizons.
However, the nutrients in the brash needles
must be released through decomposition processes before they become available for plant
uptake again. The determination of decomposition rate is therefore important in estimating
future site productivity. Key factors that influence
the accuracy of determinations of the decomposition rate include the use of appropriate mass loss
models (Wieder and Lang, 1982), application to
a realistic period of time (Harmon et al., 1990),
and appropriate methods of data analysis.
The decomposition rate k can be obtained indirectly from mass loss inventories or directly from
litterbag studies (Singh and Gupta, 1977), either
arithmetically or from the slope of a regression of
the natural logarithm of mass loss over time
(Olson, 1963). Although a single decomposition
rate is commonly used for the entire period
studied, Minderman (1968) suggested that the
theoretical course of decomposition is more like
a curve constructed from the sum of logarithmic
decay curves for the individual constituents of
litter, rather than a simple logarithmic decay
curve. A number of models for mass loss are
described in Wieder and Lang (1982), and the
authors conclude that a multi-compartmental (i.e.
double exponential) model such as that described
in Bunnell and Tait (1974) is usually most appropriate, with the decay products of one compartment sequentially beginning decay in the next
compartment with a lower k value. More
recently, Melillo et al. (1989) suggested that
decomposition takes place in two phases, with
initial mass loss occurring predominantly through
degradation of lignocellulose, and later mass loss
occurring through decomposition of lignin.
Studies of long-term decomposition of litter are
hampered by the lengthy time period required to
collect adequate data. However, use of a
chronosequence approach offers another possibility for obtaining data over a longer time period
than would otherwise be possible.
When litterbags are used to monitor mass loss,
workers either lay out litterbags randomly within
a plot and collect a given number of litterbags per
collection date (e.g. Taylor and Jones, 1990), or
lay out groups of bags at distinct collection
stations (or sampling stations, or sub-plots) and
retrieve one randomly chosen bag per station per
collection date (e.g. Berg et al., 1982a). While the
two methods each entail approximately the same
amount of sample preparation time and field
work, the latter permits a different approach to
the use of statistics in which the decomposition
rate for each individual collection station can be
accurately determined and then used to calculate
the mean decomposition rate for the plot or site
as a whole.
This study formed part of a larger investigation
of nutrient cycling following clearfelling of Sitka
spruce (Picea sitchensis (Bong.) Carr.) plantations
in the UK which describes site conditions and
nutrient capital (Titus and Malcolm, 1991), nutrient losses in leachate (Titus and Malcolm, 1992),
and the influence of fertilizer on nutrient cycling
and brash decomposition (Titus and Malcolm,
1987). The aims of this present paper are: (1) to
demonstrate the use of the litterbag technique in
chronosequence studies; (2) to demonstrate an
approach to experimental design that allows for
accurate characterization of k for individual
microsites as well as for plots or sites as a whole;
(3) to demonstrate the need for determining k
over a reasonable time period so that long-term
decomposition rates are not over-estimated; and
(4) to determine the rates of nutrient release from
decomposing Sitka spruce needles in brash.
Methods
Description of study areas
The three sites used in this study were located in
compartments 723, 720 and 227a of Kielder
Forest, Northumberland (latitude 55° 259 N, longitude 2° 309 W), and represented a chronosequence of sites of 0, 2 and 5 years from
clearfelling (Plots 0, 2 and 5, respectively). The
sites previously supported plantations of Yield
Class 10–12 (maximum mean annual increment,
m3 ha–1 a–1) Sitka spruce (Picea sitchensis (Bong.)
Carr.) which were felled at age 40–42 years. The
bench-felling system used (Low, 1985) resulted in
the banding of brash into 8-m swathes separated
by 4-m clear strips devoid of brash. The soils were
602 (4) (cr/k) 25/6/99 1:04 pm Page 209
THE LONG-TERM DECOMPOSITION OF SITKA SPRUCE NEEDLES IN BRASH
uniform peaty gleys (Pyatt, 1970) which had developed post-glacially on Scremerston Coal Group
Sandstones overlying Carboniferous limestone.
The gently sloping sites (4–7°) were at elevations
ranging from 280 to 340 m. The climate is cool and
moist, with mean monthly temperatures ranging
from 0°C to 15°C, and about 1300 mm annual
rainfall. Site details are presented in Table 1, and a
fuller description can be found in Titus (1985).
Experimental design
One rectangular 0.10 ha plot was established on
each of the three sites. Fresh brash was collected
from Plot 0, and needles removed from twigs
after air-drying at 20°C. Brash needles were collected from the surface of the litter layer on Plots
2 and 5 and air-dried. All needles were stored at
–20°C until used. A known weight of needles
(approximately 3 g) was placed in 10 3 10 cm litterbags of mesh size 0.3 3 1.0 mm, which ensured
that needles could not be lost through the mesh.
A representative sample of needles of each age
class was oven-dried at 105°C to a constant
weight to obtain an estimate of initial dry weight.
As planting takes place adjacent to stumps on
these peaty gley sites and the study was established
to quantify nutrient availability from decomposing brash to second rotation seedlings, collection
stations (35 3 35 cm) were established at planting positions next to 10 randomly chosen stumps
in the brash swathes on each of the three plots
(one station on Plot 5 was later disturbed by
209
wildlife and was therefore omitted from analysis).
Seven litterbags (five on Plot 5) were placed under
the thin layer of brash litter at each station. One
randomly chosen litterbag from each collection
station was returned to the laboratory approximately every 3.5 months over a 2-year period. The
exterior of the litterbags was carefully cleaned,
any roots were removed, and the litter was ovendried at 105°C and then weighed. The percentage
of the original mass of litter remaining in the bags
at each collection date was then calculated.
Chemical analysis
After drying, needles were ground to pass
through a 0.5-mm screen. A 0.1-g sample was
then digested in 2 ml of concentrated sulphuric
acid and 1 ml of concentrated hydrogen peroxide
using a micro-Kjeldahl technique (Allen, 1974)
and made up to 50 ml with distilled water. Total
N and P concentrations were determined colorimetrically (Murphy and Riley, 1962; Fraser and
Russell, 1969; Crooke and Simpson, 1971) using
an autoanalyser. An atomic absorption spectrophotometer was used to determine total K and
Na concentrations by flame emission, and Ca and
Mg by atomic absorption after the addition of La.
Data analysis
Linear regressions of ln (mean per cent mass
remaining) over time were obtained for each of the
three plots to obtain a ‘first approximation’ (Swift
Table 1: Site descriptions of chronosequence plots (after Titus and Malcolm, 1992)
Plot
—————–——————————————————————
0
2
5
Planting year
Original stocking density (stems ha–1)
Stocking density at felling (stems ha–1)
Felling date (month/year)
Age at felling
Mensuration date (month/year)
Top height (m)
Yield class*1 (m3 ha–1 a–1)
Restocking date (month/year)
Aspect
Slope
Elevation (m)
* maximum mean annual increment.
1939
4810
3330
01/1981
42
09/1975
14.5
10–12
04/1983
160°
6°
280–290
1939
4120
3150
11/1979
40
09/1975
15.0
10–12
05/1981
190°
8°
300–310
1936
4430
3310
11/1976
40
09/1973
14.1
10
04/1979
110°
6°
290–300
602 (4) (cr/k) 25/6/99 1:04 pm Page 210
210
F O R E S T RY
et al., 1979) of decomposition mass loss using a
simple single exponential model (Olson, 1963):
x
ln — = – kt
x0
(1)
The factors f2 and f5 by which the original per
cent weight remaining data from Plot 2 and Plot
5, respectively, would have to be multiplied to
give the appropriate y-axis intercepts for the
defined regressions were calculated from:
where:
x = mass remaining after time t; x0 = original
mass; k = decay parameter; t = time (in years).
A chronosequence was then constructed
assuming a multiple-phase exponential model
over 7 years. First, mass loss data for Plot 0 was
separated into two phases (after Bunnell and Tait,
1974) to determine the proportion of initial substrate rapidly lost from Plot 0, and regressions
were determined for both the initial (Plot 01) and
the corresponding later phase (Plot 02) for each
individual collection station. Then, starting with
the first phase for Plot 0, the mass loss curve to
year 7 was determined mathematically by substituting known values for the slopes of lines (b) and
corresponding x- and y-values in derivations of
the equation for a straight line:
y = a + bx
(2)
a = y – bx
(3)
factor
eam
——
= ——–
a
o
1
e
(4)
where:
eam = antilog of the a value for the model
eao = antilog of the initial observed a
As individual collection stations were used,
data were analysed assuming that variables were
dependent, not independent. Decomposition
rates could thus be calculated for each individual
collection station (k), and a mean calculated
–
from these for each of the three sites (k). Nutrient contents were calculated by multiplying
nutrient concentrations by needle mass estimated
from the multi-phase exponential mass loss
model for the 7-year chronosequence. All data
were analysed using SAS (SAS Institute Inc.,
1985).
and
where:
a = y-axis intercept; b = slope of line = –k.
Constructing the 7-year curve sequentially
from the end of Plot 0, the mid-point on the xaxis between the last collection from Plot 0 (day
804) and the first collection from Plot 2 (day 535)
is 1.83 years. Using a0 and b for the second phase
of Plot 0 (4.58 and –0.33, respectively; Table 2)
as a and b in equation (2) and x = 1.83, y = 3.94.
As Plot 2 litter loses weight at the rate of k = 0.28
a–1 (Table 2) from the point (1.83, 3.94) onwards,
then from equation (3), a = 4.45 for the equation
describing the line through Plot 2 data. The midpoint on the x-axis between the last collection
from Plot 2 (day 1338) and the first collection
from Plot 5 (day 1858) is 4.38 years from cutting.
Using this as the x-value in equation (2), y = 3.24
when k = 0.28 a–1. As Plot 5 litter loses weight at
the rate of k = 0.10 a–1 (Table 2) from this point
(4.38, 3.24), then the equation describing the line
for Plot 5 data passing through this point will
have the y-axis intercept 3.65 (equation (3)).
Results
Mass loss over time
Mass loss over time for needles in litterbags from
each of the three individual plots is shown in
Figure 1, and mean decomposition rates were
0.34 a–1 for Plot 0, 0.28 a–1 for Plot 2, and 0.10
a–1 for Plot 5. Long-term mass loss incorporating
a multiple-phase model to take into account the
observed decrease in k with increasing time from
felling at Kielder is shown in Figure 2, where the
original percentage mass remaining data from
Plots 2 and 5 were multiplied by factors f2 = 0.56
and f5 = 0.23 to give appropriate y-axis intercepts
for the 7-year multiple-phase model. Regressions
for individual collection stations are presented in
Table 2, along with mean values for each of the
four phases (Plots 01, 02, 2, 5). Individual
microsite (i.e. collection station) decomposition
rates (k) were in the range 0.35–0.80 a–1 for Plot
01, 0.22–0.63 a–1 for Plot 02, 0.20–0.37 a–1 for
Plot 2 and 0.05–0.15 a–1 for Plot 5. However, R2
values for individual microsite were never less
than 0.71, were as high as 0.99, and mean plot
values were 0.91, 0.94 and 0.81 for Plots 02, 2
602 (4) (cr/k) 25/6/99 1:04 pm Page 211
THE LONG-TERM DECOMPOSITION OF SITKA SPRUCE NEEDLES IN BRASH
211
Table 2: Individual regression of ln (per cent needle mass remaining) over time for each collection station on
each plot, with mean values and standard errors for each plot (ao = original untransformed y-intercept from raw
data; am = y-intercept after transformation for 7-year 4-phase exponential mass loss model when f2 = 0.56 and
f5 = 0.23; collection stations for each plot ranked in order of decreasing k)
Plot
Collection
Station
ao
am
b
R2
s.e.
P-value
n
k
01
01
01
01
01
01
01
01
01
01
–
X
s.e.
5
8
1
3
4
10
2
6
7
9
4.605
4.605
4.605
4.605
4.605
4.605
4.605
4.605
4.605
4.605
4.605
–
4.605
4.605
4.605
4.605
4.605
4.605
4.605
4.605
4.605
4.605
4.605
–
–0.797
–0.628
–0.624
–0.558
–0.511
–0.471
–0.397
–0.383
–0.379
–0.346
–0.509
0.045
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2
2
2
2
2
2
2
2
2
2
0.80
0.63
0.62
0.56
0.51
0.47
0.40
0.38
0.38
0.35
0.51
0.05
02
02
02
02
02
02
02
02
02
02
–
X
s.e.
5
1
4
7
2
3
9
8
10
6
4.709
4.605
4.591
4.566
4.609
4.573
4.587
4.464
4.528
4.518
4.575
0.021
4.709
4.605
4.591
4.566
4.609
4.573
4.587
4.464
4.528
4.518
4.575
0.021
–0.611
–0.364
–0.363
–0.337
–0.328
–0.325
–0.293
–0.270
–0.224
–0.222
–0.334
0.035
0.926
0.938
0.921
0.768
0.924
0.925
0.883
0.918
0.958
0.909
0.907
0.017
0.138
0.075
0.085
0.148
0.075
0.074
0.085
0.065
0.037
0.056
0.084
0.011
0.002
0.002
0.002
0.022
0.002
0.002
0.005
0.003
0.001
0.003
0.004
0.001
6
6
6
6
6
6
6
6
6
6
0.61
0.36
0.36
0.34
0.33
0.33
0.29
0.27
0.22
0.22
0.33
0.03
2
2
2
2
2
2
2
2
2
2
–
X
s.e.
4
5
6
9
8
7
1
3
2
10
5.184
5.104
5.064
5.030
5.066
5.056
5.045
4.968
4.901
4.900
5.032
0.028
4.602
4.522
4.482
4.448
4.484
4.474
4.463
4.386
4.319
4.318
4.450
0.028
–0.367
–0.308
–0.307
–0.293
–0.291
–0.287
–0.269
–0.258
–0.197
–0.196
–0.277
0.016
0.923
0.978
0.987
0.846
0.956
0.877
0.967
0.968
0.948
0.966
0.942
0.015
0.087
0.038
0.029
0.102
0.051
0.088
0.041
0.039
0.038
0.030
0.052
0.008
0.000
0.000
0.000
0.001
0.000
0.001
0.000
0.000
0.000
0.000
0.000
0.000
8
8
8
8
8
8
8
8
8
8
0.37
0.31
0.31
0.29
0.29
0.29
0.27
0.26
0.20
0.20
0.28
0.02
5
5
5
5
5
5
5
5
5
–
X
s.e.
5
3
8
10
6
2
7
9
1
5.397
5.369
5.255
5.173
5.103
5.038
4.967
4.893
4.889
5.121
0.064
3.929
3.901
3.787
3.705
3.635
3.570
3.499
3.425
3.421
3.653
0.064
–0.150
–0.141
–0.119
–0.104
–0.092
–0.078
–0.067
–0.056
–0.051
–0.095
0.012
0.885
0.869
0.821
0.770
0.824
0.717
0.874
0.826
0.712
0.811
0.022
0.035
0.035
0.036
0.037
0.028
0.032
0.016
0.016
0.021
0.028
0.003
0.005
0.007
0.013
0.022
0.012
0.033
0.006
0.012
0.035
0.016
0.011
6
6
6
6
6
6
6
6
6
0.15
0.14
0.12
0.10
0.09
0.08
0.07
0.06
0.05
0.10
0.01
602 (4) (cr/k) 25/6/99 1:04 pm Page 212
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F O R E S T RY
and 5, respectively, with standard errors of 0.02
for all plots (Table 2).
Annual mass loss
The percentage mass remaining at the end of each
individual year was determined from equation (2)
and from the 7-year multiple-phase exponential
mass loss model. Annual decomposition rates for
each individual year were then arithmetically
calculated from the mass, and the results are presented in Table 3 where they are compared with
data from other long-term decomposition studies.
Brash needle nutrient concentration and content
Mean needle nutrient concentrations (percentage
by weight) for all collection dates and all plots
over the 7-year chronosequence are shown
Figure 1. Per cent original mass remaining and ln
(per cent original mass remaining) over time of three
ages of Sitka spruce brash needles in litterbags on
three plots of different times since harvesting (error
bars indicate ± s.e).
graphically in Figure 3. Mean needle nutrient
content data (mg litterbag–1) are also presented as
a percentage of original nutrient content of litterbags in Figure 3, along with estimated per cent
mass remaining for the 7-year mass loss model.
Nitrogen The N concentration of the brash
needles consistently increased over the chronosequence with the exception of two collection dates
(the last collection from Plot 0 and the first collection from Plot 5). The rate of increase in
concentration was greatest at the beginning of the
study (Plot 0), and gradually declined over time.
While Plot 2 data fit this general pattern, it is
notable that the overlapping period in the
chronosequence between the end of Plot 0 and the
beginning of Plot 2 did not fully correspond, with
Plot 2 concentrations being less than those for
Plot 0. The N content of the needles generally
Figure 2. Per cent original mass remaining and ln
(per cent original mass remaining) over time of three
ages of Sitka spruce brash needles in litterbags on
three plots of different times since harvesting, corrected for chronosequence (see Table 2 for regression
parameters; error bars indicate ± s.e.).
602 (4) (cr/k) 25/6/99 1:04 pm Page 213
THE LONG-TERM DECOMPOSITION OF SITKA SPRUCE NEEDLES IN BRASH
213
Figure 3. Nutrient concentration (%) of brash needles and nutrient content of litterbags (as % of original
content) over a 7-year chronosequence. Mass of needles remaining (as per cent of original calculated from
7-year model of mass loss in Figure 2) is indicated with dotted line for comparison with loss or retention
of nutrients relative to needle mass loss (d = Plot 0, m = Plot 2, j = Plot 5).
decreased over the 7 years, but not as rapidly as
mass loss.
Phosphorus The P concentration of needles
from the three plots behaved very differently from
N. On Plot 0, the concentration generally
remained constant, and then decreased after oneand-a-half years. However, on Plot 2 the concentration rose steadily from an initial low value in
comparison with Plot 0 data. Concentrations
declined on Plot 5 and then levelled off. Overall,
the data suggests a slow decline in P concen-
tration over the 7 years, with the exception of Plot
2. However, the P contents of needles declined
over the whole chronosequence at generally the
same rate as mass was lost.
Potassium Unlike N and P, concentrations of K
generally declined exponentially over the
chronosequence. The overlap between Plots 0 and
2 was much more comparable than for the
previous two nutrients. Potassium contents also
declined in an exponential manner over the
chronosequence, and approximately 80 per cent
58.8
54.7
–
43.7
–
35.7
–
wt
kd
0.36
0.14
0.04
0.03
0.01
–
–
ki
0.36
0.25
0.18
0.14
0.11
–
–
kd
—————————–
wt
0.53†† 0.53††70.0
0.07 0.30 61.1
–
–
59.0
0.11‡‡ 0.21 57.3
–
–
56.8
0.10 0.17 –
–
–
–
ki
—————————–
Chamaecyparis
obtusa†
66.9
46.3
28.7
33.0
26.9
–
–
wt
kd
0.40 0.40
0.37 0.38
0.48 0.42
–0.14 0.28
0.20 0.26
–
–
–
–
ki
—————————–
Pinus
densifolia‡
67.6
49.4
30.8
32.2
36.1
–
–
wt
0.39
0.31
0.47
–0.04
–0.11
–
–
ki
0.39
0.35
0.39
0.28
0.20
–
–
kd
——————————
Pinus
radiata§
72.7
55.6
41.2
33.5
25.0
–
–
wt
0.32
0.27
0.30
0.21
0.29
–
–
ki
0.32
0.29
0.30
0.27
0.28
–
–
kd
——————————
Pinus
sylvestris¶
76.9
70.8
65.1
62.0
52.2
–
–
wt
0.26
0.08
0.08
0.05
0.17
–
–
ki
0.26
0.17
0.14
0.12
0.13
–
–
kd
—————————–
Populus
balsamifera||
* Edmonds (1984), Table 1; 175-year-old Abies amabilis stand near Seattle, WA, USA.
† Takeda (1995), back-calculated from k values in Table 1; Chamaecyparis obtusa stand near Kyoto, Japan.
‡ Takeda (1988), Figure 1a; mixed Pinus densiflora and Chamaecyparis obtusa stand near Kyoto, Japan.
§ Will (1967) Table 1; 33-year-old Pinus radiata stand in Kaingaroa Forest, NZ.
¶ Berg et al. (1982b), Table 1; 120–130-year-old Pinus sylvestris stand near Ivantjärnsheden, central Sweden.
|| Lousier and Parkinson (1976), Table 6; aspen woodland near Kananaskis Valley, AB, Canada.
** This study; brash needles on clearcut.
†† 14-month data, and cf. k = 0.45 in Edmonds (1984), Table 1.
‡‡ Annual rate, over 2-year period.
1
2
3
4
5
6
7
Year
Abies
amabilis*
ki
kd
wt
kd
0.36 0.36
0.32 0.34
0.28 0.32
0.28 0.31
0.16 0.28
0.10 0.25
0.10 0.23
ki
——————————–
Picea
sitchensis**
74.6 0.29 0.29 69.5
72.5 0.03 0.16 50.2
66.2 0.09 0.14 38.1
48.2 0.32 0.18 28.9
59.7 –0.21 0.10 24.5
–
–
–
22.3
–
–
–
20.2
wt
——————————–
Populus
tremuloides||
Table 3. Comparisons of arithmetically derived decomposition rates (k = –ln(x/x0)/t where x = % wt. remaining, x0 = % wt. remaining in previous year, and
t = 1; after Olson, 1963) for each individual year (ki) and over the duration of the study period (kd) calculated from per cent of original mass remaining (wt)
at time t (negative rates are the result of unexplained increases in litterbag mass)
602 (4) (cr/k) 25/6/99 1:04 pm Page 214
602 (4) (cr/k) 25/6/99 1:04 pm Page 215
THE LONG-TERM DECOMPOSITION OF SITKA SPRUCE NEEDLES IN BRASH
of the K was lost in the first year. Rates of K losses
from needles greatly exceeded rates of needle
mass loss.
Sodium Concentrations of Na showed an initial
sharp decline from 0.05 to 0.02 per cent over the
first 31/2 months. The concentrations then fluctuated for the rest of the study between 0.004 and
0.018 per cent. The Na content of litter behaved
similarly, with over 65 per cent lost over the first
31/2 months. As with K, Na content was lost
rapidly as compared with needle mass.
Calcium Calcium concentrations were erratic
and fluctuated with little pattern over the
chronosequence. Values oscillated between lows
of 0.25 per cent and highs of 0.45 per cent.
Calcium contents rose to above initial values in
year 1, and then declined over 7 years at a rate
that was comparable to the rate of mass loss.
Magnesium After an initial small increase, Mg
concentrations generally decreased over the
chronosequence in approximately an exponential
pattern. Comparing the slopes of the lines of Mg
content with the slopes of the 7-year mass loss
model shows that while Mg may have been
retained relative to needle mass for the first year
following clearfelling, it was then lost very
rapidly. For Plots 2 and 5 the rates of Mg loss
were the same as the rates of needle mass loss.
Discussion
Use of chronosequences for litterbag studies
Chronosequences have been used in many ecological studies over the past few decades (Cole
and Van Miegroet, 1989; Yarie et al., 1989).
While the main advantage is a contraction of the
time required for the study, disadvantages include
the need for: (1) similar climatic conditions on all
plots, (2) similar biotic factors on all plots, (3)
similar ecosystem and/or site properties, and (4)
data points that extend to the end of the
chronosequence being studied (Cole and Van
Miegroet, 1989). When these conditions are not
met, results can be misleading (Turvey and
Smethurst, 1989). The plots at Kielder were
chosen on the assumption that proximity and
common site conditions and history (Table 1)
215
would ensure comparability and construction of
a valid chronosequence. While the needle mass
and nutrient concentration changes over the
chronosequence were largely credible (with the
exception of P), and the nutrient content changes
generally followed on well over successive plots,
it is still notable that all the nutrient contents at
the beginning of Plot 2 were consistently below
those at the end of Plot 0. This may be the result
of a combination of interacting factors such as
differences in initial needle quality, differences in
site quality that led to differences in needle nutrient concentrations and substrate qualities, or
differences in microclimatic conditions or decomposer populations. Furthermore, any site differences that led to errors in developing the mass loss
model would also be reflected in needle nutrient
content calculations. Regardless of the source of
errors, the initial assumptions made in the construction of the chronosequence appear to be
generally valid, and provide a method of estimating both the mass and nutrient content of litter
over a long time span from 2 years of field data.
However, as the plots were not replicated, application of the data to other sites should be done
cautiously.
Determination of decomposition rates
The simplicity of the calculation of the decomposition rate constant k has led to its widespread
acceptance and use. The relative ease with which
k can be determined and the widespread use of
litterbags from distinct collection stations within
a plot or site can lead to two possible shortcomings: (1) the determination of k as though
variables were independent when in fact they
were dependent because of the use of discrete collection stations, and (2) the application of k to a
wider time span than is warranted. Because substrate quality changes as decomposition progresses, there is a corresponding decrease in k
over time.
Site vs. microsite decomposition rates Although
some authors lay out litterbags randomly within
a plot and therefore mass loss nutrient variables
are independent (e.g. Taylor and Jones, 1990),
other researchers establish randomly located collection stations (or sampling stations, or subplots) within a plot and place a number of
602 (4) (cr/k) 25/6/99 1:04 pm Page 216
216
F O R E S T RY
litterbags within each collection station, and thus
variables are dependent rather than independent
(e.g. Berg et al., 1982a). Where individual collection stations have been used, a ‘best fit’ line can
be determined for each station, and then a mean
–
k (or k) calculated for the entire plot from the
individual regressions. There are several advantages to this, in that not only can a single mean
–
for the plot (k) with a corresponding measure of
–
––
variation (s.e.) and goodness of fit (R2) be
obtained, but a precise decomposition rate (k)
with appropriate measures of variation (s.e., R2)
can be determined for individual microsites
(Table 2). An examination of the range of k for
individual collection stations shows that decomposition rates can vary by up to 300 per cent
between microsites on the same plot (Table 2).
However, the high R2 values for individual collection station regressions confirms that decomposition within a given microsite proceeds
temporally in a consistent manner. Logistically,
this added information on spatial variation can be
gained for little, if any, extra effort in the field.
However, collection stations must be small
enough that microsite conditions within an individual collection station are uniform.
Temporal considerations in determination of
decomposition rate Care must be taken in the
–
application of a single k (or k) value to a wider
time span than is warranted because of changes
in substrate quality over time (Minderman,
1968). In long-term experiments k can be calculated individually for each year (ki, where i is the
ith year), especially for comparative purposes.
The results from a number of long-term studies
are presented in Table 3 with re-calculations of k
for increasing durations of time from the beginning of the studies (kd), as well as for each individual year (ki). Several trends are immediately
apparent. First, there is a great range in the
number of years required to approach a relatively
constant ki value (approximately 2 years for
Abies amabilis, Populus balsamifera and P.
tremuloides; 3 years for Chamaecyparis obtusa;
and 6–7 years for Picea sitchensis). However,
these constant values vary from approximately
0.1 a–1 for A. amabilis and P. sitchensis to
0.03–0.09 a–1 for Populus, to 0.01–0.03 a–1 for
C. obtusa. Data for other species were too irregular to discern if a constant decomposition rate
had been attained. More long-term studies are
required to determine when different litter types
ultimately attain a constant annual decomposition rate or ‘final limit value’ (Berg et al., 1996),
what this rate is, and what the implications of the
time taken to reach this rate are for humus
development. Second, decomposition rates in
individual later years (ki) can often be over half
those calculated for the duration of the study up
to that year (kd) for all species examined. Third,
periods of net accumulation (i.e. negative values)
and increases (rather than expected decreases) in
consecutive annual rates of decomposition (ki)
can be masked or reduced in calculations of kd
over long-term periods (Pinus densifolia, P.
radiata, Populus balsamifera, P. tremuloides).
It is possible that later negative values or
increases in annual k values reflect year-to-year
variation in climate (Bunnell and Tait, 1974).
However, it is also possible that as litterbags bags
became better incorporated into the soil profile,
the hydraulic conductivity of the system increased
so that needles within the bags became better supplied with moisture from the surrounding litter
layer over time and hence decomposed faster.
Fragments from the surrounding litter may also
have entered the bags once the outside litter had
decomposed to a small enough size to pass
through the mesh. Certainly the evidence suggests
that there can be problems with long-term
decomposition studies using litterbags, and
models constructed from a chronosequence study
using litters of different ages may reduce these
experimental errors in that the actual field incubation time can be reduced. A related advantage
is that observed increases in variance of mass loss
with time in the field (e.g. Takeda, 1988) can be
minimized.
Litter of a given species usually loses weight
more quickly in stands than on clear-cuts (Whitford et al., 1981). First-year decomposition rates
of needles from litterfall in a Sitka spruce stand in
Wales ranged from 0.43 to 0.63 a–1 (derived from
Hayes, 1962 and 1965), compared with 0.36 a–1
for brash needles on a cutover at Kielder (Table
3). To estimate 30-year weight loss from brash
needles at Kielder, an exponential asymptotic
equation (Howard and Howard, 1974; Berg et
al., 1996) was fitted (Jandel Scientific, 1991)
through the first 7 years of brash needle weight
loss data for Kielder (Table 3), and weight
602 (4) (cr/k) 25/6/99 1:04 pm Page 217
THE LONG-TERM DECOMPOSITION OF SITKA SPRUCE NEEDLES IN BRASH
remaining = 16.5 + 83.6 3 e(–x years/2.18) (R2 =
0.9997; s.e. = 0.606). This line becomes asymptotic after 17 years, and predicts that 16.5 per
cent (± 1; 95 per cent confidence interval) of the
original needle weight will remain after 30 years,
for a kd value of 0.06 a–1 as compared with kd
values of 0.08–0.13 a–1 found by Miller et al.
(1996) for 30-year-old Sitka spruce stands. While
the differences between the Kielder clear-cut and
the five stands are in the expected order (Whitford et al., 1981), it is acknowledged by Miller et
al. (1996) that their stand estimates assume that
the only inputs to the forest floor are needle litter.
If it is assumed that one-quarter of the forest floor
is made up of fine root biomass and litter then kd
= 0.10–0.17 a–1; if the root biomass and litter
component is 50 per cent of the forest floor, then
kd increases to 0.15–0.25 a–1 (derived from Miller
et al., 1996, Table 8). These latter estimates may
be more in keeping with the nature of forest floor
material.
Brash needle nutrient concentration and content
Concentrations of nutrients limiting litter decomposition remain constant or increase until the
carbon : nutrient ratio falls below a critical level
required for microbial decomposition of the litter
(Swift et al., 1979). Other elements may be easily
leached from litter (Bogatyrev et al., 1983), while
some bound elements such as Ca may require
decomposition to take place before they are
released from the substrate (Lousier and Parkinson, 1978). The role of some limiting nutrients in
controlling mass loss is discussed in Berg and
Staaf (1981) and Melillo et al. (1982). The
dynamics of a range of nutrients in decomposing
litter under stands is discussed in Lousier and
Parkinson (1978), Staff and Berg (1982), de
Catanzaro and Kimmins (1985) and Weber
(1987). The dynamics of nutrients in decomposing brash needles is discussed in Berg et al.
(1982a) and Berg and Staff (1983).
Nitrogen Concentrations of N in litterbags
increased from 1.2 to 2.3 per cent over 7 years at
Kielder (Figure 2), and this indicates that this
nutrient is being accumulated in the litter (cf. Berg
and Staaf, 1981) and/or that it is not being lost at
the same rate as litter mass. Similar increases in
concentration (but over a shorter time period)
217
have also been observed under Scots pine brash
on some Swedish clearfelled sites, where N concentrations of brown needles rose from 0.29 per
cent (by weight) at the onset of an experiment to
1.06 per cent by 3 years, and N concentrations of
green needles rose from 1.16 to 2.2 per cent by 3
years (calculated from regressions in Berg et al.,
1982a). However, on other clearfelled sites in
Sweden with twice the amount of slash as normal,
N concentrations in Scots pine litter decreased
from 1.15 to 1.11 per cent over 2 years (Berg and
Staff, 1983), indicating that N was not limiting
decomposition on this latter site.
The actual total N content of the litterbags
decreased with time, indicating that although N
was being retained relative to litter mass, it was
not actually accumulated in brash needles during
the first 2 years. The rate of retention decreased
over the next 2 years, and on Plot 5 (years 5 to 7)
the virtually constant concentration yet decreasing
content indicates that N was being lost at the same
rate as litter mass (Figure 1). While there may have
been some initial leaching of N (Berg and Staaf,
1981) it is unlikely that this would have remained
the major process through which this element was
lost from the litter. Although N is initially the
nutrient most easily leached from Scots pine
needles (Bogatyrev et al., 1983), amounts lost in
laboratory experiments quickly decline, and are
relatively low compared with amounts leached
from broadleaves. In the only other known study
on nutrient dynamics in decomposing Sitka spruce
litter, N concentrations were also observed to
increase with time (Hayes, 1962 and 1965).
Phosphorus Even though the P concentration
data from Plots 0 and 2 at Kielder do not follow
on well from each other in the chronosequence
(Figure 3), the data suggest that P concentrations
remained constant at 0.11–0.12 per cent or
increased to this range over the first 4 years after
harvesting, and then later decreased. Staaf and
Berg (1982) likewise found that P concentrations
in Scots pine litter decomposing in a 120–130year-old stand in Sweden increased over a 4-year
period.
As with N, the actual P content of the litterbags
decreased over the duration of the chronosequence. However, the loss of P compared with
mass indicates that this element was released at
the same rate as litter mass was lost.
602 (4) (cr/k) 25/6/99 1:04 pm Page 218
218
F O R E S T RY
Potassium Potassium is a highly mobile cation
that is rapidly leached from foliage (Tukey, 1970;
Bogatyrev et al., 1983). Both the concentration
and content data (Figure 3) show rapid exponential decreases in this nutrient, with approximately
80 per cent of the K lost from the litterbags within
the first year of decomposition compared with a
mass loss of only 30 per cent. A similar rapid loss
has been observed in Scots pine brash needles in
Sweden (Berg and Staaf, 1983) as well as in litter
decomposing under tree canopies (Lousier and
Parkinson, 1978; Moore, 1984). The mobility of
K in brash needles is similar to that observed for
LFH horizons at Kielder, where leachate outputs
of K were occasionally the same as precipitation
inputs by year 6 of the chronosequence even
though the LFH capital was approximately 30 kg
ha–1 (Titus and Malcolm, 1992). This, and the
lack of change in either content or concentration
of K in litterbags by year 6, suggests that relatively little of this element will be made available
to seedlings from the litter after the first year.
Sodium The rapid decrease in initial Na
concentration (Figure 3) is not surprising, given
the ease with which this element is leached from
plant tissue (Tukey, 1970) and some litters
(Lousier and Parkinson, 1978). Erratic concentrations of this element in litter have been
observed elsewhere (Lousier and Parkinson,
1978), and may partly be the result of the great
variation in precipitation inputs of this element at
Kielder (Titus and Malcolm, 1992), and the
ability of litter to retain this element for short
periods on exchange sites. Sodium, as a monovalent cation, can predominate on forest floor
exchange sites (Nye and Tinker, 1977) and can be
easily removed through leaching (Duchaufour,
1982).
Calcium Calcium leaches slowly from pine
litter (Bogatyrev et al., 1983) and some deciduous
litter (Lousier and Parkinson, 1978), and this lack
of mobility is thought to be because of the
incorporation of Ca into cell wall structures. The
initial increase in the concentration of this
element (Figure 3) and fairly steady level of
content over the first year suggest that Ca was
neither leached nor released through the breakdown of the initial easily decomposable fraction
of the litter. Indeed, where Ca was added to brash
in fertilizer at Kielder the Ca content of brash
litter was increased, suggesting that Ca can be
immobilized in a recalcitrant form in litter (Titus
and Malcolm, 1987). However, once the more
recalcitrant fraction of the litter began to decompose, Ca was released at the same rate as mass
was lost, indicating that mobilization of this
element from litter is dependent on microbial
decomposition. Calcium loss from litter in mature
Scots pine stands in Sweden (Staff and Berg,
1982) was similar, except that Ca was lost at a
slightly faster rate than mass.
Magnesium Magnesium has been found to be
more readily leached from some coniferous litter
than Ca, but less readily than K (Bogatyrev et al.,
1983). This intermediate mobility is reflected in
an exponential decrease in both litter concentration and content (Figure 3) over 7 years, but
with a slower rate of decline than that observed
for K. However, leaching of Mg from Scots pine
litter can be low (Staff and Berg, 1982).
Implications for management of brash
Although the presence of brash can be an impediment to planting, it contains a potential
supply of nutrients for the next crop rotation. At
Kielder, this amounts to a total of 68, 6 and 25
kg ha–1 of N, P and K and in the brash needles,
respectively, compared with 879, 51 and 86 kg
ha–1 of N, P and K in the LFH horizon (Plot 0,
clear strip only; Titus and Malcolm, 1991). The
relative retention of N led to approximately 37 kg
ha–1 remaining in the brash needles after 3 years
and 24 kg ha–1 after 7 years (55 per cent and 35
per cent of the original amount, respectively;
Figure 3). Movement of P out of the brash needle
litter was greater, with approximately 2 and 1 kg
ha–1 remaining after 3 and 7 years, respectively
(32 per cent and 15 per cent of the original
amount; Figure 3). However, the greatest movement was for K, with only about 1 and 0.5 kg ha–1
remaining after 3 and 7 years (5 per cent and 2.5
per cent of the original amount; Figure 3).
Approximately 14 kg ha–1 N is retained in
brash needles over the first 7 years, relative to
needle mass loss (i.e. the difference between N
content and per cent original mass remaining;
Figure 3). Although immobilized in needles, this
N should eventually become available for tree
602 (4) (cr/k) 25/6/99 1:04 pm Page 219
THE LONG-TERM DECOMPOSITION OF SITKA SPRUCE NEEDLES IN BRASH
219
growth as it is released through decomposition
processes. The 44 kg ha–1 N released over 7 years
represents about 5 per cent of the total LFH
capital (Titus and Malcolm, 1991). The input of
P to the site from brash needles is not large, but
the 4–5 kg ha–1 over 7 years represents about 10
per cent of the total LFH capital (Titus and
Malcolm, 1991). By contrast, the 23–24 kg ha–1
of K released from the brash needles over 7 years
represents about 50 per cent of the total amount
potentially available in the LFH horizons. This,
combined with the rapid leaching of K from the
LFH horizons (Titus and Malcolm, 1992), suggests that very little K may be available to regenerating seedlings from either the brash or the old
LFH horizons by the time their new root systems
have begun to occupy the site, and demand for
nutrients increases. While decomposition processes may provide a steady supply of N and P, it
is unlikely that much K will become available
from either brash needles or the LFH 7 years after
felling. Care should be taken to monitor second
rotation plantations to ensure that they do not
become K-deficient, although P and N deficiencies
are less likely.
well for most nutrients studied, but some nutrient
concentrations and contents varied between
plots. This suggests that the sites may have differed in some respect which was not estimated,
indicating the inherent problem of finding suitable sites for chronosequence studies. However,
the alternative of incubating litterbags for long
time periods is also not without problems, and a
chronosequence approach may be preferable,
especially if replicated sites are used.
Differences in the dynamics of nutrients in
brash needles was evident at Kielder. Initially
both N and P were retained. Potassium, Na and
Mg were rapidly lost from litter, indicating that
they were not limiting decomposition and could
be readily leached from needles in litter.
Calcium was lost at the same rate as litter mass
so was neither limiting nor readily mobile. The
amounts of all nutrients after 5 years were low
compared with the initial amounts in the brash,
and nutrient loss became proportional to mass
loss. Retaining brash needles on site will ensure
an added supply of N and P to young seedlings,
but K will rapidly be lost. Second rotation plantations should be monitored to ensure that they
do not become K-deficient.
Conclusions
Acknowledgements
Litterbag studies have traditionally been used to
determine litter decomposition rates. However,
where individual collection stations are used,
microsite decomposition rates can also be determined. This requires little extra logistical
resources, and provides detailed information on
the spatial variation in decomposition rates.
Determination of k for longer time periods does
not take into account changes in k over time as
labile components of litter are decomposed and
residual substrate quality becomes more recalcitrant. The determination of k for individual years
(ki, where i is the ith year) gives a useful parameter for comparing different studies, and indicates
that in some long-term litterbag studies decomposition rates appear to increase after 4–5 years.
Where there are concerns that this is due to problems inherent in long-term use of litterbags, a
chronosequence approach that reduces incubation times in the field may be of use when the
age of the litter is known.
The use of a chronosequence approach worked
This work was completed while the senior author was
a graduate student in receipt of a University of Edinburgh Student Fellowship. The financial assistance,
cooperation and advice of the Forestry Commission,
and the technical assistance of Mr A. Gray (University
of Edinburgh) are gratefully acknowledged. The
authors wish to thank Dr M. Weber (Canadian Forest
Service, Edmonton, Alberta), Dr G. Bird (AECL,
Pinawa, Manitoba) and Dr C. Prescott (UBC) for
reviewing an initial draft of the manuscript.
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Received 18 March 1998