Leaf litter decomposition and nutrient dynamics in a subtropical forestafter typhoon disturbance

Xiaoniu Xu1,2,*, Eiji Hirata3, Tsutomu Enoki3 and Yoshihiro Tokashiki31Faculty of Agriculture, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan; 2Department ofForestry, Anhui Agricultural University, Hefei, Anhui 230036, P.R. China (Present address: Field ScienceCenter for Northern Biosphere, Hokkaido University, 250 Tokuda, Nayoro, Hokkaido 096-0071, Japan);3Faculty of Agriculture, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan; *Author forcorrespondence (tel: +81-1654-2-4264; fax: +81-1654-3-7522; e-mail: xiaoniu61@yahoo.co.jp)

Received 9 April 2002; accepted in revised form 13 June 2003

Key words: Castanopsis sieboldii, Leaf decomposition, Litter quality, Nutrient immobilization, Schima wallichii,Typhoon disturbance

Abstract

Decomposition of typhoon-generated and normal leaf litter and their release patterns for eight nutrient elementswere investigated over 3 yr using the litterbag technique in a subtropical evergreen broad-leaved forest on Oki-nawa Island, Japan. Two common tree species, Castanopsis sieboldii and Schima wallichii, representative of thevegetation and differing in their foliar traits, were selected. The elements analyzed were N, P, K, Ca, Mg, Na, Al,Fe and Mn. Dry mass loss at the end of study varied in the order: typhoon green leaves � typhoon yellowleaves � normal leaves falling for both species. For the same litter type, Schima decomposed faster thanCastanopsis. Dry mass remaining after 2 yr of decomposition was positively correlated with initial C:N and C:Pratios. There was a wide range in patterns of nutrient concentration, from a net accumulation to a rapid loss indecomposition. Leaf litter generated by typhoons decomposed more rapidly than did the normal litter, with rapidlosses for N and P. Analysis of initial quality for the different litter types showed that the C:P ratios were ex-tremely high �range 896 � 2467� but the P:N ratios were � 0.05 �range 0.02 � 0.04�, indicating a likelyP-limitation for this forest. On average 32% less N and 60% less P was retranslocated from the typhoon-gener-ated green leaves than from the normal litter for the two species, Castanopsis and Schima. An estimated 2.13 gm–2 yr–1 more N and 0.07 g m–2 yr–1 more P was transferred to the soil as result of typhoon disturbances, whichwere as high as 52% of N and 74% of P inputted from leaf litter annually in a normal year. Typhoon-drivenmaintenance of rapid P cycling appears to be an important mechanism by which growth of this Okinawan sub-tropical forest is maintained.

Introduction

Decomposition of plant litter refers to the physicaland chemical processes involved in reducing litter tosimpler chemical constituents. As such it is a majordeterminant of the nutrient cycles of most terrestrialecosystems �Meentemeyer 1978; Swift et al. 1979;Van Vuuren et al. 1993; Aerts and De Caluwe 1997�.Litter decomposition rates are controlled by environ-

mental conditions, the chemical composition of thelitter, and by soil organisms �Swift et al. 1979; Beareet al. 1992; Vitousek et al. 1994; Zhang et al. 1997;Sullivan et al. 1999�.

The subtropical evergreen broad-leaved forest inOkinawa has a high diversity of tree species �Xu etal. 2001a�. The dominant and sub-dominant speciesdiffer in their foliar traits of morphology and chemi-cal quality. In addition, typhoon disturbances have

161© 2004 Kluwer Academic Publishers. Printed in the Netherlands.Plant Ecology 173: 161–170, 2004.

significant impact on the annual litter production.Litter generated from typhoon has a rather differentchemical quality to the normal litterfall �Xu 2001�. Toa certain extent, the characteristics of the leaves de-termine their rates of decomposition. Although thedecomposition process has frequently been studied inforest ecosystems, data on the decomposition processof litter for Okinawan subtropical forest, particularlyon the differences between typhoon-generated litterand normal litter, are few. Thus, the main objectivesof this study were: to determine the differences inpatterns of dry mass loss and nutrient release betweentyphoon-generated leaf litter and normal leaf litter forthe dominant species and to examine the effect of ty-phoon disturbance on nutrient dynamics in the sub-tropical forest ecosystem in Okinawa.

Study area and methods

Study area

This study was conducted at Yona Experimental For-est of the University of the Ryukyus, located in thenorthern part of Okinawa Island, southwest Japan�26°45'N and 128°10'E�. The area is characterized bya subtropical climate and abundant rainfall through-out the year. Annual mean temperature is c. 21.8 °C.Annual mean rainfall is 2680 mm over 30 yrs from1968 to 1997 �unpubl. data, Experimental forest,University of the Ryukyus, 1998�. Mean annual rela-tive humidity reaches 82%. Typhoons frequently oc-cur between July and October, bringing high rainfalland strong winds to the island. Monsoons, from thesouth or southwest, bring a rainy season betweenspring and early summer, and from the north ornorthwest create a relatively dry season in winter.

The topography of the area is hilly, and a mountainrange runs from the northeast to the southwest in thecentre of the northern part of the Island. The highestpeak, Mt. Yonaha is 498 m a.s.l.. Deep valleys dis-sect the area and steep slopes predominate. The bed-rock is composed of tertiary sandstone and palaeozoicclay-slate except for a narrow area of palaeozoiclimestone along the coastline where a yellow soil hasdeveloped �Kojima 1980�.

The mountainous landscape is covered with orangeplantations and a wide range of forest ecosystems.The majority of Okinawan subtropical forest is domi-nated by a secondary forest that was harvested for fu-elwood, particularly for charcoal production during

the Second World War �Hirata, pers. comm.�; most ofthe lower slopes once covered by subtropical foresthave also been converted to orange and pine planta-tions �Kojima 1980; Yamamori 1994�. This subtropi-cal forest is characterized by short canopy with adense understorey �Xu 2001�.

Material

Decomposition rates of leaf litter were measured fortwo tree species: Castanopsis sieboldii Hatusima exYamazaki et Mashiba and Schima wallichii Kort., thedominant and sub-dominant canopy species. The twospecies contributed c. 60% of annual total leaf litter-fall �Xu et al. 2000�. Three types of leaf litter wereused: typhoon-generated green leaf, typhoon-gener-ated yellow leaf, and normal leaf litter. Typhoon-gen-erated litter was collected from litter traps aftertyphoons occurred in August 1996 and 1997. Normalleaf litter was collected from litter traps during theperiod of maximum leaf fall in March of the sameyears. Litter samples were oven-dried at 70 °C, thensealed in polythene bags and preserved at 15 °C in thelaboratory.

Litterbag experiment

Litter decomposition studies were carried out usingthe litterbag technique �Bocock et al. 1960�. Litterbags �20 cm�15 cm� were made of 1-mm polyestermesh. Before the experiment, four subsamples fromthe respective litter samples were taken to determinemoisture and initial chemical concentrations. Theequivalent of 10 g of dry litter was sealed in each bag.One hundred and thirty two litterbags per litter typefor each species were randomly placed in six blockson the soil surface at the study site used for litterfallstudies. The experiment, lasting 3 yrs, started on 3July 1998. Collections were made every month in thefirst 8-mo period, and then they were made in 2-mointervals. Six replicate litter bags were sampled ateach time, and the leaf litter was oven-dried as soonas possible at 70 °C to a constant weight, and thenmilled for chemical analysis.

Chemical analysis

All samples in the present study were analyzed for C,N, P, K, Ca, Mg, Na, Al, Fe and Mn. The concentra-tions of total C and N were determined by dry com-

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bustion with a C-N analyzer �Yanaco, MT-500,Kyoto, Japan�. The subsamples of 1.0 g of the groundsamples were digested with HNO3-HClO4 reagent,and analyzed for the concentrations of P, K, Ca, Mg,Na, Al, Fe and Mn, by inductively coupled plasmaspectrometer �Shimadzu, ICPS-2000, Kyoto, Japan�.The procedure for nutrient analysis followed the Edi-torial Committee of Experimental Methods for PlantNutrition, Japan �1990�.

Statistical analysis

There were six replicates of each litter type and alllitterbags were randomly selected for collection. Alldata were analyzed by Statistica �StatSoft, Japan Inc.1999�. The decomposition rate �k� was calculatedfrom the percentage of dry mass remaining �ash free�using an exponential decay model �Olson 1963�:Wt/W0 � e–kt , where Wt/W0 is the fraction of initialmass remaining at time t, and t is the elapsed time�yr� and k is the decomposition constant �yr–1�. Singlevariable regressions were used to correlate dry massremaining with the initial C:N and C:P ratios of thelitter. Differences in mass loss, decomposition con-stant and substrate chemistry between species andamong litter types were tested using one-way analy-sis of variance �ANOVA�. The multiple comparisonsof litter type means were made using Tukey and Stu-dent-Newman-Keuls tests. In all analyses, P � 0.05was the criterion for significant differences.

Results

Initial nutrient concentration

The initial nutrient concentrations for the differentlitter types of the two species, Castanopsis andSchima, are given in Table 1. There are significantdifferences for C:N ratios and the concentrations ofN and K among litter types for the two species. Theconcentrations for all elements measured in typhoon-green litter are significantly higher than those in thenormal litter. However, there are no differences for P,Ca, Mg, Fe and Mn between typhoon-yellow and thenormal litters.

Weight loss and decomposition rate

Litter mass loss displayed characteristic decomposi-tion patterns, which appeared to approximate a nega-tive exponential for the two species measured �Figure1�. The dry mass remaining after 2 yr varied signifi-cantly among the litter types by an order of magni-tude: normal leaf litter � typhoon-generated yellowleaves � typhoon-generated green leaves for bothCastanopsis and Schima. In addition, there was a be-tween-species difference �F � 2.43; df � 1, 5; P �0.04�.

Litter mass loss and decomposition constants �kvalues� after 1 and 2 yr are given in Table 2. The de-composition constants �k� were 1.15 and 1.31 yr–1 fortyphoon-generated green leaves of Castanopsis and

Table 1. Mean initial elemental concentrations �mg g-1, dry mass� of leaf litter from two species of subtropical evergreen broad-leaved foreston Okinawa Island, Japan, at the start of the decomposition experiment. Standard errors are in the parentheses �n � 5�. Means for a specieswithin a column followed by different small letters are significantly different at P � 0.05

Species N P K Ca Mg Al Fe Mn C:N

Castanopsis sieboldiiTyphoon-green 13.85a 0.52a 6.28a 7.27a 2.72a 0.43a 0.05a 0.31a 38a

�1.13� �0.07� �0.41� �1.03� �0.36� �0.26� �0.02� �0.04� �1.54�Typhoon-yellow 11.66b 0.29b 3.66b 7.91b 2.63a 0.93b 0.08ab 0.42b 45b

�0.96� �0.05� �0.35� �1.11� �0.27� �0.34� �0.03� �0.05� �1.57�Normal 9.29c 0.23b 3.08c 7.72b 2.59a 1.12c 0.09b 0.44b 56c

�0.57� �0.03� �0.18� �0.54� �0.13� �0.31� �0.02� �0.03� �1.82�

Schima wallichiiTyphoon-green 14.93a 0.60a 7.36a 7.35a 2.24a 0.83a 0.06a 0.35a 36a

�1.19� �0.11� �0.47� �0.92� �0.41� �0.29� �0.02� �0.04� �1.23�Typhoon-yellow 12.91b 0.29b 5.23b 8.36b 2.32ab 1.42b 0.07a 0.50b 41b

�1.06� �0.04� �0.41� �1.05� �0.32� �0.37� �0.03� �0.07� �2.13�Normal 10.16c 0.21b 4.35c 8.18b 2.46b 1.53b 0.07a 0.51b 51c

�0.77� �0.04� �0.29� �0.82� �0.15� �0.33� �0.02� �0.04� �1.48�

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Schima, respectively; while they were 0.87 and 1.06yr–1 for the normal leaf litter of two species after 1 yrof decomposition. After 2 yr of decomposition, thetyphoon-generated green leaves of Schima were com-pletely decomposed.

A rapid initial phase of mass loss, which may beattributed to initial leaching of the readily solublecomponents of the litter, was observed for all littertypes measured, but was more pronounced fortyphoon-generated green leaves �Figure 1�.

Nutrient dynamics

Patterns of nutrient transfer indicate how rapidly ele-ments are lost from decomposing leaf litter. Nutrient

transfer patterns for elements varied greatly �Figure2�.

Nitrogen. N immobilization was pronounced innormal leaf litter of Castanopsis �over 21% of its ini-tial mass after 1 yr of decomposition�. However, therewas no net gain of N in decomposition for other littertypes in both Castanopsis and Schima. N massdecreased linearly and sharply in typhoon-generatedgreen leaves �Figure 2a�.

Phosphorus. P was released quickly during the first3-mo of decomposition and afterwards releasedgradually for the typhoon-generated leaf litter. How-ever, normal leaf litter demonstrated net immobiliza-tion after 1 yr of decomposition, especially forCastanopsis �c. 17% of its initial mass; Figure 2b�.

Potassium. The result from the present studyshowed that K was subject to extensive leaching fromlitter in decomposition. Mass loss of K was rapid inthe first 3-mo, and was very low afterwards. No sig-nificant differences appeared among litter types forthe same species �Figure 2c, Figure 2d�.

Magnesium and calcium. The release of Mg and Cawas somewhat rapid over the 3-yr study period, es-pecially in the first year, in which 60-80% of theirinitial masses were lost. No difference appeared in thepatterns of Mg release between two species �Figure2e�. However, in the initial 3-mo of decomposition,Ca release in Castanopsis was greater than in Schima�Figure 2f�.

Aluminium and iron. Significant accumulations ofAl and Fe were found in decomposing leaf litter inthe present study �Figure 2g, Figure 2h�. Dynamicalpatterns for Al in decomposition varied significantlybetween species and between litter types as well.However, no significant differences appeared for thepattern of Fe between species and between litter typesin the early stage �c. 1 yr�, but in the late stage sig-nificant differences occurred.

Manganese. Schima demonstrated significant netaccumulation of Mn in the first year decomposition,and then rapid loss for all litter types. Castanopsisshowed a significantly different pattern of Mn releasein decomposition. However, after 2-yr of decomposi-tion, net accumulation of Mn was observed for bothnormal leaf litter and typhoon-generated yellowleaves in Castanopsis �37 and 12% of their initialmasses, respectively�.

Figure 1. Percent dry mass remaining of different litter types oftwo species, �a� Castanopsis sieboldii, and �b� Schima wallichii ofsubtropical forest on Okinawa Island, Japan. Open circles,typhoon-green; open squares, typhoon-yellow; open triangles, nor-mal leaf litter. The litter-bag experiment last for 3 yr and started inJuly 1998. Each point represents the mean of six replicate bags

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Nutrient ratio and dry mass loss

There were some differences in the initial C:N andC:P ratios among the three litter types and the twospecies. Figure 3 shows the relationships betweeninitial C:N, and C:P, ratios with dry mass loss duringthe study period. Percentage of dry mass loss wassignificantly correlated to initial C:N and C:P ratiosafter 2 yr decomposition �r2 � 0.706, 0.726; df � 5;P � 0.05�. However, after 1 yr decomposition, per-centage of dry mass loss was significantly correlatedonly with initial C:N ratios �r2 � 0.712; df � 5; P� 0.05�, but not with initial C:P ratios �r2 � 0.446;df � 5; P � 0.147�.

Discussion

Dry mass loss and decomposition rate

The percentage of dry mass loss from leaf litter var-ied significantly with species and litter types. The de-composition rate of the typhoon-generated greenleaves was significantly higher than that of normalleaf litter �Table 2; Figure 1�. This result was inagreement to that found in a tropical forest in Indo-nesia �Yoneda 1997� in studies conducted on tropicalcyclone-generated litterfall. Rapid decomposition ofthe typhoon-generated leaf litter may be attributed toits distinct features of both anatomical structure andinitial chemical concentrations �Schlesinger 1985;Gallardo and Merino 1993�. The typhoon-generated

leaves, particularly the green ones, were usually pre-mature, and should not be as hard as the normallyfalling leaves on grounds of their anatomical struc-ture. Moreover, these leaves had higher concentra-tions of nutrients, especially for N and P �Table 1�.These special features for the typhoon-generated leaflitter may have increased their decomposition rates.

The decomposition rates of normal leaf litter forthe two dominant species lie within the range of thosereported for some tropical and subtropical humidbroad-leaved forests �Brown and Lugo 1982;Schlesinger 1985; Cuevas et al. 1991; Gallardo andMerino 1993�.

Initial litter quality and nutrient dynamics

Nutrient content in litter �mainly N� appears to regu-late the early stages of decomposition �in first 1-2 yr�whereas later stages appear to be regulated by thepercentage of lignin and other resistant components�Berg and Ekbohm 1983�. Consequently, initial C:Nratio has been used as a predictor of mass loss forshort-term decomposition. The present study showeda good fit for the relationship between initial C:N ra-tio and percentage of dry mass loss after 1 and 2 yr.This result agrees with the findings of other authorsfor different species �Aerts 1997; Moro and Domingo2000�.

The increase in N concentration in all species andlitter types is a common observation in decomposinglitter �Berg and Staaf 1981; Mellilo et al. 1982�. Suchan increase could be attributed to the addition of N

Table 2. Mean percent loss in dry mass, decomposition constant �k; yr-1� and half-life time �t1/2� for leaf litter of two tree species in sub-tropical evergreen broad-leaved forest on Okinawan Island, Japan. Standard errors are in parentheses �n � 6�. Means for a species within acolumn followed by different small letters are significantly different at P � 0.05

1 yr of decomposition 2 yr of decomposition

Species Litter type Loss �%� k Loss �%� k t1/2

Castanopsis sieboldii Typhoon-green 68.7a 1.15a 90.2a 0.92a 0,76�3.53� �0.03� �3.19� �0.05�

Typhoon-yellow 60.6b 0.92b 80.1b 0.81ab 0,86�3.51� �0.07� �4.12� �0.05�

Normal 55.4b 0.87b 75.1b 0.74b 0,94�3.13� �0.06� �3.86� �0.06�

Schima wallichii Typhoon-green 73.1a 1.31a 100a 1.21a 0,57�2.92� �0.04� �0.00� �0.04�

Typhoon-yellow 69.7ab 1.19a 83.4b 0.91b 0,76�3.14� �0.08� �3.76� �0.03�

Normal 65.6b 1.06b 82.5b 0.87b 0,80�2.95� �0.03� �3.48� �0.05�

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Figure 2. Changes in the mass of nutrients in different litter types of two species, Castanopsis sieboldii and Schima wallichii contained in thelitter bags. Mass of nutrient remaining was obtained by multiplying nutrient concentration by remaining litter mass. Due to over-crowded,only 18 samples were used �in a 2-mo interval� and the other 4 samples were omitted, however, the patterns are the same to those using 22samples; open symbols, Castanopsis sieboldii; solid symbols, Schima wallichii. Circles, typhoon-green; squares, typhoon-yellow; triangles,normal leaf litter.

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from exogenous sources into microbial biomass�Mellilo et al. 1982; Sinsabaugh et al. 1994�. The fol-lowing mass loss of N is biologically mediated bydecomposer organisms �Swift et al. 1979�. In thisstudy, Castanopsis and Schima showed significantdifferences in their patterns of change in N during leafdecomposition, especially for the normal leaf litter.The former species displayed an evident N immobi-lization while the latter one demonstrated a rapid loss,which could be attributed to the initial quality of theirlitter �Moro and Domingo 2000�.

The behaviour of P during the decomposition pro-cess varies among species �Gosz et al. 1973;Schlesinger 1985; Moro and Domingo 2000; Baker etal. 2001�. The results from the present study showedthat the release of P in decomposition was slower inCastanopsis than Schima for the same litter type. Aslight net increase in P mass in the first 1.5 yr of de-composition was observed in normal leaf litter ofCastanopsis. The possible mechanisms behind this

immobilization may be retention in microbial bio-mass or translocation from fungal hyphae. CriticalC:P ratios reported in the literatures have a range of200 to 480 �Gozs et al. 1973; Dziadowiec 1987�.However, many other studies �e.g., Berendse et al.1989; Moro and Domingo 2000� showed much higherinitial C:P ratios. Our results showed that the initialC:P ratios in leaf litter ranged from 896 to 2467. Thecorrelation analysis showed that the dry mass loss af-ter 2-yr decomposition was significantly correlatedwith the initial C:P ratio. The low initial C:P ratios intyphoon-generated leaf litter probably allowed theirnet release from the start of incubation. Similar pat-terns have been found in other species with low ini-tial C:P ratios �Lousier and Parkinson 1978; Rustad1994; Moro and Domingo 2000�.

The release pattern of K and Na in decomposingleaf litter was distinctly different from that of theother nutrients. Loss of K and Na was immediate andrapid for the first 1-3 mo. Thereafter, the loss rate de-clined slightly. This pattern is characteristic of K andNa �Gosz et al. 1973; Lousier and Parkinson 1978�since K and Na are not structural components of plantlitter and are subject to physical removal by leaching.

The pattern of Ca release was somewhat similar tothe dry matter loss because Ca is a structural compo-nent and thus protected from physical leaching �Goszet al. 1973; Edmonds and Thomas 1995�. However,Lousier and Parkinson �1978� and Klemmedson et al.�1985� found an accumulation of Ca and a slow re-lease in the later stage of decomposition. The possiblereason for this pattern may be attributed to the trans-fer of Ca from the forest floor to decomposing litterthrough fungal hyphae �Fahey 1983�.

The dynamics of Mg is similar to the pattern of K.Gosz et al. �1973� reported the same result. In plants,c. 70% of the total Mg is readily extracted with water�Marshner 1995�, which indicates that Mg could beleached out by rainwater in the decompositionprocess.

The increase in concentrations and amounts of Aland Fe, during litter decomposition, is not a rareobservation �Gosz et al. 1973; Lousier and Parkinson1978; Rustad and Cronan 1988; Rustad 1994�. Theaccumulation of heavy metal ions such as Al and Feduring decomposition process is ascribed to the for-mation of highly stable complexes with humic sub-stances, or to them being taken up by microorganisms�Laskowski and Berg 1993; Tate et al. 1995�. Thesenutrients are finally mineralized in the very late stages�Rustad and Cronan 1988�.

Figure 3. Relationship between �a� initial C:N ratio, and �b� initialC:P ratio and the percentage of initial dry mass loss after 1 yr�closed symbols� and 2 yr �open symbols� of decomposition;Circles, Castanopsis sieboldii; triangles, Schima wallichii

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During litter decomposition, Mn is thought to beless mobile than Mg and Ca �Lousier and Parkinson1978; Edmonds and Thomas 1995�. In this study, Mnwas noted to decrease except for a temporary increasefor typhoon-generated green leaves in both species.Gosz et al. �1973� also reported a decrease in Mn asdecomposition proceeds. However, an increase in Mnmass was observed for normal leaf litter andtyphoon-generated yellow leaves in both Schima andCastanopsis in early stages �c. 1 or 1.5 yr�. This wasprobably due to microbial immobilization and/or ad-dition of Mn from exogenous sources �Lousier andParkinson 1978�.

Ecosystem implication of typhoon disturbance

Mineral elemental concentrations in leaves may beindicators of soil mineral concentrations �Vitousekand sanford 1986�, although other factors such as wa-ter availability, disturbance, and storage of elementsin short supply can confound patterns. Nitrogen andphosphorus concentrations in leaf litter of the twospecies in the study were similar to those for tropicalforests in infertile soils �Vitousek and Sanford 1986;Villela and Proctor 1999�. Medina et al. �1990�showed that a P:N ratio in leaves below 0.05 indicatesa low soil P supply. The P:N ratios of different littertypes in the study were all lower than 0.05 �range0.022-0.042�. In addition, the C:P ratio for the forestfoliage was exceptionally high �Xu 2001�, falling wellwithin the range of tropical forest ecosystems that areexpected to be P-limited �Vitousek and Sanford1986�. Available P is extremely low in soils under theOkinawan subtropical forests �Yamamori 1994; Xu etal. 2001b�. These results further suggest that the Oki-nawan subtropical forest is P-limited.

Typhoon disturbances can return large amounts ofplant material into the forest floor. Litterfall, particu-larly green leaves resulting from the typhoons hadhigher nutrient concentrations than the normal litterfor those nutrients that are translocated during senes-cence. The results from the study demonstrated thatpercentage retranslocation of N and P was rather highin both Castanopsis and Schima �Table 1�. Typhoon-generated green leaves retranslocated an average of34% less N and 60% less P than the normal ones forthe two dominant species studied. The previous studyof Xu �2001� revealed that the annual rates of leaf lit-ter were 571 and 426 g m–2 yr–1, respectively, in theyear following a strong typhoon and in the normalyear. The related annual N and P inputs were, respec-

tively, 6.13 and 0.16 g m–2 yr–1, and 4.10 and 0.09 gm–2 yr–1. Consequently, an estimation of about 2.03 gm–2 yr–1 more N and 0.07 g m–2 yr–1 more P wastransferred to the forest floor in the year with the ty-phoon disturbance, this presumably being normallyreabsorbed into the trees as a conservation mecha-nism �Jordan 1985�. Instead of being highly conser-vative, typhoon disturbances redirect nutrient ele-ments �especially N and P that are usually bound upin wood� into a more mobile form, cycling them atsomewhat higher rates. On the other hand, leaf litter,particularly the green type resulting from the typhoondecomposed more rapidly than did the normal leaflitter, with nutrient transfer patterns of rapid loss forP and N. This leads to an increase in P and N avail-ability in soil after typhoons. As mentioned before Pis inferred as being the main element limitingproductivity. Therefore, to a certain extent, typhoon-driven maintenance of rapid cycling of P and highavailability of P appears to be an important mecha-nism to maintain growth for this subtropical forest onOkinawa.

Acknowledgements

This study was made possible by partial support fromthe Japanese Ministry of Education, Sciences, Sportsand Culture. We thank Prof. K. Nogami, MiyazakiUniversity, for invaluable suggestions. Mr. M. Asatohelped with laboratory analysis.

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