Annals of Botany 86: 1065±1071, 2000
doi:10.1006/anbo.2000.1269, available online at http://www.idealibrary.com on
Litter Decomposition in Four Woody Species in a Mediterranean Climate: Weight Loss,
N and P Dynamics
M A R IÂ A J O S EÂ M O RO * { and F R A N C I S CO D O M I N G O{
{Departamento de EcologõÂa, Universidad de Alicante, 03080 Alicante, Spain and {EstacioÂn Experimental de Zonas
AÂridas, CSIC, General Segura 1, 04001 AlmerõÂa, Spain
Received: 18 January 2000 Returned for revision: 17 February 2000 Accepted: 28 July 2000 Published electronically: 24 October 2000
Decomposition dynamics in leaves and needles of two Mediterranean shrubs and two pine species growing in the
Sierra de Filabres (AlmerõÂ a, Spain) was investigated during 2 years using the litter bag technique. The species studied
are representative of the vegetation of the study area and di€er greatly in their foliar traits. Results are discussed in
relation to the initial litter quality (C, N and P) and through the application of the exponential decay model. The mass
lost at the end of study varied in the order: Pinus pinaster 5 Pinus nigra 5 Cistus laurifolius 5 Adenocarpus
decorticans. Di€erences in annual rates of decomposition among species are consistent with the particular chemical
and structural attributes of their leaves. The mass of decomposing litter remaining after 2 years was positively
associated with the initial C : N ratio. Weight loss and nutrient release were fastest in the leguminous A. decorticans.
The results suggest the importance of both structure and elemental concentration of initial litter for decomposition
# 2000 Annals of Botany Company
dynamics in Mediterranean species.
Key words: Adenocarpus decorticans Boiss., C : N ratio, Cistus laurifolius L., litter decomposition, litter quality,
Mediterranean environments, nutrient dynamics, Pinus pinaster Aiton, Pinus nigra Arnold, single exponential model,
Southern Spain.
I N T RO D U C T I O N
Mediterranean type ecosystems are characterized by hot,
dry summers and warm, wet winters. This temporal
asynchrony of favourable temperature and moisture conditions could limit rates of litter decomposition (Read and
Mitchell, 1983; Hart et al., 1992). This e€ect would be
enhanced in ecosystems with species whose leaves are rich in
resistant components, such as the coniferous and sclerophyllous forests and shrublands that are widely distributed
in the Mediterranean area. However, the decomposition
process could be accelerated during the favourable autumn
and spring seasons and thus the average decomposition rate
could be similar to that attained by other temperate
ecosystems (Gray and Schlesinger, 1981).
To a large extent, morphological, structural and chemical
characteristics of leaves and needles determine their rates of
decomposition (Swift et al., 1979). Evergreen sclerophyllous
leaves of Mediterranean vegetation are typically poor in
nutrients and rich in recalcitrant compounds (Read and
Mitchell, 1983). The withdrawal of nutrients before leaf fall
reduces quality of litter, and both conserves nutrients
within aerial biomass and reduces the dependence of
individual plants on the soil system (Enright and Ogden,
1987; Del Arco et al., 1991). Although the decomposition
process has frequently been studied in temperate areas, data
on the decomposition process of litter in the Mediterranean
area are infrequent. Thus, the aim of this study is to
determine weight loss and N and P dynamics from leaf and
* For correspondence. E-mail [email protected]
0305-7364/00/121065+07 $35.00/00
needle litter representing Mediterranean species that di€er
greatly in their foliar traits.
M AT E R I A L S A N D M E T H O D S
Study site
The study was conducted in a small microcatchment on the
northern slopes of the Sierra de los Filabres (Almeria
Province, south-eastern Spain). The elevation of the
watershed ranges from 1540±1740 m above sea level. The
climate is Mediterranean with a mean annual precipitation
of 450 mm and an average annual temperature of 118C.
The mean annual precipitation during the study period was
higher than average: 500, 1100 and 900 mm in 1988, 1989
and 1990, respectively. Parent material is composed of
siliceous rock with quartz micaschists. Soils are eutric
cambisol-regosol. The vegetation consisted of a 15-year-old
mixed pine a€orestation of Pinus pinaster Aiton and Pinus
nigra Arnold which occupy di€erent sites within the
watershed. Within the gaps in the forest, which has an
open canopy, dense vegetation consisting mainly of two
evergreen shrubs, Adenocarpus decorticans Boiss. and Cistus
laurifolius L. has developed. A. decorticans is a native
N-®xing (Moro et al., 1992), deep-rooted leguminous
species with mesophyllous leaves and high concentrations
of nutrients in living tissues (Moro and Domingo, 1996).
C. laurifolius is considered to be a stress-tolerant species,
having super®cial roots, moderately sclerophyllous leaves
and relatively low foliar concentrations of nutrients (Moro
et al., 1996).
# 2000 Annals of Botany Company
1066
Moro and DomingoÐDecomposition in Mediterranean Species
Field and analytical procedures
Litter decomposition studies were carried out using
the litter bag technique (Bocock, 1960). Litter bags
(15 20 cm) were made with 1 mm polyethylene mesh.
In 1988, leaf and needle litter was collected from litter traps
during the period of maximum phenological litterfall. Litter
samples were air-dried in the laboratory. Oven-dry weight
was determined for three subsamples per species after
drying at 758C for 48 h. The equivalent of 5 g of litter taken
from the air dried stock was placed in litter bags, except for
C. laurifolius, 10 g of which was used. In the same plots
used for litterfall studies (Moro et al., 1996), 30 bags per
species were placed on the forest ¯oor below randomly
selected individuals of each species. The study of shrub
litter decomposition started on 28 Aug. 1988 whereas the
corresponding study of needle decomposition started on 25
Oct. 1988. The study was conducted for 2 years.
Two samples per species were taken randomly at 2 month
intervals. The bagged litter samples collected were cleaned
of soil material and oven-dried at 758C. Samples were
analysed individually for ash, N and P. Residual ash was
determined in a set of subsamples which were combusted
overnight at 4508C in a mu‚e furnace after adding 2 ml of
sulfuric alcohol to prevent volatilization of P. Ash content
was acid digested and analysed for phosphorus using the
molybdenum-blue method (Allen et al., 1974). Another set
of 0.20 g subsamples was analysed for total nitrogen by
Kjeldahl digestion (Allen et al., 1974). C was assumed to
constitute 50 % of ash-free dry mass (Gallardo and Merino,
1993).
Statistical analysis
The decomposition rate (k) was calculated from the
percentage of mass remaining using an exponential decay
model: Wt/Wo ˆ e ÿkt (Olson, 1963), where Wt/Wo is the
fraction of initial mass remaining at time t, t is the elapsed
time (years) and k is the decay constant (years ÿ1). The
model was ®tted to each of the litter bag sets by least
squares regression of the natural logarithm of percentage
mass remaining over time, using the SPSS statistics
package. Fit of the model was tested using the ratio of
mean squares (RMS) (Ezcurra and Becerra, 1987) equal to
the deviations mean squares from the model alone divided
by the pure error mean square. The smallest ratio of mean
squares indicates the best ®tting model. Half-life values
were calculated from the Olson equation (Olson, 1963).
To test for signi®cant di€erences among slopes (decay
rates) of regression functions, we used the procedure
described by Zar (1996, pp. 362±366), followed by a
Tukey multiple comparison test to determine di€erences
between slopes. This analysis was used to compare decay
rates among species for the overall study period, as well as
after the ®rst year of decomposition. For each species, a ttest was used to compare the corresponding slopes (decay
rates) between the above mentioned periods (i.e. after the
®rst year, and for the overall period). Analysis of variance,
followed by the Newman±Keuls multiple range tests, was
used to compare the initial chemistry of the litter samples.
Paired-sample t-test analysis was applied to compare the
average dry mass loss between shrub and pine species.
Exponential models were utilized to describe C : N evolution over time. Di€erent models were explored to describe
relationships between percentage of remaining dry mass to
nitrogen concentration or to C : N ratio. Only the best®tting equations for each species are presented here.
R E S U LT S
Weight loss and decomposition rate
The average weight loss from litter contained in bags is
shown in Fig. 1. The proportion of the initial dry weight
released from the litter after 2 years varied by an order of
magnitude: Pinus pinaster 5 Pinus nigra 5 Cistus laurifolius 5 Adenocarpus decorticans. The mass lost by the pine
species was similar after the ®rst year (14 % in P. pinaster,
18 % in P. nigra). However, di€erences between the two
species became more marked during the second year of
decomposition. At the end of the study period, mass loss
from needles of P. nigra was approx. 26 % higher than for
P. pinaster.
Di€erences were greater between shrub species. After 2
years, A. decorticans leaf litter had lost 70 % of its original
dry weight whereas that of C. laurifolius had lost only 38 %.
The paired-sample t-test showed that the average dry mass
loss from Adenocarpus was signi®cantly higher than
C. laurifolius for the overall period of incubation
(t ˆ 13.4, P 5 0.001). However, no signi®cant di€erences
were found between the pine species (t ˆ 2.00, P 4 0.05). A
rapid initial phase of mass loss, which may be attributed to
initial leaching of the readily soluble components of the
litter, was observed for the four species studied, but was
longer and more pronounced in the shrubs. Much mass loss
occurred in Adenocarpus litter after the ®rst 3 months when
47 % of its original dry weight was lost compared to only
16 % in Cistus.
Decomposition parameters for the incubated leaf/needle
litter are presented in Table 1 and the corresponding ®tted
curves in Fig. 1. The best ®t (lower RMS) was found in
P. nigra and C. laurifolius. Half-life times varied from 4±5
years in pines and 3.2 years in Cistus, to 1.3 years in
Adenocarpus.
Tukey-test analysis showed a signi®cant e€ect of species
on litter decomposition rates for the overall 2 year period
(kt , Table 2). A. decorticans showed the highest rate
(ÿ0.534 year ÿ1) and P. pinaster the lowest (ÿ0.124
year ÿ1). Decay constants for the ®rst year (k1) also di€ered
signi®cantly among species except between P. pinaster and
P. nigra. K1 was slightly lower than kt in P. nigra litter, while
the opposite was found in P. pinaster. Nevertheless
di€erences were not signi®cant (Table 2). A. decorticans
decomposed very fast during the ®rst year (ÿ0.83 year ÿ1)
having a decay rate more than 1.5-times as great as the
decay constant for the overall period (t ˆ ÿ2.6, P 5 0.05)
(Table 2). The decomposition rate in C. laurifolius also
di€ered signi®cantly between periods (t ˆ ÿ2.93,
P 5 0.01).
Proportion of initial mass remaining
Moro and DomingoÐDecomposition in Mediterranean Species
1.0
1.0
0.9
0.8
0.8
0.6
0.7
0.4
0.6
0.0
0.5
1 .0
1.5
2.0
2.5
0.2
0.0
0.5
1.0
1.5
2.0
1067
2.5
Elapsed time/years
F I G . 1. Proportion of dry-mass remaining as a function of incubation period in decomposing leaf and needle litter. Fitted curves are derived from
the exponential decay model. s, P. pinaster; h, P. nigra; j, A. decorticans; d, C. laurifolius.
T A B L E 1. Decomposition rate (k) and half-life time (T‰) for litter of the studied species incubated during a 2 year study in
Sierra de los Filabres
Species
P. pinaster
P. nigra
C. laurifolius
A. decorticans
k (yearÿ1)
T‰ (years)
RMS
r2
P
n
ÿ0.124
ÿ0.168
ÿ0.213
ÿ0.534
5.45
4.10
3.16
1.29
4.16
2.33
2.65
16.15
0.83
0.92
0.90
0.85
***
***
***
***
24
24
26
24
*** P 5 0.001. k was calculated from a single exponential model. Half-life time ˆ 0.693/k. RMS, Mean square ratio for each equation; r2,
coecient of determination.
T A B L E 2. Decomposition rates (year ÿ1) from leaf/needle
litter of the species studied calculated from a negative
exponential model at di€erent collection periods
correlated less well with the mass remaining after the ®rst
year (r ˆ 0.98, P 5 0.05, n ˆ 4).
Species
Release of nitrogen and phosphorus
P. pinaster
P. nigra
C. laurifolius
A. decorticans
After 1 year (k1)
Total 2 year period (kt)
ÿ0.1301,a
ÿ0.1641,a
ÿ0.3181,b
ÿ0.8291,c
ÿ0.1241,a
ÿ0.1691,b
ÿ0.2132,c
ÿ0.5342,d
Values with the same superscript letter in a column are not
signi®cantly di€erent. Values with the same superscript number in a
row are not signi®cantly di€erent. Signi®cance of Tukey-test given at
P 5 0.01 for the ®rst year period and P 5 0.001 for the total 2 year
period, excepting PP±PN and PN±CL: P 5 0.05. For each species,
time-di€erences given by t-test (P 5 0.05).
Initial leaf litter quality
ANOVA showed a signi®cant e€ect of species on initial
litter quality (P 5 0.001 for N, P and K; P 5 0.005 for Ca)
(Table 3). Abscissed needles had the lowest overall nutrient
concentrations and no signi®cant di€erences were found for
any element between the two pine species. Initial quality of
shrub litter di€ered markedly between the two species.
Signi®cant di€erences were found in essential nutrients. The
concentration of N and K was higher in leaves of
A. decorticans (1.55 % N, 0.69 % K), while C. laurifolius
had the highest levels of P and Ca (Table 3).
The initial C : N ratio was a good predictor of the 2 year
weight loss, (r ˆ 0.999, P 5 0.001, n ˆ 4) (Fig. 2) but it
N and P mass dynamics varied considerably among
species (Fig. 3). N immobilization was pronounced in
P. pinaster (over 140 % of their original mass after 568 d).
P. nigra and C. laurifolius immobilized over 24 and 45 %,
respectively, of their initial nitrogen mass. Moreover, N
mineralization in A. decorticans started immediately after
the leaves were placed on the forest ¯oor. P. nigra needles
exhibited a slow, gradual mineralization of P excluding a
short period of absolute increase at day 450. P was released
quickly during the ®rst 2 months of decomposition in
P. pinaster. This phase was followed by a period of
stabilization and net immobilization at the end of
incubation. The two shrubs showed di€erent patterns with
regard to P dynamics. An initial period of net retention
followed by a phase of net liberation was detected in
A. decorticans leaf litter. However, in C. laurifolius, a quick
release of P was followed by stabilization.
N concentration and C : N changes over time
N concentration increased with mass loss in all species. In
the leaf litter of the species studied, N concentration
increased 1.5 to 3-fold with regard to initial values. A
strong negative correlation between N concentration and
mass remaining was found for most species throughout the
study period (Table 4). This relationship agrees with the
1068
Moro and DomingoÐDecomposition in Mediterranean Species
T A B L E 3. Initial chemical composition (mg g ÿ1 d.wt) of needle and leaf litter from species of the Sierra de Filabres
N
P
0.39 + 0.08a
0.33 + 5.7E ÿ3a
1.40 + 0.18b
0.41 + 0.03a
2.52 + 0.09a
3.22 + 0.30a
3.43 + 0.32a
15.45 + 1.09b
P. pinaster
P. nigra
C. laurifolius
A. decorticans
K
Ca
Mg
C:N
1.04 + 0.12a
0.79 + 0.23a
3.38 + 0.37b
6.89 + 0.40c
11.74 + 1.01a
9.64 + 1.2a
20.60 + 1.59b
13.78 + 1.25a
3.18 + 1.03a
2.90 + 0.62a
3.58 + 0.49a
2.95 + 0.44a
195 + 4.44a
161 + 6.32a
148 + 4.25a
33 + 1.54b
Values are means + 1 s.e.; (n ˆ 5). Values with the same superscript letter in a column are not signi®cantly di€erent (Newman±Keuls test;
P 4 0.05).
Percentage of mass remaining
time course of the C : N ratio for each species (Table 5).
Data have been ®tted to an exponential decay model and
the ®t is highly signi®cant (P 5 0.001) in all species. A
comparison of coecients of determination (r2) suggests
that this statistical relationship is stronger for P. pinaster
and C. laurifolius. The relationships of biomass remaining
80
70
r = 0.999 P < 0.001
PP
60
CL
50
40
PN
DISCUSSION
AD
30
20
to the C : N, C : P and N : P ratios were tested by correlation
analysis. Only the correlation between the C : N ratio and
the remaining dry mass was signi®cant in pines. The
remaining dry weight is positively related to C : N and
inversely to N : P in C. laurifolius. A positive signi®cant
correlation was found for C : N and C : P ratios with regard
to mass remaining in A. decorticans. However, the
correlation to ratios other than C : N disappeared when
the statistical e€ect of the C : N ratio was eliminated. Thus a
simple regression was selected to establish the relationship
between dry mass and C : N ratio in both shrub and pine
species (Table 6).
Weight loss and decomposition rate
40
80
120
160
200
Initial C : N ratio
F I G . 2. Relationship between the initial C : N ratio and the proportion
of initial mass remaining after 2 years. PP, P. pinaster; PN, P. nigra;
CL, C. laurifolius; AD, A. decorticans.
1.6
2.8
N
2.4
Proportion of mass remaining
1.6
0.8
1.2
1.2
0.8
0.0
0.4
0.8
1.2
1.6
2.0
0.0
1.2
P
0.4
0.8
1.2
1.6
2.0
0.4
0.8
1.2
1.6
2.0
P
1.0
0.8
0.8
0.4
0.0
N
2.0
1.2
0.4
The percentage of weight lost from needle litter (14±17 %
after ®rst year, 25±34 % after second year), was similar to
that found in other Mediterranean pine a€orestations and
coniferous forests (Hart et al., 1992; Garcõ a-Ple et al., 1995)
in studies conducted over 1±2 years. Moreover, these values
scarcely di€ered from those found in some temperate pine
0.6
0.0
0.4
0.8
1.2
1.6
2.0
0.0
Elapsed time/years
F I G . 3. Time evolution of the nutrient mass remainingÐexpressed as a proportion of initial nutrient massÐfrom decomposing litter in species of
Sierra de los Filabres. s, P. pinaster; h, P. nigra; j, A. decorticans; d, C. laurifolius.
Moro and DomingoÐDecomposition in Mediterranean Species
T A B L E 4. Relationships between percentage of remaining
dry mass (y) and nitrogen concentration (in % d.wt) (x) in
leaf litter of the four species
r2
Species
0.70
0.61
0.87
0.87
P. pinaster
P. nigra
C. laurifolius
A. decorticans
Equation
P
ln(y) ˆ 4.28 ÿ 0.164ln(x)
ln(y) ˆ 4.78e ÿ0909x
y ˆ 119.24 ÿ 82.6x
y ˆ 156.4 ÿ 45.18x
50.001
50.001
50.001
50.001
Only the best-®tting equation is shown for each species. n ˆ 9 for
pines; n ˆ 10 for shrubs.
T A B L E 5. Changes in C : N ratio (y) over time (x) in leaf
litter of the four species
C : N ratio
Species
P. pinaster
P. nigra
C. laurifolius
A. decorticans
r2
0.90
0.79
0.91
0.85
Equation
y
y
y
y
ˆ 206.4e ÿ0002x
ˆ 159.2e ÿ00008x
ˆ 129.1e ÿ00012x
ˆ 26.6e ÿ000097x
Initial
After 2 years
195
161
148
33
59
92
66
15
n ˆ 9 for pines; n ˆ 10 for shrubs.
T A B L E 6. Relationships of C : N ratio (x) to percent biomass
remaining (y) throughout 2 years in species studied in Sierra
de los Filabres
Species
P. pinaster
P. nigra
C. laurifolius
A. decorticans
r
0.85
0.75
0.96
0.97
Equation
ln(y) ˆ 3.49 ‡ 0.199ln(x)
ln(y) ˆ 2.51 ‡ 0.399ln(x)
y ˆ 39.68 ‡ 0.402x
y ˆ ÿ31.91 ‡ 4.098x
F
P
38.8
21.2
181
247
50.001
50.005
50.001
50.001
Only the best-®tting equation is shown; shrubs: n ˆ 10; pines: n ˆ 9.
forests (Fahey, 1983; Stohlgren, 1988), supporting the idea
that some Mediterranean and temperate ecosystems have
similar decay rates despite their climatic di€erences (Gray
and Schlesinger, 1981). The decomposition rate in
C. laurifolius is in the order of that reported for Mediterranean hard or moderately sclerophyllous species (Schlesinger, 1985; Rosich et al., 1989; Gallardo and Merino,
1993). The special features of A. decorticans (N-®xing and
mesophyllous leaves) probably enable the rapid decay
observed in this study. Rates of mass loss are similar to
those found in some deciduous species in both temperate
(Dziadowiec, 1987) and Mediterranean areas (Gallardo and
Merino, 1993) as well as in other N-®xing species
(Schwintzer, 1984; Sharma and Ambasht, 1987).
Our results showed a signi®cant species and time e€ect
on decomposition dynamics in shrubs. Both species
exhibited a pattern characterized by a conspicuous phase
of rapid decay followed by stabilization. Nevertheless, litter
from each of the pine species was found to decompose at
1069
similar rates during the ®rst year and the overall 2 year
period as shown by the lack of signi®cant di€erences
between k1 and kt. These results suggest that the leaching
phase of decomposition in the pines studied is irrelevant to
determining their decomposition dynamics in the longer
term. Coniferous litter is known to be rich in recalcitrant
compared to labile, more readily leachable compounds.
Although the labile fraction was not calculated, our data
suggest that it could be a minor component of litter in the
pines studied. Moreover, litter material utilized in this
investigation derives from litterfall instead of fresh material
as in other studies; thus some of the easily leached
compounds might have been withdrawn before abscission
by retranslocation mechanisms.
Initial litter quality and nutrient dynamics
Chemical di€erences in initial litter quality of the species
studied re¯ect the combined e€ect of chemical content in
living tissues and the eciency of retranslocation mechanisms before abscission. The pine species produce similar,
low quality needle litter with low levels of N, P and other
elements (Moro et al., 1996). As expected, N was highest in
the ®xing legume. Surprisingly, the P concentration in
C. laurifolius leaf litter was relatively high and similar to
leaves in that signi®cant P resorption was not observed
before abscission (Moro, 1992). As a consequence of these
di€erences, decomposition dynamics in species of the Sierra
de Filabres di€ered markedly for some nutrients (Fig. 3).
Nutrient content in litter (mainly N) appears to regulate
the early stages of decomposition (1±2 years) whereas later
stages appear to be regulated by the percentage of lignin
and other resistant components (Berg and Ekbohm, 1983).
Consequently, the initial C : N ratio has been used as a mass
loss predictor for short-term decomposition. Our study
showed a good ®t for the relationship between C : N ratio
and percentage of mass remaining after 2 years (Fig. 2).
These results agree with the ®ndings of other authors for
di€erent species (Edmonds, 1980).
A `critical C : N ratio' for mineralization seems to be
between about 20 : 1 to 30 : 1 (Lutz and Chandler, 1946).
However, other authors have pointed out that this critical
index changes greatly depending on a wide variety of
factors (Dziadowiec, 1987; Stohlgren, 1988). In conifers,
critical C : N ratios ranging from 109 to 19 have been found
(Edmonds, 1980; Berg, 1986). N is an essential element
strongly retained by plants, and so freshly fallen litter
normally has a higher C : N ratio than the critical threshold
for mineralization, especially in nutrient-poor coniferous or
sclerophyllous Mediterranean species (Read and Mitchell,
1983; Hart et al., 1992). The initial C : N ratios for
P. pinaster and P. nigra were lower than those reported in
the literature for other coniferous species (Schlesinger,
1985; Stohlgren, 1988; Klemmedson, 1992). Although there
was a pronounced decline in C : N ratio throughout the
incubation period, pines and C. laurifolius continued to
immobilize N after 1.5 years and, evidently, critical
mineralization values were not attained in these species.
In A. decorticans, however, N in leaf litter was observed to
mineralize from the start of incubation with a C : N ratio of
1070
Moro and DomingoÐDecomposition in Mediterranean Species
33 : 1, in agreement with the critical value proposed by Lutz
and Chandler (1946). A similar pattern has been found in
other N-®xing species (Sharma and Ambasht, 1987) and
annuals (Parker et al., 1984).
The behaviour of P during the decomposition process
varies greatly among species, as demonstrated by the results
of many authors (Gosz et al., 1973; Lousier and Parkinson,
1978; Schlesinger, 1985; Enright and Ogden, 1987; Stohlgren, 1988). As shown in Fig. 3, this variability was also
found among the species studied here. With the exception
of C. laurifolius, the species showed periods of net increase
in P mass. Possible mechanisms behind this immobilization
are: (1) retention in microbial biomass or (2) translocation
from fungal hyphae. Critical C : P ratios reported in the
literature range from 200 : 1 to 480 : 1 (Gosz et al., 1973;
Dziadowiec, 1987). As stated above, P concentration in
C. laurifolius leaf litter was relatively high and similar to
living leaves. The low C : P ratio in initial C. laurifolius leaf
litter probably allows its net liberation from the start of
incubation. Similar patterns have been found in other
species (normally deciduous) having an initial C : P ratio
similar to that of C. laurifolius (Lousier and Parkinson,
1978; Dziadowiec, 1987; Enright and Ogden, 1987; Sharma
and Ambasht, 1987).
A signi®cant inverse correlation between N concentration
and percentage of mass remaining was found for species in
this study. This pattern seems to be a general trend, since it
has been described by many workers in a large variety of
species and can occur with or without release of N (Carlyle,
1986). Eventually, enough C is lost for the C : N ratio of
litter to be equivalent to that of bacterial and fungal tissues.
The relatively rapid respiration of C and the immobilization
of N by microbial tissue produce a decrease in the C : N
ratio over time. In this study, C : N ratios decreased over
time in a predictable manner for all species.
The patterns of decomposition observed here are
consistent with the general ®gures for C and N cycles
described by Moro et al. (1996) in populations of the same
species. Pines and C. laurifolius are able to cope with the
environmental stress of Mediterranean mountain areas,
having closed, self-regulated C and N-cycles. Although A.
decorticans shows an opposite strategy, it coexists in time
and space with C. laurifolius but mainly colonizes more
fertile microsites (Moro et al., 1996). The conserving
behaviour for N and C shown by C. laurifolius contrasts
with its less ecient utilization and quick circulation of P
(Moro, 1992). The coexistence of these opposite nutrientcycling strategies, in particular the P and N dynamics
during decomposition exhibited by the two shrub species,
may help increase the fertility in soils such as those of the
study area which are lacking in these nutrients.
No obvious di€erences emerged when decay rates and
nutrient dynamics of the species studied here were compared with analogous species growing in temperate, more
mesic ecosystems. This agrees with the hypothesis of Gray
and Schlesinger (1981), and highlights the relevance of litter
attributes as well as climatic factors to the decomposition
dynamics of Mediterranean species.
AC K N OW L E D G E M E N T S
This work was funded by the Commission of the European
Communities (EV4V-0109-E) and by ICONA-CSIC
through the LUCDEME project. We thank A. DuraÂn.
N. Gimenez and S. Ivars for technical support.
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