Global Change Biology (2003) 9, 575±584
Climatic effects on litter decomposition from arctic tundra
to tropical rainforest
J A R I L I S K I *, A R I N I S S I N E N {, M A R K U S E R H A R D { and O L L I T A S K I N E N §
*European Forest Institute, Torikatu 34, FIN-80100 Joensuu, Finland, and The Department of Forest Ecology, University of Helsinki,
Finland, {Finnish Environment Institute, PO Box 140, FIN-00251 Helsinki, Finland, {Potsdam-Institute for Climate Impact
Research, PO Box 60 12 03, D-14412 Potsdam, Germany, §The Department of Forest Ecology, PO Box 27, FIN-00014 University of
Helsinki, Finland
Abstract
Climatic effects on the decomposition rates of various litter types in different environments must be known to predict how climatic changes would affect key functions of
terrestrial ecosystems, such as nutrient and carbon cycling and plant growth. We developed regression models of the climatic effects on the first-year mass loss of Scots pine
needle litter in boreal and temperate forests across Europe (34 sites), and tested the
applicability of these models for other litter types in different ecosystems from arctic
tundra to tropical rainforest in Canada (average three year mass loss of 11 litter types at 18
sites), the USA and Central America (four litter types at 26 sites).
A temperature variable (annual mean temperature, effective temperature sum or its
logarithm) combined with a summer drought indicator (precipitation minus potential
evapotranspiration between May and September) explained the first-year mass loss of
the Scots pine needle litter across Europe with a higher R2 value than actual evapotranspiration (0.68±0.74 vs. 0.51) and with less systematic error for any sub-region. The model
with temperature sum and the summer drought indicator appeared best suited to the
other litter types and environments. It predicted the climatic effects on the decomposition rates in North and Central America with least systematic error and highest R2 values
(0.72±0.80). Compared with Europe, the decomposition rate was significantly less sensitive to annual mean temperature in Canada, and to changes in actual evapotranspiration
in the USA and Central America.
A simple model distinguishing temperature and drought effects was able to explain
the majority of climatic effects on the decomposition rates of the various litter types
tested in the varying environments over the large geographical areas. Actual evapotranspiration summarizing the temperature and drought effects was not as general climatic
predictor of the decomposition rate.
Keywords: boreal forest, decomposition, drought, litter, temperate, temperature forest
Received 22 January 2002; revised version received 5 September 2002 and accepted 4 October 2002
Introduction
Decomposition of plant litter is a key process in nutrient
and carbon cycling in terrestrial ecosystems. It makes
nutrients available for growth again and releases carbon
to be returned to the atmosphere. Any changes in the
Correspondence: J. Liski, European Forest Institute,
c/o Department of Forest Ecology, PO Box 27, FIN-00014
University of Helsinki, Finland, fax ‡ 358 9 191 58100,
e-mail: jari.liski@efi.fi
ß 2003 Blackwell Publishing Ltd
rate of decomposition may thus change the nutrient
availability, plant growth and the carbon budget of terrestrial ecosystems.
The decomposition rate depends on the quality of the
litter, as well as favourable temperature and moisture
conditions; therefore, climatic changes would change
its rate. To predict these changes, the effects of temperature and moisture on the decomposition rate must be
known.
Actual evapotranspiration is often ranked as the best
climatic index of the decomposition rate (Meentemeyer,
575
576 J . L I S K I et al.
1978; Berg et al. 1993; Aerts 1997). It summarises temperature and moisture effects, increasing with temperature as
long as there is water to be evaporated (Rosenberg et al.,
1983). Actual evapotranspiration values are low where
temperatures are low or soils are dry. Thus, cool northern
forests may have similar evapotranspiration values as
dry southern forests, but for different reasons.
Actual evapotranspiration has not, however, always
proved to be the best climatic indicator of the decomposition rate. Annual mean temperature and precipitation explained the climatic effects on the average
three-year mass loss of 11 litter types across Canada so
well that actual evapotranspiration provided little additional explanation (Moore et al., 1999). Along a transect
from northern Fennoscandia to the west coast of Portugal
in Europe, the first-year mass loss of Scots pine (Pinus
sylvestris L.) needle litter was closely related to actual
evapotranspiration but, in eastern central Europe and
around the Mediterranean, this mass loss was lower
than expected (Berg et al., 1993). Nevertheless, the
authors associated the decomposition rate to actual evapotranspiration concluding that the relationship was different in these two regions. Such discrete region-specific
relationships would be difficult to use in predicting the
possible changes in the decomposition rate in response to
climatic changes. Moreover, these differences suggest
that there may be a more direct and general way to relate
the decomposition rate to climate.
The objective of this study was to evaluate how accurately and generally the effects of climate on the decomposition rate can be quantified by distinguishing the
effects of temperature and drought compared to summarising these effects in actual evapotranspiration. First, we
quantified the effects of temperature, drought and actual
evapotranspiration on the first-year mass loss of Scots pine
needle litter in boreal and temperate upland forests in
Europe; then, second, we tested the applicability of these
quantified effects on the decomposition rates of other litter
types in other ecosystem types from arctic tundra to tropical rainforest in North and Central America.
Materials and methods
Modelling mass loss in Europe
We developed regression models of the first-year mass
loss of Scots pine needle litter using data from 34 forest
sites across Europe (Berg et al., 1993). This data was
collected specifically to study the effects of climate on
the mass loss values. The experimental design and laboratory analyses were standardised to ensure that data
from all sites were comparable.
These sites included most of the variation in temperature, seasonality, and continentality in Europe, extending
from northern Fennoscandia to the Mediterranean and
from the Atlantic coast to eastern Poland. Annual mean
temperature at the sites ranged from
1.7±16.6 8C,
annual precipitation varied from 402 to 1500 mm and
annual actual evapotranspiration ranged between 328
and 654 mm. At each site, senescent Scots pine needles
were allowed to decompose in 25 mesh bags for a year,
and the decomposition rate was measured as an average
mass loss from these bags. According to the measured
decomposition rates and climatic variables explaining
these rates, Berg et al. (1993) grouped these sites into
four groups: Scandinavian-NW-continental transect (n
sites), the European west coast (w sites), the Mediterranean (m sites) and the Central European (c sites) sites.
We calculated effective temperature sum of the year
(0 8C threshold), and precipitation and potential evapotranspiration from May to September for these study sites
to enable our analyses. With reference to geographical
coordinates and altitude, we derived mean monthly temperature and precipitation for each site using 10' longitude-latitude-referenced climate data and the BIOM
model (Sykes et al., 1996). Effective temperature sum
was calculated by assuming that mean monthly temperatures occurred in the middle of each month with daily
values linearly interpolated; and potential evapotranspiration was calculated from mean monthly temperatures using the algorithms of the BIOM model.
To evaluate how accurately our estimates represented
the climate at the study sites, we also calculated climate
variables reported for these sites by Berg et al. (1993).
Linear correlation coefficients for estimated and observed
annual mean temperature, annual actual evapotranspiration and total annual precipitation were 0.98, 0.88 and
0.77, respectively. The lower correlation coefficient for
annual precipitation was due to larger than observed
estimates for two sites in south-west Spain and
smaller estimates for two sites in south-west Sweden;
without these sites the coefficient was 0.94. Because the
results of this study were not sensitive to precipitation
estimates of these four sites, we concluded that our estimates were adequate.
Linear regression models were used to explain the
mass loss, k,
k ˆ a ‡ bT ‡ cD or
…1a†
k ˆ a ‡ bE:
…1b†
We used annual mean temperature, temperature sum or
logarithm of temperature sum to describe temperature
(T), the difference between precipitation and potential
evapotranspiration from May to September to describe
summer drought (D), or actual evapotranspiration (E) to
describe the combined effects of temperature and
drought as independent variables in these models. At
ß 2003 Blackwell Publishing Ltd, Global Change Biology, 9, 575±584
C L I M A T I C E F F E C T S O N L I T T E R D E C O M P O S I T I O N 577
three sites, the summer drought indicator had positive
values, which we set equal to zero, assuming no drought
effect. Logarithm of temperature sum was included in the
analyses to study if mass loss was more sensitive to
temperature under cold than warm conditions, a phenomenon suggested based on soil respiration and nitrogen mineralisation measurements (Raich & Schlesinger,
1992; Kirschbaum, 1995).
Testing the models in North and Central America
We tested the regression models, which we developed
using the European data, against data collected from 18
sites across Canada (Moore et al., 1999) and 26 sites across
the USA and Central America (Gholz et al., 2000). These
data covered and exceeded variation in temperature and
summer drought found in the European data (Fig. 1). The
annual precipitation of the Canadian sites varied from
261 to 1782 mm (Trofymow & CIDET Working Group,
1998) and that of the US-Central American sites from 260
to 3914 mm (Gholz et al., 2000).
At each Canadian site, 11 litter types were set to decompose in mesh bags and the average mass remaining
after three years was reported (Moore et al., 1999). For the
US-Central American sites, data on the mass remaining
Summer drought indicator (mm)
1500
1000
500
0
−500
−1000
−10
0
10
20
30
Annual mean temperature (˚C)
European data (Berg et al., 1993)
Canadian data (Moore et al., 1999)
US and Central American data (Gholz et al., 2000)
Fig. 1 Annual mean temperature and summer drought (precipitation minus potential evapotranspiration from May to September) at the study sites across Europe (Berg et al., 1993) used to
develop regression models of the climatic effects on litter decomposition rate, and at the study sites across Canada (Moore et al.,
1999) and the USA and Central America (Gholz et al., 2000) used
to test these models.
ß 2003 Blackwell Publishing Ltd, Global Change Biology, 9, 575±584
after the first year of decomposition in mesh bags were
available for the needles of a pine species (Pinus resinosa),
fine roots of another pine species (Pinus elliottii), and
leaves and fine roots of a tropical deciduous tree
(Dypetes glauca) (Gholz et al., 2000).
We derived values for the independent variables of our
regression models (Eqns 1a, b) at the Canadian and the
US-Central American study sites from climate data
reported (Trofymow & CIDET Working Group, 1998;
Forest Science Data Bank, 2000). We calculated temperature sum and the accumulated potential evapotranspiration between May and September from the mean
monthly temperatures like we did for the European
study sites.
Because the quality of the experimental litter in
Canada, the USA and Central America differed from
that in Europe, we tested our regression models in relative instead of absolute terms. Consequently, we
reformed Eqns 1a and b for
k ˆ k0 …1 ‡ b…T
T0 † ‡ g…D
k ˆ k0 …1 ‡ b…E
E0 ††
D0 †† or
…2a†
…2b†
where k0 expresses mass loss under temperature,
summer drought or actual evapotranspiration conditions
T0, D0 or E0, and b and g express the proportional change
in mass loss when changing from these climate conditions to other temperature, summer drought or actual
evapotranspiration conditions T, D or E. The values of k0,
b and g were derived from Eqn 1a as k0 ˆ a ‡ bT0 ‡ cD0,
b ˆ b/k0 and g ˆ c/k0 or from Eqn 1b as k0 ˆ a ‡ bE0 and
b ˆ b/k0.
We tested this model in two ways. First, we calculated
mass loss values for the Canadian and the US-Central
American study sites using Eqn 2 with coefficients b
and g determined from the European data, and compared
these estimates to measured values. The mass loss value
(k0) of the Canadian and the US-Central American litter
types at certain temperature (T0), summer drought (D0)
and actual evapotranspiration conditions (E0) was estimated by using the averages of these variables for the
Canadian and the US-Central American data; a useful
feature of linear regression models is that they always
include this average point.
The Canadian average three year mass loss was 0.360
(3 years) 1, annual mean temperature was 1.47 8C, temperature sum was 2120 8C days, logarithm of temperature
sum was 3.31 log (8C days), summer drought indicator
was
61.9 mm and actual evapotranspiration was
466 mm. The US-Central American average first-year
mass losses were 0.200, 0.484, 0.211 and 0.328 year 1 for
Pinus resinosa needles, Drypetes leaves, Pinus elliottii roots
and Drypetes roots, respectively, annual mean temperature was 11.2 8C, temperature sum was 4610 8C days,
578 J . L I S K I et al.
logarithm of temperature sum was 3.58 log (8C days),
summer drought indicator was 93.9 mm and actual
evapotranspiration was 642 mm.
We estimated the first-year mass loss of the European
litter at these same conditions using Eqn 1. Then, we
calculated the values of coefficients b and g using this
value for k0 in Eqn 2. These b and g-values expressed how
much the decomposition rate of the European litter
changed in relative terms as climate diverged from the
Canadian or the US-Central American average conditions. Finally, we used Eqn 2 with these b and g-values
and the Canadian or the US-Central American average
mass loss values (see above) to calculate the mass loss
estimates for the Canadian and the US-Central American
study sites and litter types.
To evaluate the reliability of the calculated values, in
addition to visually inspecting scatterplots, we fitted a
linear regression between the calculated and the measured mass loss values. Deviation of the slopes of these
regressions from one indicate a systematic difference
between the calculated and the measured values while
the R2 values of these regressions measure the proportion
of variation in the measurements explained by the
models.
The second approach to testing was to fit Eqn 2 to the
European, the Canadian and the US-Central American
data separately, and to compare the resulting estimates
of b and g. Because these coefficients depended on T0, D0
and E0, we did the fittings for the Canadian and the USCentral American average temperature, drought and
actual evapotranspiration conditions. To estimate the
statistical significance of the difference in the coefficients
between Europe and the other regions, we estimated
normal distributions for the coefficients based on their
95% confidence limits and calculated the overlapping
areas.
Table 1
Results
Modelling mass loss in Europe
Temperature explained the first-year mass loss of Scots
pine needle litter along the Scandinavian-NW-continental transect (n sites) as well as did actual evapotranspiration (Table 1, Fig. 2). This was expected because water
deficit did not limit actual evapotranspiration at these
sites, and so actual evapotranspiration depended only
on temperature. For these sites, the linear correlation
coefficient between actual evapotranspiration and annual
mean temperature was 0.99, and that between actual
evapotranspiration and temperature sum was 0.97.
Some of the sites on the European west coast (w sites)
followed the same relationships but the mass loss values
were lower at most of the central European (c sites) and
the Mediterranean sites (m sites) (Fig. 2).
We found that these low mass loss values at the central
European (c sites) and the Mediterranean sites (m sites)
were associated with summer drought. Drought severity
was estimated as monthly precipitation minus potential
evapotranspiration from May to September. The drier the
summer the smaller the mass loss compared to expectations based on temperature or actual evapotranspiration
relationships along the Scandinavian-NW-continental
transect (n sites) (see Fig. 2). Adding this summer
drought indicator to the regression models as another
independent variable explained the mass loss values
across Europe with considerably less systematic error
for any site group and R2 values from 0.68 to 0.74,
depending on the temperature variable (Table 1, Fig. 3).
The systematic error of the mass loss estimates along the
Scandinavian-NW-continental transect (n sites) calculated using the model with temperature sum and the
summer drought indicator (Fig. 3b) was further
Regression models for the first-year mass loss (year 1) of Scots pine needle litter in Europe (data from Berg et al., 1993)
Model
Model #
R2
n
Sites included
0.189 ‡ 0.0241 Tamt
0.0381 ‡ 0.114 10 3 Tdd0
1.73 ‡ 0.610 Tlog(dd0)
± 0.228 ‡ 1.10 10 3 E
0.195 ‡ 0.0277 Tamt ‡ 0.721 10 3 D
0.0943 ‡ 0.0995 10 3 Tdd0 ‡ 0.835 10
1.83 ‡ 0.644 Tlog(dd0) ‡ 0.705 10 3 D
0.123 ‡ 0.813 10 3 E
(1)
(2)
(3)
(4)
(5)
(6)
(7)
(8)
0.87
0.86
0.89
0.87
0.74
0.68
0.69
0.51
16
16
16
16
34
34
34
34
n sites
n sites
n sites
n sites
all sites
all sites
all sites
all sites
3
D
Tamt, annual mean temperature (8C); Tdd0, effective temperature sum with 0 8C threshold (8C days); Tlog(dd0), the logarithm of the
temperature sum (log (8C days)); D, the difference between precipitation and potential evapotranspiration from May to September (mm);
E, annual actual evapotranspiration (mm); and `n sites', a subgroub of the sites on a Scandinavian-NW-continental transect.
ß 2003 Blackwell Publishing Ltd, Global Change Biology, 9, 575±584
C L I M A T I C E F F E C T S O N L I T T E R D E C O M P O S I T I O N 579
(b)
(a)
0.6
0.6
n
0.3
0.2
0.1
0.0
−5
w
nw w
w
n
n
nw n
n
c
n n
c
nnn
n cccm
m
n
n
0.4
n
0.5
w
Mass loss (year−1)
0.5
Mass loss (year−1)
Fig. 2 The first-year mass loss of Scots
pine needle litter measured across Europe
plotted against (a) annual mean temperature and (b) annual actual evapotranspiration (data from Berg et al., 1993). The letter
symbols indicate grouping of the sites by
Berg et al., into the Scandinavian-NW-continental transect (n), the European west
coast (w), the Mediterranean (m) and Central European (c) site groups. The lines
show regression models (a) #1 and (b) #4
shown in Table 1 for to the n sites.
m
m
m mm
Measured mass loss (year−1)
Measured mass loss (year−1)
w w
n
w
n
w
w
n
n
w
0.4
0.3
0.2
n
0.1
n
n
n
c
nm
ncc c
n
nn
ncm
m
m
m n mm
n
0.1 0.2 0.3 0.4 0.5
Modelled mass loss (year−1)
m
w
n
ww
w
n
n w
w
n
0.4
n
0.3
n
n
c
nm
n
c c c
nn
nncm
m
m
mm nm
n
n
0.2
0.1
0.1 0.2 0.3 0.4 0.5
Modelled mass loss (year−1)
0.6
(d) E
0.6
0.6
0.5
0.4
n
0.3
0.2
n
0.1
n
n
n
Measured mass loss (year−1)
Measured mass loss (year−1)
n
0.5
0.0
0.0
0.6
(c) Tlog(dd0), D
0.0
0.0
0.2
c
c
cc m c
m m
n
n
nn n n
m
m
nm
n
0.6
0.5
0.1
0.3
n
n nw
n
(b) Tdd0, D
0.6
Fig. 3 The first-year mass loss of Scots
pine needle litter measured across Europe
(Berg et al., 1993) plotted against the
values calculated using the regression
models: (a) #5, (b) #6, (c) #7 and (d) #8
shown in Table 1. The letter symbols indicate grouping of the sites by Berg et al.,
into the Scandinavian-NW-continental
transect (n), the European west coast (w),
the Mediterranean (m) and Central European (c) site groups.
n
0.0
400
500
600
700
300
Annual actual evapotranspiration (mm)
20
(a) Tamt, D
0.0
0.0
0.4
0.1
0
5
10
15
Mean annual temperature (8 C)
w w
n
w
n
w
w
w w
n
n ww
w
n
n w
c
n mn c c c
n nn n cm
m mm
m
n
0.2
0.3
0.4
0.5
Modelled mass loss (year−1)
decreased by using log-transformed temperature sum
values (Fig. 3c).
Testing the models in North and Central America
The effects of temperature and summer drought on mass
loss were not statistically different between Europe and
Canada when temperature sum or log-transformed temperature sum were used together with the summer
ß 2003 Blackwell Publishing Ltd, Global Change Biology, 9, 575±584
0.6
0.5
ww
n
w
n
w w
0.4
0.3
0.2
n
0.1
0.0
0.0
0.1
n
n
n nw
n
c
n n
c m
nnn n ccmc
m
m
mm
nm
n
0.2
0.3
0.4
0.5
0.6
Modelled mass loss (year−1)
drought indicator as climate variables (Table 2). The
effect of actual evapotranspiration was not different
either. Mass loss estimates for the Canadian study sites,
which were calculated using models with these climatic
effects quantified based on the European data, were comparable to measured values (Figs 4b, c, d). The slopes of
the regressions between these calculated and measured
values were close to one, indicative of small systematic
difference (Table 4). The models with temperature sum or
580 J . L I S K I et al.
Table 2 95% confidence limits for the effects of temperature or actual evapotranspiration (b) and summer drought (g) on litter mass loss
rate in Europe and Canada. b and g are coefficients of Eqns 2a and b, which were fitted to data on the first-year mass loss of Scots pine
needle litter across Europe (n ˆ 34, Berg et al., 1993) and data on the average three-year mass loss of 11 litter types across Canada (n ˆ 18,
Moore et al., 1999)
b
g
Climate variables
in the model
Europe
Tamt, D
Tdd0, D
Tlog(dd0), D
E
0.091±0.199
0.27±0.52 10
1.7±3.3
1.8±4.5 10 3
Canada
Europe
0.048±0.086**
0.38±0.68 10
1.6±3.1
3
3
2.5±5.1 10
2.3±4.3 10
1.9±3.6 10
1.7±4.0 10
Canada
3
3
3
3
0.25±2.7 10 3**
0.93±3.4 10 3
0.5±3.3 10 3
Statistically significant differences in the climatic effects (P < 0.01) are indicated by **. The abbreviations of the climate variables are
explained in Table 1.
0.5
0.0
1.0
0.5
0.0
−0.5
−0.5
0.0
0.5
1.0
Modelled mass loss ((3 years)−1)
(c) Tlog(dd0), D
(d) E
Measured mass loss ((3 years)−1)
−0.5
−0.5
0.0
0.5
1.0
Modelled mass loss ((3 years)−1)
Measured mass loss ((3 years)−1)
1.0
(b) Tdd0, D
Measured mass loss ((3 years)−1)
Measured mass loss ((3 years)−1)
(a) Tamt, D
1.0
0.5
0.0
−0.5
−0.5
0.0
0.5
1.0
Modelled mass loss ((3 years)−1)
1.0
0.5
0.0
−0.5
−0.5
0.0
0.5
1.0
Modelled mass loss ((3 years)−1)
log-transformed temperature sum and the summer
drought indicator explained a little more of the variation
in the data (R2 ˆ 0.71 and 0.76) than the model with actual
evapotranspiration alone (R2 ˆ 0.64). When annual mean
temperature was used to indicate temperature, mass loss
was significantly more tolerant of it and summer drought
in Canada than in Europe (Table 2), and this model
Fig. 4 Average three-year mass loss of 11
experimental litter types, as measured
across Canada (Moore et al., 1999), plotted
against the values calculated using regression models with the effects of climate
determined from measurements taken in
Europe (Berg et al., 1993). Climate variables used in the models were (a) annual
mean temperature and summer drought
(precipitation minus potential evapotranspiration from May to September),
(b) temprature sum and summer drought,
(c) log-transformed temperature sum and
summer drought and (d) annual actual
evapotranspiration. The models were
adjusted to pass through the average
measured mass loss value (0.36
(3 years) 1). The lines indicate linear regressions fitted to the measured and the
model-calculated values; the slopes and
the R2 values of these regressions are
given in Table 4.
clearly underestimated the low mass loss values and
overestimated the high values (Fig. 4a).
The effects of climate on mass loss were not statistically
different between Europe and the USA-Central America
either when log-transformed temperature sum was used
together with the summer drought indicator as climate
variables (Table 3). When the other temperature variables
ß 2003 Blackwell Publishing Ltd, Global Change Biology, 9, 575±584
3
3
3
3
0.042±0.055
0.14±0.18 10
1.0±1.4
1.5±2.6 10 3
Statistically significant differences in the climatic effects, P < 0.01 or P < 0.05, are indicated by ** or *, respectively. The abbreviations of the climate variables are explained in Table 1.
3
3
3
0.32±1.5 10
0.40±1.5 10
0.18±1.6 10
3
3
0.52±1.7 10
0.65±1.6 10
0.38±1.7 10
3
3
1.2±3.0 10 3* 0.82±2.1 10
1.3±3.0 10 3* 0.90±2.1 10
1.1±3.0 10 3 0.71±2.2 10
3
0.027±0.049
0.11±0.17 10 3*
0.65±1.4
0.44±1.1 10 3**
0.028±0.053
0.10±0.18 10 3
0.70±1.5
0.65±1.3 10 3**
0.034±0.068
0.12±0.24 10 3
0.91±2.0
0.98±1.7 10 3*
Tamt, D
Tdd0, D
Tlog(dd0), D
E
0.027±0.050
1.1±1.4 10
0.10±0.18 10 3 1.1±1.5 10
0.66±1.4
1.1±1.5 10
0.31±1.1 10 3**
Pinus
elliottii
roots
Drypetes
glauca
leaves
Pinus
resinosa
needles
USA and Central America
Europe
Pinus
Climate
variables used sylvestris
in the model needles
Drypetes
glauca
roots
3
Drypetes
glauca
roots
Pinus
elliottii
roots
Drypetes
glauca
leaves
Pinus
resinosa
needles
USA and Central America
Europe
Pinus
sylvestris
needles
g
b
Table 3 95% confidence limits for the effects of temperature or actual evapotranspiration (b) and summer drought (g) on litter mass loss rate in Europe and the USA and Central
America. b and g are coefficients of Eqns (2a) and (2b), which were fitted to data on the first-year mass loss of Scots pine needle litter across Europe (n ˆ 34, Berg et al., 1993) and data on
the first-year mass loss of four litter types across the USA and Central America (n = 26, Gholz et al., 2000)
C L I M A T I C E F F E C T S O N L I T T E R D E C O M P O S I T I O N 581
ß 2003 Blackwell Publishing Ltd, Global Change Biology, 9, 575±584
were used, statistical differences in the climatic effects
were found for two litter types. Mass loss of Pinus resinosa
needle litter was slightly more sensitive to summer
drought when annual mean temperature (P-value ˆ
0.028) or temperature sum (P-value ˆ 0.032) was used as
a temperature variable, and the risk level for the difference in the temperature effects on the mass loss of Pinus
elliottii root litter was just below 5% (P-value ˆ 0.048)
when temperature sum was used as a temperature variable. In contrast, mass loss of all four litter types was
statistically significantly more tolerant of changes in
actual evapotranspiration in the USA and Central
America than in the Europe (P < 0.001±0.019).
Mass loss of the litter types at the US-Central American
study sites were most accurately calculated when temperature sum and the summer drought indicator were
used as climate variables. The slopes of the regressions
between the calculated and the measured values were
0.71 or 0.91 for the foliar litter types and 0.64 or 0.66 for
the root litter types (Table 4, Fig. 5b). The R2 values of
these regressions ranged from 0.72 to 0.80. Although the
slopes were a little closer to one for both foliar litter types
(0.73 and 0.97) when log-transformed temperature sum
was used as a temperature variable, the scatterplots of the
calculated and the measured values revealed another
kind of systematic error in the calculated values. The
lowest and the highest calculated values were underestimations of the measured values while the calculated
values in the middle were overestimations (Fig. 5c).
When annual mean temperature was used as a temperature variable, mass loss of each litter type at the three
coldest sites was underestimated (Fig. 5a). When actual
evapotranspiration was used alone as a climate index, the
sensitivity of mass loss of three litter types, Drypetes
glauca leaves and the both root litter types, to climate
was substantially overestimated, and the R2 values were
low for the root litter types (Table 4, Fig. 5d).
Discussion
Among the climate variables tested, temperature sum and
summer drought were those that best explained climatic
effects on litter decomposition rates in various conditions.
They explained the first-year mass loss of Scots pine
needle litter across Europe essentially as well as did
annual mean temperature or logarithm of temperature
sum combined with summer drought (Table 1, Fig. 3),
and, in addition, a regression model, which was fitted to
the European data and had these climate variables, predicted variation in the mass loss of different litter types
across Canada, the USA and Central America with least
systematic error and, on average, highest R2 values
(Table 4, Figs 4 and 5). We considered these advantages
more important than the weak statistical differences
582 J . L I S K I et al.
Table 4 Reliability of regression models fitted to data on the first-year mass loss of Scots pine needle litter across Europe (data from Berg
et al., 1993) in predicting the climatic effects on mass loss values measured across Canada (average three-year mass loss of 11 litter types,
Moore et al., 1999) and across the USA and Central America (first-year mass loss of four litter types, Gholz et al., 2000)
Climate variables used in the model
Region
Litter type
Canada
Average of 11
litter types
USA and Central America
Pinus resinosa needles
Drypetes glauca leaves
Pinus elliottii roots
Drypetes glauca roots
Tamt, D
Tdd0, D
Tlog(dd0), D
E
Slope
R2
Slope
R2
Slope
R2
Slope
R2
0.45
0.81
1.0
0.71
0.89
0.76
0.89
0.64
0.87
0.67
0.61
0.60
0.68
0.69
0.70
0.68
0.91
0.71
0.66
0.64
0.72
0.75
0.80
0.75
0.97
0.73
0.65
0.63
0.66
0.64
0.62
0.59
0.67
0.47
0.38
0.33
0.74
0.65
0.50
0.39
The slopes and the R2 values are from regressions fitted between the measured and the model-calculated values (Figs 5, 8). The
slopes indicate how much the measured value changed, on average, for a unit change in the model-calculated value, and the R2 values
indicate the share of variation in the measured values explained by the models. The abbreviations of the climate variables are explained in
Table 1.
1.0
1.0
0.7
0.4
0.1
−0.2
0.1
0.4
0.7
1.0
−0.2
Modelled mass loss (year−1)
(c) Tlog(dd0), D
Measured mass loss (year−1)
(b) Tdd0, D
Measured mass loss (year−1)
(a) Tamt, D
0.1
−0.2
0.1
0.4
0.7
1.0
−0.2
Modelled mass loss (year−1)
0.1
(d) E
1.0
Measured mass loss (year−1)
Measured mass loss (year−1)
0.4
0.4
−0.2
0.1
0.4
0.7
1.0
−0.2
Modelled mass loss (year−1)
1.0
0.7
0.7
0.7
0.4
0.1
−0.2
0.1
0.4
0.7
1.0
−0.2
Modelled mass loss (year−1)
Fig. 5 The first-year mass loss of Pinus
elliottii root litter, as measured across the
USA and Central America (Gholz et al.,
2000), plotted against the values calculated using regression models with the
effects of climate determined from measurements taken in Europe (Berg et al.,
1993). Climate variables used in the
models were (a) annual mean temperature and summer drought (precipitation
minus potential evapotranspiration from
May to September), (b) temperature sum
and summer drought, (c) log-transformed
temperature sum and summer drought
and (d) annual actual evapotranspiration.
The models were adjusted to pass through
the average measured mass loss value
(0.211 year 1). The lines indicate linear regressions fitted to the measured and the
model-calculated values; the slopes and
the R2 values of these regressions are
given in Table 4. In terms of these slopes
and R2 values, this litter type represents
an average of the four litter types tested.
ß 2003 Blackwell Publishing Ltd, Global Change Biology, 9, 575±584
C L I M A T I C E F F E C T S O N L I T T E R D E C O M P O S I T I O N 583
between Europe and USA-Central America in the temperature sum effect on Pinus elliottii root litter and the
summer drought effect on Pinus resinosa needle litter
(Table 3).
On the other hand, decomposition rate was found substantially more tolerant of annual mean temperature in
Canada than in Europe (Table 2, Fig. 4a). This was probably because many of the Canadian sites had a very
continental climate. Although cold winters decreased
annual mean temperature at these sites, summers were
still warm enough to provide favourable conditions. Logtransformed temperature sum was included in the analyses to study if decomposition was more sensitive to
temperature in cold conditions, like suggested based on
soil respiration and nitrogen mineralisation measurements (Raich & Schlesinger, 1992; Kirschbaum, 1995).
The results were controversial. Replacing temperature
sum with its logarithm improved the mass loss estimates
for the coldest sites in Europe (Fig. 3) but had little effect
in Canada (Fig. 4) and led to underestimations of mass
loss at the coldest sites in the USA (Fig. 5).
Our results suggest that a simple model can explain the
majority of climatic effects on the decomposition rates of
different litter types in widely varying environments
from arctic tundra to tropical rainforest. The applicability
of such a model does not seem to be limited to any
specific year in the start of decomposition, since models
developed using the first-year mass loss data from
Europe predicted the three-year mass loss values accurately across Canada (Table 4, Fig. 4). At later stages,
decomposition may become more tolerant of climate
(Johansson et al., 1995), and decomposition of old soil
organic matter has been found to be rather insensitive
to temperature (Liski et al., 1999; Giardina & Ryan, 2000).
Such a wide applicability of our simple models was not
expected. Earlier, when actual evapotranspiration had
been used as a climate index, it had been concluded
that climatic effects on litter decomposition were specific
to geographical regions (Berg et al., 1993), litter types
(Berg et al., 2000) and decomposition years (Johansson
et al., 1995). We also found that the relationship between
actual evapotranspiration and the decomposition rate
was significantly different in Europe than in the USA
and Central America (Table 3). It seems possible to explain climatic effects on litter decomposition rates more
generally and accurately when temperature and drought
effects are distinguished than when they are summarised
in actual evapotranspiration.
This conclusion contradicts the prevailing view that
climatic effects on litter decomposition are best explained
by actual evapotranspiration at regional or larger scales
(e.g. Meentemeyer, 1978; Berg et al., 1993; Aerts, 1997).
Our conclusion is probably different, because we used
different criteria to evaluate the climate indexes. In
ß 2003 Blackwell Publishing Ltd, Global Change Biology, 9, 575±584
addition to the R2 values of the models, we paid attention
to systematic differences between regions and litter types
to see how widely applicable the models were. The
earlier studies have evaluated the models based on the
R2 values only. Despite a high R2 value, a model may be
systematically invalid for individual regions or environmental conditions.
In the present study, we investigated the direct effects
of climate on decomposition rate. Climate may also affect
decomposition rate indirectly by changing litter chemistry (Aerts, 1997). To predict the total effects of climate on
litter decomposition, the regression models developed
here must be coupled with information on the possible
changes in litter quality.
Acknowledgements
We thank Taru Palosuo, Ari Pussinen, Raija Laiho, Raisa
MaÈkipaÈaÈ and Timo Karjalainen for comments, and three anonymous reviewers for constructive criticism on an earlier version of this paper. This study was partly funded by the European
Commission through ATEAM (EVK2-CT-2000-00075) and
CASFOR-II (ICA4-CT-2001-10100) projects.
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