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Annals of Botany 86: 1065±1071, 2000doi: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
MARIÂ A JOSEÂ MORO*{ and FRANCISCO DOMINGO{
{Departamento de EcologõÂa, Universidad de Alicante, 03080 Alicante, Spain and {EstacioÂn Experimental de Zonas
AÂridas, CSIC, General Segura 1, 04001 AlmerõÂa, SpainReceived: 18 January 2000 Returned for revision: 17 February 2000 Accepted: 28 July 2000 Published electronically: 24 October 2000
determine w
0305-7364/0
* For corre
Decomposition dynamics in leaves and needles of two Mediterranean shrubs and two pine species growing in theSierra de Filabres (AlmerõÂ a, Spain) was investigated during 2 years using the litter bag technique. The species studiedare representative of the vegetation of the study area and di�er greatly in their foliar traits. Results are discussed inrelation to the initial litter quality (C, N and P) and through the application of the exponential decay model. The masslost at the end of study varied in the order: Pinus pinaster 5 Pinus nigra5 Cistus laurifolius5 Adenocarpusdecorticans. Di�erences in annual rates of decomposition among species are consistent with the particular chemicaland structural attributes of their leaves. The mass of decomposing litter remaining after 2 years was positivelyassociated 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 decompositiondynamics in Mediterranean species. # 2000 Annals of Botany Company
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.greatly in their foliar traits.
INTRODUCTIONMediterranean type ecosystems are characterized by hot,dry summers and warm, wet winters. This temporalasynchrony of favourable temperature and moisture con-ditions could limit rates of litter decomposition (Read andMitchell, 1983; Hart et al., 1992). This e�ect would beenhanced in ecosystems with species whose leaves are rich inresistant components, such as the coniferous and sclero-phyllous forests and shrublands that are widely distributedin the Mediterranean area. However, the decompositionprocess could be accelerated during the favourable autumnand spring seasons and thus the average decomposition ratecould be similar to that attained by other temperateecosystems (Gray and Schlesinger, 1981).
To a large extent, morphological, structural and chemicalcharacteristics of leaves and needles determine their rates ofdecomposition (Swift et al., 1979). Evergreen sclerophyllousleaves of Mediterranean vegetation are typically poor innutrients and rich in recalcitrant compounds (Read andMitchell, 1983). The withdrawal of nutrients before leaf fallreduces quality of litter, and both conserves nutrientswithin aerial biomass and reduces the dependence ofindividual plants on the soil system (Enright and Ogden,1987; Del Arco et al., 1991). Although the decompositionprocess has frequently been studied in temperate areas, dataon the decomposition process of litter in the Mediterraneanarea are infrequent. Thus, the aim of this study is to
eight loss and N and P dynamics from leaf and
0/121065+07 $35.00/00
spondence. E-mail [email protected]
needle litter representing Mediterranean species that di�er
MATERIALS AND METHODS
Study site
The study was conducted in a small microcatchment on thenorthern slopes of the Sierra de los Filabres (AlmeriaProvince, south-eastern Spain). The elevation of thewatershed ranges from 1540±1740 m above sea level. Theclimate is Mediterranean with a mean annual precipitationof 450 mm and an average annual temperature of 118C.The mean annual precipitation during the study period washigher than average: 500, 1100 and 900 mm in 1988, 1989and 1990, respectively. Parent material is composed ofsiliceous rock with quartz micaschists. Soils are eutriccambisol-regosol. The vegetation consisted of a 15-year-oldmixed pine a�orestation of Pinus pinaster Aiton and Pinusnigra Arnold which occupy di�erent sites within thewatershed. Within the gaps in the forest, which has anopen canopy, dense vegetation consisting mainly of twoevergreen shrubs, Adenocarpus decorticans Boiss. and Cistuslaurifolius L. has developed. A. decorticans is a nativeN-®xing (Moro et al., 1992), deep-rooted leguminousspecies with mesophyllous leaves and high concentrationsof nutrients in living tissues (Moro and Domingo, 1996).C. laurifolius is considered to be a stress-tolerant species,having super®cial roots, moderately sclerophyllous leavesand relatively low foliar concentrations of nutrients (Moro
et al., 1996).# 2000 Annals of Botany Company
®tting equations for each species are presented here.
s
Field and analytical procedures
Litter decomposition studies were carried out usingthe 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 trapsduring the period of maximum phenological litterfall. Littersamples were air-dried in the laboratory. Oven-dry weightwas determined for three subsamples per species afterdrying at 758C for 48 h. The equivalent of 5 g of litter takenfrom the air dried stock was placed in litter bags, except forC. laurifolius, 10 g of which was used. In the same plotsused for litterfall studies (Moro et al., 1996), 30 bags perspecies were placed on the forest ¯oor below randomlyselected individuals of each species. The study of shrublitter decomposition started on 28 Aug. 1988 whereas thecorresponding study of needle decomposition started on 25Oct. 1988. The study was conducted for 2 years.
Two samples per species were taken randomly at 2 monthintervals. The bagged litter samples collected were cleanedof soil material and oven-dried at 758C. Samples wereanalysed individually for ash, N and P. Residual ash wasdetermined in a set of subsamples which were combustedovernight at 4508C in a mu�e furnace after adding 2 ml ofsulfuric alcohol to prevent volatilization of P. Ash contentwas acid digested and analysed for phosphorus using themolybdenum-blue method (Allen et al., 1974). Another setof 0.20 g subsamples was analysed for total nitrogen byKjeldahl digestion (Allen et al., 1974). C was assumed toconstitute 50% of ash-free dry mass (Gallardo and Merino,
1066 Moro and DomingoÐDecompo
1993).
Statistical analysis
The decomposition rate (k) was calculated from thepercentage of mass remaining using an exponential decaymodel: Wt/Wo � eÿkt (Olson, 1963), where Wt/Wo is thefraction of initial mass remaining at time t, t is the elapsedtime (years) and k is the decay constant (yearsÿ1). Themodel was ®tted to each of the litter bag sets by leastsquares regression of the natural logarithm of percentagemass remaining over time, using the SPSS statisticspackage. Fit of the model was tested using the ratio ofmean squares (RMS) (Ezcurra and Becerra, 1987) equal tothe deviations mean squares from the model alone dividedby the pure error mean square. The smallest ratio of meansquares indicates the best ®tting model. Half-life valueswere calculated from the Olson equation (Olson, 1963).
To test for signi®cant di�erences among slopes (decayrates) of regression functions, we used the proceduredescribed by Zar (1996, pp. 362±366), followed by aTukey multiple comparison test to determine di�erencesbetween slopes. This analysis was used to compare decayrates among species for the overall study period, as well asafter the ®rst year of decomposition. For each species, a t-test was used to compare the corresponding slopes (decayrates) 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 theaverage dry mass loss between shrub and pine species.Exponential models were utilized to describe C:N evolu-tion over time. Di�erent models were explored to describerelationships between percentage of remaining dry mass tonitrogen concentration or to C:N ratio. Only the best-
ition in Mediterranean Species
RESULTS
Weight loss and decomposition rate
The average weight loss from litter contained in bags isshown in Fig. 1. The proportion of the initial dry weightreleased from the litter after 2 years varied by an order ofmagnitude: Pinus pinaster 5 Pinus nigra5 Cistus laurifo-lius 5 Adenocarpus decorticans. The mass lost by the pinespecies was similar after the ®rst year (14% in P. pinaster,18% in P. nigra). However, di�erences between the twospecies became more marked during the second year ofdecomposition. At the end of the study period, mass lossfrom needles of P. nigra was approx. 26% higher than forP. pinaster.
Di�erences were greater between shrub species. After 2years, A. decorticans leaf litter had lost 70% of its originaldry weight whereas that of C. laurifolius had lost only 38%.The paired-sample t-test showed that the average dry massloss from Adenocarpus was signi®cantly higher thanC. laurifolius for the overall period of incubation(t � 13.4, P 5 0.001). However, no signi®cant di�erenceswere found between the pine species (t � 2.00, P 4 0.05). Arapid initial phase of mass loss, which may be attributed toinitial leaching of the readily soluble components of thelitter, was observed for the four species studied, but waslonger and more pronounced in the shrubs. Much mass lossoccurred in Adenocarpus litter after the ®rst 3 months when47% of its original dry weight was lost compared to only16% in Cistus.
Decomposition parameters for the incubated leaf/needlelitter are presented in Table 1 and the corresponding ®ttedcurves in Fig. 1. The best ®t (lower RMS) was found inP. nigra and C. laurifolius. Half-life times varied from 4±5years in pines and 3.2 years in Cistus, to 1.3 years inAdenocarpus.
Tukey-test analysis showed a signi®cant e�ect of specieson litter decomposition rates for the overall 2 year period(kt , Table 2). A. decorticans showed the highest rate(ÿ0.534 yearÿ1) and P. pinaster the lowest (ÿ0.124yearÿ1). Decay constants for the ®rst year (k1) also di�eredsigni®cantly among species except between P. pinaster andP. nigra. K1 was slightly lower than kt in P. nigra litter, whilethe opposite was found in P. pinaster. Neverthelessdi�erences were not signi®cant (Table 2). A. decorticansdecomposed very fast during the ®rst year (ÿ0.83 yearÿ1)having a decay rate more than 1.5-times as great as thedecay constant for the overall period (t � ÿ2.6, P 5 0.05)(Table 2). The decomposition rate in C. laurifolius alsodi�ered signi®cantly between periods (t � ÿ2.93,
P 5 0.01).
year (r � 0.98, P 5 0.05, n � 4).
release of P was followed by stabilization.
1.0
0.9
0.8
0.7
0.6
Pro
port
ion
of
init
ial m
ass
rem
ain
ing
0.2
1.0
0.8
0.6
0.4
0.0 0.5 1.0 1.5 2.0 2.5 2.52.01.51.00.50.0
Elapsed time/years
FIG. 1. Proportion of dry-mass remaining as a function of incubation period in decomposing leaf and needle litter. Fitted curves are derived fromthe exponential decay model. s, P. pinaster; h, P. nigra; j, A. decorticans; d, C. laurifolius.
TABLE 1. Decomposition rate (k) and half-life time (T�) for litter of the studied species incubated during a 2 year study inSierra de los Filabres
Species k (yearÿ1) T� (years) RMS r2 P n
P. pinaster ÿ0.124 5.45 4.16 0.83 *** 24P. nigra ÿ0.168 4.10 2.33 0.92 *** 24C. laurifolius ÿ0.213 3.16 2.65 0.90 *** 26A. decorticans ÿ0.534 1.29 16.15 0.85 *** 24
*** P5 0.001. k was calculated from a single exponential model. Half-life time � 0.693/k. RMS, Mean square ratio for each equation; r2,coe�cient of determination.
TABLE 2. Decomposition rates (yearÿ1) from leaf/needlelitter of the species studied calculated from a negative
exponential model at di�erent collection periods
Species After 1 year (k1) Total 2 year period (kt)
P. pinaster ÿ0.1301,a ÿ0.1241,aP. nigra ÿ0.1641,a ÿ0.1691,bC. laurifolius ÿ0.3181,b ÿ0.2132,cA. decorticans ÿ0.8291,c ÿ0.5342,d
Values with the same superscript letter in a column are notsigni®cantly di�erent. Values with the same superscript number in arow are not signi®cantly di�erent. Signi®cance of Tukey-test given atP 5 0.01 for the ®rst year period and P5 0.001 for the total 2 yearperiod, excepting PP±PN and PN±CL: P 5 0.05. For each species,time-di�erences given by t-test (P 5 0.05).
Moro and DomingoÐDecomposition in Mediterranean Species 1067
Initial leaf litter quality
ANOVA showed a signi®cant e�ect of species on initiallitter quality (P 5 0.001 for N, P and K; P 5 0.005 for Ca)(Table 3). Abscissed needles had the lowest overall nutrientconcentrations and no signi®cant di�erences were found forany element between the two pine species. Initial quality ofshrub litter di�ered markedly between the two species.Signi®cant di�erences were found in essential nutrients. Theconcentration of N and K was higher in leaves ofA. decorticans (1.55% N, 0.69% K), while C. laurifoliushad 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 � 0.999, P 5 0.001, n � 4) (Fig. 2) but itcorrelated less well with the mass remaining after the ®rst
Release of nitrogen and phosphorus
N and P mass dynamics varied considerably amongspecies (Fig. 3). N immobilization was pronounced inP. 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, Nmineralization in A. decorticans started immediately afterthe leaves were placed on the forest ¯oor. P. nigra needlesexhibited a slow, gradual mineralization of P excluding ashort period of absolute increase at day 450. P was releasedquickly during the ®rst 2 months of decomposition inP. pinaster. This phase was followed by a period ofstabilization and net immobilization at the end ofincubation. The two shrubs showed di�erent patterns withregard to P dynamics. An initial period of net retentionfollowed by a phase of net liberation was detected inA. decorticans leaf litter. However, in C. laurifolius, a quick
N concentration and C:N changes over time
N concentration increased with mass loss in all species. Inthe leaf litter of the species studied, N concentrationincreased 1.5 to 3-fold with regard to initial values. Astrong negative correlation between N concentration andmass remaining was found for most species throughout the
study period (Table 4). This relationship agrees with the
TABLE 3. Initial chemical composition (mg gÿ1 d.wt) of needle and leaf litter from species of the Sierra de Filabres
N P K Ca Mg C:N
P. pinaster 2.52+ 0.09a 0.39+ 0.08a 1.04+ 0.12a 11.74+ 1.01a 3.18+ 1.03a 195+ 4.44a
P. nigra 3.22+ 0.30a 0.33+ 5.7Eÿ3a 0.79+ 0.23a 9.64+ 1.2a 2.90+ 0.62a 161+ 6.32a
C. laurifolius 3.43+ 0.32a 1.40+ 0.18b 3.38+ 0.37b 20.60+ 1.59b 3.58+ 0.49a 148+ 4.25a
A. decorticans 15.45+ 1.09b 0.41+ 0.03a 6.89+ 0.40c 13.78+ 1.25a 2.95+ 0.44a 33+ 1.54b
Values are means+ 1 s.e.; (n � 5). Values with the same superscript letter in a column are not signi®cantly di�erent (Newman±Keuls test;
1068 Moro and DomingoÐDecomposition in Mediterranean Species
time course of the C:N ratio for each species (Table 5).Data have been ®tted to an exponential decay model andthe ®t is highly signi®cant (P 5 0.001) in all species. Acomparison of coe�cients of determination (r2) suggeststhat this statistical relationship is stronger for P. pinaster
P 4 0.05).
and C. laurifolius. The relationships of biomass remaining
2001601208040
30
20
40
60
50
70
80
Initial C : N ratio
CLPN
PP
AD
r = 0.999 P < 0.001
Per
cen
tage
of
mas
s re
mai
nin
g
FIG. 2. Relationship between the initial C :N ratio and the proportionof initial mass remaining after 2 years. PP, P. pinaster; PN, P. nigra;
CL, C. laurifolius; AD, A. decorticans.
N
P
1.6
1.2
0.8
0.40.0 0.4 0.8 1.2 1.6 2.0
1.2
0.8
0.4
0.00.0 0.4 0.8 1.2 1.6 2.0
Elapsed
Pro
port
ion
of
mas
s re
mai
nin
g
FIG. 3. Time evolution of the nutrient mass remainingÐexpressed as a prSierra de los Filabres. s, P. pinaster; h, P.
to the C:N, C:P and N:P ratios were tested by correlationanalysis. Only the correlation between the C:N ratio andthe remaining dry mass was signi®cant in pines. Theremaining dry weight is positively related to C:N andinversely to N:P in C. laurifolius. A positive signi®cantcorrelation was found for C:N and C:P ratios with regardto mass remaining in A. decorticans. However, thecorrelation to ratios other than C:N disappeared whenthe statistical e�ect of the C:N ratio was eliminated. Thus asimple regression was selected to establish the relationshipbetween dry mass and C:N ratio in both shrub and pine
species (Table 6).DISCUSSION
Weight loss and decomposition rate
The percentage of weight lost from needle litter (14±17%after ®rst year, 25±34% after second year), was similar tothat found in other Mediterranean pine a�orestations andconiferous 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 pineN
P
0.8
1.2
1.6
2.4
2.0
2.8
0.6
0.8
1.0
1.2
0.0 0.4 0.8 1.2 1.6 2.0
0.0 0.4 0.8 1.2 1.6 2.0
time/years
oportion of initial nutrient massÐfrom decomposing litter in species ofnigra; j, A. decorticans; d, C. laurifolius.
by retranslocation mechanisms.
TABLE 4. Relationships between percentage of remainingdry mass (y) and nitrogen concentration (in % d.wt) (x) in
leaf litter of the four species
Species r2 Equation P
P. pinaster 0.70 ln(y) � 4.28 ÿ 0.164ln(x) 50.001P. nigra 0.61 ln(y) � 4.78eÿ0�909x 50.001C. laurifolius 0.87 y � 119.24 ÿ 82.6x 50.001A. decorticans 0.87 y � 156.4 ÿ 45.18x 50.001
Only the best-®tting equation is shown for each species. n � 9 forpines; n � 10 for shrubs.
TABLE 5. Changes in C:N ratio (y) over time (x) in leaflitter of the four species
Species r2 Equation
C:N ratio
Initial After 2 years
P. pinaster 0.90 y � 206.4eÿ0�002x 195 59P. nigra 0.79 y � 159.2eÿ0�0008x 161 92C. laurifolius 0.91 y � 129.1eÿ0�0012x 148 66A. decorticans 0.85 y � 26.6eÿ0�00097x 33 15
n � 9 for pines; n � 10 for shrubs.
TABLE 6. Relationships of C:N ratio (x) to percent biomassremaining (y) throughout 2 years in species studied in Sierra
de los Filabres
Species r Equation F P
P. pinaster 0.85 ln(y) � 3.49 � 0.199ln(x) 38.8 50.001P. nigra 0.75 ln(y) � 2.51 � 0.399ln(x) 21.2 50.005C. laurifolius 0.96 y � 39.68 � 0.402x 181 50.001A. decorticans 0.97 y � ÿ31.91 � 4.098x 247 50.001
Moro and DomingoÐDecompos
forests (Fahey, 1983; Stohlgren, 1988), supporting the ideathat some Mediterranean and temperate ecosystems havesimilar decay rates despite their climatic di�erences (Grayand Schlesinger, 1981). The decomposition rate inC. laurifolius is in the order of that reported for Mediter-ranean hard or moderately sclerophyllous species (Schle-singer, 1985; Rosich et al., 1989; Gallardo and Merino,1993). The special features of A. decorticans (N-®xing andmesophyllous leaves) probably enable the rapid decayobserved in this study. Rates of mass loss are similar tothose found in some deciduous species in both temperate(Dziadowiec, 1987) and Mediterranean areas (Gallardo andMerino, 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�ecton decomposition dynamics in shrubs. Both speciesexhibited a pattern characterized by a conspicuous phaseof rapid decay followed by stabilization. Nevertheless, litter
Only the best-®tting equation is shown; shrubs: n � 10; pines: n � 9.
from each of the pine species was found to decompose at
similar rates during the ®rst year and the overall 2 yearperiod as shown by the lack of signi®cant di�erencesbetween k1 and kt. These results suggest that the leachingphase of decomposition in the pines studied is irrelevant todetermining their decomposition dynamics in the longerterm. Coniferous litter is known to be rich in recalcitrantcompared to labile, more readily leachable compounds.Although the labile fraction was not calculated, our datasuggest that it could be a minor component of litter in thepines studied. Moreover, litter material utilized in thisinvestigation derives from litterfall instead of fresh materialas in other studies; thus some of the easily leachedcompounds might have been withdrawn before abscission
ition in Mediterranean Species 1069
Initial litter quality and nutrient dynamics
Chemical di�erences in initial litter quality of the speciesstudied re¯ect the combined e�ect of chemical content inliving tissues and the e�ciency of retranslocation mechan-isms before abscission. The pine species produce similar,low quality needle litter with low levels of N, P and otherelements (Moro et al., 1996). As expected, N was highest inthe ®xing legume. Surprisingly, the P concentration inC. laurifolius leaf litter was relatively high and similar toleaves in that signi®cant P resorption was not observedbefore abscission (Moro, 1992). As a consequence of thesedi�erences, decomposition dynamics in species of the Sierrade Filabres di�ered markedly for some nutrients (Fig. 3).
Nutrient content in litter (mainly N) appears to regulatethe early stages of decomposition (1±2 years) whereas laterstages appear to be regulated by the percentage of ligninand other resistant components (Berg and Ekbohm, 1983).Consequently, the initial C :N ratio has been used as a massloss predictor for short-term decomposition. Our studyshowed a good ®t for the relationship between C:N ratioand percentage of mass remaining after 2 years (Fig. 2).These results agree with the ®ndings of other authors fordi�erent species (Edmonds, 1980).
A `critical C :N ratio' for mineralization seems to bebetween about 20 :1 to 30 :1 (Lutz and Chandler, 1946).However, other authors have pointed out that this criticalindex changes greatly depending on a wide variety offactors (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 elementstrongly retained by plants, and so freshly fallen litternormally has a higher C:N ratio than the critical thresholdfor mineralization, especially in nutrient-poor coniferous orsclerophyllous Mediterranean species (Read and Mitchell,1983; Hart et al., 1992). The initial C :N ratios forP. pinaster and P. nigra were lower than those reported inthe literature for other coniferous species (Schlesinger,1985; Stohlgren, 1988; Klemmedson, 1992). Although therewas a pronounced decline in C:N ratio throughout theincubation period, pines and C. laurifolius continued toimmobilize N after 1.5 years and, evidently, criticalmineralization 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
N. Gimenez and S. Ivars for technical support.
Cistus laurifolius y Adenocarpus decorticans en la Sierra de los
s
33 :1, in agreement with the critical value proposed by Lutzand Chandler (1946). A similar pattern has been found inother N-®xing species (Sharma and Ambasht, 1987) andannuals (Parker et al., 1984).
The behaviour of P during the decomposition processvaries greatly among species, as demonstrated by the resultsof many authors (Gosz et al., 1973; Lousier and Parkinson,1978; Schlesinger, 1985; Enright and Ogden, 1987; Stohl-gren, 1988). As shown in Fig. 3, this variability was alsofound among the species studied here. With the exceptionof C. laurifolius, the species showed periods of net increasein P mass. Possible mechanisms behind this immobilizationare: (1) retention in microbial biomass or (2) translocationfrom fungal hyphae. Critical C :P ratios reported in theliterature range from 200 :1 to 480 :1 (Gosz et al., 1973;Dziadowiec, 1987). As stated above, P concentration inC. laurifolius leaf litter was relatively high and similar toliving leaves. The low C:P ratio in initial C. laurifolius leaflitter probably allows its net liberation from the start ofincubation. Similar patterns have been found in otherspecies (normally deciduous) having an initial C :P ratiosimilar to that of C. laurifolius (Lousier and Parkinson,1978; Dziadowiec, 1987; Enright and Ogden, 1987; Sharmaand Ambasht, 1987).
A signi®cant inverse correlation between N concentrationand percentage of mass remaining was found for species inthis study. This pattern seems to be a general trend, since ithas been described by many workers in a large variety ofspecies and can occur with or without release of N (Carlyle,1986). Eventually, enough C is lost for the C:N ratio oflitter to be equivalent to that of bacterial and fungal tissues.The relatively rapid respiration of C and the immobilizationof N by microbial tissue produce a decrease in the C:Nratio over time. In this study, C :N ratios decreased overtime in a predictable manner for all species.
The patterns of decomposition observed here areconsistent with the general ®gures for C and N cyclesdescribed by Moro et al. (1996) in populations of the samespecies. Pines and C. laurifolius are able to cope with theenvironmental stress of Mediterranean mountain areas,having closed, self-regulated C and N-cycles. Although A.decorticans shows an opposite strategy, it coexists in timeand space with C. laurifolius but mainly colonizes morefertile microsites (Moro et al., 1996). The conservingbehaviour for N and C shown by C. laurifolius contrastswith its less e�cient utilization and quick circulation of P(Moro, 1992). The coexistence of these opposite nutrient-cycling strategies, in particular the P and N dynamicsduring decomposition exhibited by the two shrub species,may help increase the fertility in soils such as those of thestudy area which are lacking in these nutrients.
No obvious di�erences emerged when decay rates andnutrient dynamics of the species studied here were com-pared with analogous species growing in temperate, moremesic ecosystems. This agrees with the hypothesis of Grayand Schlesinger (1981), and highlights the relevance of litterattributes as well as climatic factors to the decomposition
1070 Moro and DomingoÐDecompo
dynamics of Mediterranean species.
ACKNOWLEDGEMENTS
This work was funded by the Commission of the EuropeanCommunities (EV4V-0109-E) and by ICONA-CSICthrough the LUCDEME project. We thank A. Dura n.
ition in Mediterranean Species
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