litter production and decomposition dynamics in moist deciduous forests of the western ghats in...

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I;bresr Ecocology ad Mnmqewrn~. ,:I ( 1992) 18 I-201 Elsevier Science Publishers B.V.. Amsterdam B. Mohan Kumar and Jose K. Deep Colle‘?e ofForesiry, Kera!a Agrrcultxal L’niversity. Veilanikkara. Thrmur 680 654, India (Accepted 24 June 199 1) ABSTRACT Mohan Kumar, B. and Deepu, J.K., 1992. Litter production dnd deccmposition dynamics in moist deciduous forests of the Westerr. Ghats in Peninsular India. For. Ecol. Manage., 50: ! 8 l-201. A field study was conducted in the moist deciduous forests of the Western Ghats (India) to test the following three hypotheses: ( 1 ) Litter production in tropical forests is a function of the floristic com- position. density, basal area and disturbance intensity; (2) Decay rate constants of tropical species is an inverse function of the initial lignin/nitrogen ratio; (3) Decomposition rates in tropical forests are faster than temperate forests. Litter fall was estimated by installing 63 litter traps in the moist deciduous forests of Thrissur Forest Division in the Western Ghats at three sites. Litter fail followed a monomodal distribution pattern with a distinct peak during the dry period from November-December to March-A.pril. Dilleniapen- lagyna, Grewia tiliaqfolia, hfacrosoletl spp., Xylia xylocarpa, Terrninalia spp.. Lagwrroemia ianceo- lata. Cleistanthus collinus, Bride!ia retusa, and Helicleres isora were the principal litter produr,-g species at these sites. The annual litter fall ranged from 12.18 to 14.43 t ha-i. Structural attributes of vegetation such as floristic composition, basal area, density and disturbance intensity did not directly influence litter fall rates. Leaf litter decay rates for six dominant tree species were assessed following the s?andard litter bag !echnique. One hundred and eight litter bags per species containing 20 g samples were installed in the rarest floor litter layer at the same three sites selected for the litter fall quantification exercise. The residual litter mass decreased lmearly with time for all species. In general, less disiurbed sites and species adapted to higher nitrogen availabilities exhi’oited relativeiy higher decay rate coefficients (ic). The rapid organic matter turnover observed in comparison with pubhshed temperate forest litter decay rates confirms that tropical moist deciduous forest species are characterised by faster decom- position rates. Mean concentratioss of N. P and K in the litter were profoundly variable amongst the dominant species. Initial nitrogen content of the leaflitter varied from 0.65 to 1.6%, phosphorus from 0.034 to 0.077% and potassium from 9.25 to 0.62%. C. collinus. an understorey shrub consistently recorded the lowest litter concentrsticnr for ali nutrients. The overriding pattern is one of higher nutrient leveis in the overstorey leaf litter- and lower concentrations in the undcrstorey litter. Furthermore, as decom- position proceeded, the nitrogen concentration of the residual biomass increased. Correspondence to: 53. Mohan Kumar. College of Forestt-yy, Kerala Agricultural University. Vel- lanikkara, Thrissur 680 654, India. 0 1992 Elsevier Science Publishers B.V. Ail rights reserved 0378-i 127/92/$05.00

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I;bresr Ecocology ad Mnmqewrn~. ,:I ( 1992) 18 I-201 Elsevier Science Publishers B.V.. Amsterdam

B. Mohan Kumar and Jose K. Deep Colle‘?e ofForesiry, Kera!a Agrrcultxal L’niversity. Veilanikkara. Thrmur 680 654, India

(Accepted 24 June 199 1)

ABSTRACT

Mohan Kumar, B. and Deepu, J.K., 1992. Litter production dnd deccmposition dynamics in moist deciduous forests of the Westerr. Ghats in Peninsular India. For. Ecol. Manage., 50: ! 8 l-201.

A field study was conducted in the moist deciduous forests of the Western Ghats (India) to test the following three hypotheses: ( 1 ) Litter production in tropical forests is a function of the floristic com- position. density, basal area and disturbance intensity; (2) Decay rate constants of tropical species is an inverse function of the initial lignin/nitrogen ratio; (3) Decomposition rates in tropical forests are faster than temperate forests.

Litter fall was estimated by installing 63 litter traps in the moist deciduous forests of Thrissur Forest Division in the Western Ghats at three sites. Litter fail followed a monomodal distribution pattern with a distinct peak during the dry period from November-December to March-A.pril. Dilleniapen- lagyna, Grewia tiliaqfolia, hfacrosoletl spp., Xylia xylocarpa, Terrninalia spp.. Lagwrroemia ianceo- lata. Cleistanthus collinus, Bride!ia retusa, and Helicleres isora were the principal litter produr,-g species at these sites. The annual litter fall ranged from 12.18 to 14.43 t ha-i. Structural attributes of vegetation such as floristic composition, basal area, density and disturbance intensity did not directly influence litter fall rates.

Leaf litter decay rates for six dominant tree species were assessed following the s?andard litter bag !echnique. One hundred and eight litter bags per species containing 20 g samples were installed in the rarest floor litter layer at the same three sites selected for the litter fall quantification exercise. The residual litter mass decreased lmearly with time for all species. In general, less disiurbed sites and species adapted to higher nitrogen availabilities exhi’oited relativeiy higher decay rate coefficients (ic). The rapid organic matter turnover observed in comparison with pubhshed temperate forest litter decay rates confirms that tropical moist deciduous forest species are characterised by faster decom- position rates.

Mean concentratioss of N. P and K in the litter were profoundly variable amongst the dominant species. Initial nitrogen content of the leaflitter varied from 0.65 to 1.6%, phosphorus from 0.034 to 0.077% and potassium from 9.25 to 0.62%. C. collinus. an understorey shrub consistently recorded the lowest litter concentrsticnr for ali nutrients. The overriding pattern is one of higher nutrient leveis in the overstorey leaf litter- and lower concentrations in the undcrstorey litter. Furthermore, as decom- position proceeded, the nitrogen concentration of the residual biomass increased.

Correspondence to: 53. Mohan Kumar. College of Forestt-yy, Kerala Agricultural University. Vel- lanikkara, Thrissur 680 654, India.

0 1992 Elsevier Science Publishers B.V. Ail rights reserved 0378-i 127/92/$05.00

182 B. MOHAN KUMAR AND J.K. DEEPU

INTRODUCTION

Litter fall and decomposition are two primary mechanisms by which the forest ecosystems’ nutrient pool is maintained. The litter on the forest floor acts as an input-output system for nutrients (Das and Ramakrishnan, 1985 ), and the rates at which forest litter falls and subsequently decays regulate en- ergy flow, primary productivity and nutrient cycling in forested ecosystems (Karnas, 1970; Waring and Schlesinger, 1985 ). It is particularly important in the nutrient budget of tropical forest ecosystems on nutrient-poor soils where vegetation depends on the recycling of nutrients contained in the plant detri- tus (Singh, 1968; Cole and Johnson, 1978; Prichett and Fisher, 1987).

In recent years, there has been an increase in the number of studies con- cerning litter dynamics, although a majority of these deal with temperate and/ or homogenous forests (Das and Ramakrishnan, 1985; Pande and Sharma, 1966; Giil et al., 198’7; Stohlgren, 1988a,b; Harmon et al., 1990). The studies on mineral nutrient dynamics of tropical forest ecosystems are, however, few and far between (Herrera et al., 1978; Jordan et al., 1980; Luizao and Schu- bart, 1987; Pascal, 1988 ).

Many authors reported that species composition, density, basal. area, age structure (Stohlgren, 1988a), altitude (Reiners and Lang, 1987), latitude (Bray and Gorham, 1964) and season (Luizao and Schubart, 1987 ) are fac- tors that strongly influence litter fall dynamics in natural forests. Litter p-o- duction by individual tree species in a natural forest stand is dependent on their dominance in the stand and the total amount reflects its stocking df:n- sity, according to Kotwal and Mall ( 1977).

Litter decomposition is by and large a substrate-dependent property. How- ever, eu/ironmental conditions under which decomposition occurs, such as temperature and moisture supply, can play a vital role in deciding decay rates (Singh :nd Gupta, 1977; Pastor and Post, 1986: Luizao and Schubart, 1987). Several workers have found strong negative linear relationships between ini- tial lignm/nitrogen ratios and the mass disappearance rates of leaf litter (Aber and Me!ilio, 1982; Melillo et al., I982 ). Various workers have also suggested that nitrogen (Rlelin, 1930; Heal and French, 1974; Schlesinger and Hasey, 198 1; Flangan and Van Cieve, 1983 ), lignin (Meentemeyer, 1978) and C/N ratio (Taylor et al., 1989) are better predictors of decay rates than lignin/ nitrogen ratio.

In this study we attempted to characterise the litter dynamics of a tropical moist deciduous forest ecosystem. Three hypotheses were tested: First, litter fall is a function of floristic composition, density, basal area and disturbance Intensity. In this connection, Stohlgren (1988a) observed that mean annual litter-faii rates in two Sierran mixed coniferous forests did nrn have a direct relationship with stand basal area, density and bole volume. However, this hypothesis remains untested for tropical moist deciduous forest ecosysiems.

LITTER DYN4MICS OF A TROPIC4L MOIST DECIDUOUS FO :EST 183

Secondly, the decay rate constant is an inverse function of the initial lignin! nitrogen ratio as proposed by Melillo et al. ( I982 ). Our approach differed from their study in that we selected six tropical species having reasonably wide lignin/nitrogen ratios. Thirdly, tropical species and tropical forests ex- hibit faster rates of organic matter decomposition and nutrient turnover com- pared with temperate forests.

STUDY AREAS

Study sites were located in the moist deciduous forests of Peechi and Pat- tikkad forest ranges of Thrissur Forest Division, Kerala (between I O”25’ and 10”45’ N Iatitude and 76’05’ and 76”30’ % longitude) in the Western Ghats (Fig. I ), The biogeographic region, Western Ghats, is the mountain range running along the Western margin of the Deccan plateau in the peninsular India. The Thrissur Forest Division is situated on the ‘TV-shaped strip of the Western Ghats south of Palghat gap.

1’ 76’ 76”10’7692c)‘76~30’76”40‘76~50’

Fig I. Map of the study area ! A. kmt ians of the expkmlta: ; kits)

184 E. MOHAN KUMAR AND J.K. DEEPU

100

, 90

,. 2 80 I /’

s----o- -o-__o___o ‘.

0” ‘, ,’ 3.

, -0 -0’

O’.. /’ O-0 RaInfall ,a* m-

x--x Max temperature o-o Min temperalure O---o Relotlve hum,d,ty

1 1CS

1 90

80

TO ^ Y

-60 - :

-50 2 k

1

40 ;i

,a

30

i

20

10

Jan Feb hlar Apr May June July Aug Scot Ckt Nov Dee

Months

Fig. 2. Mean values of precipitation, maximum temperature, minimum temperature and rela- tive humidity at Peechi for the period from 1985 to 1990.

The area enjoys a warm humid climate with a mean amnral precipitation of 2680 mm ( 1985- 1990 ) at Peechi. The mean maximum monthly temper- ature ranges from 32.O”C (August) to 38.O”C (April) and the minimum from 2O.O”C (January) to 23.5”C (April; Fig. 2). Generally May to October are wet mon;hs and November to April are dry. Soils at the experimental loca- tions are oxisols. The predominant parent material is metamorphic rocks of the gneiss series.

Moist deciduous forests are characterised by a distinct leafless period dur- ing the dry season. However, during the wet season, because of the thick foli- age, they do not permit much light to reach the ground. The trees of moist deciduous forests exhibit a stratified vertical structure (Chandraseknaran, 1962 ). Regarding horizontal structure, the area does not show any character- istic species association (Menon and Balasubramanyan, 1985 ). Dominant sylvan community comprises of Xylia xylocarpa Taub.. Dillenia pentagynu Roxb., Tectorza grandis Lf., Grewia tiliaefolia Vahl., Term&ah paniculata Roth., Terminalia crenulata Roth. and Lagerstroemia micro:arpa Wt. Lian- aous species like Acacia spp., Butea parviflora Roxb.. Caljmpterisfloribinda Lamk., Dalbergia volubilis Roxb., and Zyzyphus rugma Lamk. are also en- countered in these forest areas.

MATERIALS AND METWODS

Based on the forest type map prepared by Menon and Balasubramanyan ( I985 ) and visual observations on stand composition, density and degree of

LITTER DYNAMKS OF A TROPICAL MOIST DECKNOW FOREST 185

TABLE I

Basal area, density per hectare, continuum index, and disturbance index of the experimental locations

Location basal area (m’ha-‘)

Density 20 cm OF

greater DBH

Continuum rndex

Relative Litter disturbance fall index (g m-l)

Kuthiran (Ku) 18.04 138.1 623.7 3 1443.6 Kalluchal (K1 ) IS.48 182.8 660.8 4 1217.9 Karadippara (Kp) 25.89 161.4 512.0 1 1346.0

variability of stands, three localities, Kuthiran (Ku) in Pattikkad Range and Kalloochal (Kl) and Karadippara (Kp) in Peechi Forest Rang: were se- lected, to represent different canopy opening and disturbance levels. Prior to the commencement of the present s:udq’ permanent plots (2 ha in extent) were established in these locations and their relative disturbance indices (RDI ) worked out (Narayanan, i 988 ) by arranging the Curt% and McIntosh ( 135 1) continuum index on a 1 (least) to 8 (maximum) scale. The RDI, basal area, density per hectare and continuum index of these three sites are given in Table 1.

Each site was broadly divided into three sub-sites (replications) taking into consideration species composition, stand density, etc. Fire lines were also traced around the plots.

Litter collectim

Since the forest areas in Thrissur Division are generally subjected to human and livestock interferences, locally available, inexpensive materials were se- lected for the construction of litter traps. Other design considerations in- cluded stability and easy drainage of ram water. The litter traps wtzre made of interwoven, plated coconut leaves, the loose tips being fastened with ropes. The floor and sides of the traps were covered by coarse cloth. The circular aperture of the traps had an area approximately equat to 0.2 m2. Seven traps were installed at each sub-site, amongst the trees in such a manner that the base of the trap was approximately 60-70 cm above the ground between 2 1 and 24 March, 1987. The traps were fastened onto three wooden stakes each. The basal portion of the traps and the StdkeS were dipped in a dilute formu- lation of Aldrin to control termites.

Litter was collected at monthly intervals from the traps for I year comm- encing in April, 1987. The catch from each trap was taken to the laboratory, and separated into leaves and other materials. The leaf litter was then sorted out according to species. Dry weights of each component were determined by drying to constant weights at 80°C and the mean monthly value for each sub-

I86 B. MOHAN KUMAR AND J.K. DEEPLI

site was worked out on a unit area basis (g m-* ). The diameters of the indi- vidual traps were used to determine litter fall on a unit area basis. Occasion- ally a few traps were destroyed by animals and/or fire. In such cases the me-n values correspond to the available traps per sub-site. However, the damaged traps were subsequently replaced.

Lifter decomposition

Freshly fallen leaves of six dominant tree species of the area (D. pentugyna, G. tiliaefolia, Pterocarpus marsupium Roxb., T. grandis, T. panicuiata and X xylocarpa) were collected from the experimental area during February 1987. All leaves were dried at 45-50°C for about 48 h after species-wise pool- ing the samples from different locations in a ventilation oven. Six sub-sam- ples from each species were analysed for initial lignin and nitrogen contents.

The standard litter bag technique was used for the decomposition studies, wherein 20 g of dried leaf sample of the six sdected species were placed in nylor mesh bags (4 mm) and placed in the litter layer of the forest floor, anchored to small painted stakes to facilitate easy detection. A total of 72 samples each (six species by 12 monthly sampling intervals) were placed at each of the sub-sites under Ku, Kl and Kp sites, between 21 and 24 March, 1987.

At monthly intervals, litter samples from all sub-sites were collected and returned to the laboratory. After removing large arthropods, the contents of each bag were analysed for oven dry mass and total nitrogen content.

Chemical analyses

The periodical residual litter mass of each species from a sub-site was pooled and six sub-samples were drawn for nitrogen analysis. The leaf litter samples of the prominent species were also pooled over sites and over time. Six sub- samples per species were drawn for phyrochemical analysis from this com- posite sample. Total nitrogen content of the litter was determined colorimet- rically following the method of Wolf ( 1982 ), after the plant samples were ground to pass through a 70 mm mesh sieve. Phosphorus and potassium con- tents of the litter samples were estimated after trip17 acid digestion. Phospho- rus was determined by following the vanado molybdo phosphoric yellow col- our method, potassium by flame photometry (Jackson, 1967 ) and lignin by the Association of Official Analytica.! Chemists’ ( 1980) method.

Statistical analyses

Statistical treatment of the data included analysis of variance to compare litter-fall rates using the software SPSS/PC after log, transformation The

LITTER DYN.kMlCS OF A TROPICAL MOlST DECIDUOUS FOREST i8?

model for constant potential weight ioss (Won, i 363 ) represented by the equation: x/x0= eVk’, where x is the weight remaining at time t, x0 is the original weight, e is the base of natural Iogarithm, k is the decay rate coeffi- cient and t, is time, was fitted to the data on mass disappearance. The decay coefficients were compared following analysis of variance and Duncan’s Mul- tiple Range Test.

RESULTS

Litterfall

The total litter fall was highiy variable (P<O.QI f at different collection intervals (Table 2 ). Litter fall followed a characteristic monomodal distri- bution pattern (Fig. 3 ) with a conspicuous peak during the period from No- vember-December to March-April. The variation between sites in this re- spect was not pronounced. Monthly litter fail, if averaged over the coflection period, was about 120 g mV2, 101 g m-‘, and I12 g m-j, respectively, for the Ku Kl and Kp sites. Despite the marked variation in tree basal areas, density and disturbance intensities between the Kp site and the others (Table 1). the annual iitter fall rates did not differ substantially (F test: non-significant,

TABLE 2

Monthly litter fall (iog,) transformed values at three Iocatioss in the moist deciduous forests of the Western Ghars (Figures in parentheses indicate retransformed valoes in g mm2 )

Months Locations Mt23-6

KU K! KP

1987 April May June July August September October November December

5.26 4.68 4.19 3.66 3.92 4.44 4.28 4.47 4.3s 4.28 4.15 3.9 I 3.78 4.00 5.52 4.89 4.95 4.80

5.79 4.53 4.4; 4.57 4.25 3.37 3.50

-1 - $.I, 4.49

i 3’ ,.A

4.06 4.27 4.44 4.30 ?.3! 3.70 5.05 4.75

1988 January 5.97 5.64 5.44 2.75 February 5.62 5.74 0.05 5.80 March 5.47 4.89 5.43 5.17

CD (0.05) for comparing months: 0.36

B. MOHAN KUMAR AND J.K. DEEPIJ

Fig. 3. Seasonal variations in the litter fail at three locations in the moist deciduous forests of Western Ghats.

Means: 1443.6 g me2 year-‘, 1217.9 g m-’ year-’ and 1346.0 g m-’ year-’ respectively for Ku, K1 and Kp sites).

Major species contributing to litter fall and the magnitude of their contri- butions are given in Table 3. Xylia, Terminalia, Grewia: Lagerstroewzia, LX!- lenia, Bridelia, Macrosolen and Helicteres constituted the important contrib- utors of litter fall at all sites. All species, in general, followed the monomcdal seasonal distribution pattern. Nevertheless, three species like Di/lenia, Ma- crosolen and Grewia sta,-ted leaf fall early in October-November, in turn fol- lowed by trees such as X_vlia, Terminalia, Bridelia, Lagerstroemia and Cleis- tanthus, thus indicating a relay sequence in litter fall within the community (Figs. 4a, 4b and 4~). Interestingly, sites having low basal areas and high rel- ative disturbance indices were characterised by z relative!y higher amount of leaf fall in the catch. Tissue type analysis showed that leaf litter f;iii consti- tuted as much as 74%, 67% and 65%, respectively of tile total annual litter fall at the Ku, Kl and Kp sites. Twigs, bark and miscellaneous materials (such as fruits, seeds and herb litter) made up the rest of the catch.

Leaf litter decomposition

Results of the litter bag study reveal that the mass remaining in thy litter bags decreased linearly with time for a!1 species and sites (Figs. 5a, 5~ and 5~). The regressions describing decay rates over time were highly significant (P~0.01) for all species and locations (r* ranged from 0.74 to 0.99 with most values exceeding 0.90). All leaves lost mass completely within periods ranging from 5 to 8 months. Of the six species investigated, Fterocarpus leaves lost mass most rapidly (5 months at the Ku and Kl sites and 4 months on the

LITTER DYNAMICS OF A TROPICAL MOiST ilECIDUOUS FOREST is9

TABLE 3

Litter fall of dominant species at the three sites -

Species Litter fall Percent contribution to (g m2) total litter fail

(a) Ku site Biilenia peniagyna Roxb. Grewia tiliaefoiia Vahl. Lagersiroemia lanceoiata Be&. Macrosolen spp. Terminalla spp. X_v!ia xybcarpa Taub. Total litter fall

(b) KJ site Cieistanthus cobs Benth. Dilienia pentagyna Roxh. rl;z+,ia tiiiaefolia Vahl. Helertris isora Linn. Lagerstroemia lanceolata Bedd. Terminalia spp.

Xyira xq’locarpa Taub. Total litter fall

(c) KQ site Bridelia refusa A. Juss. Diilenia pentagyna Roxb. Grewia riliaejblia Vahl. Lagerstroemia lanceolata Bed& Termlnaiia spp. A’vlia xyiocarpa Taub. Total litter fall

SO.40 175.70 41.52

103.19 253.13 310.69

1443.60

44.64 42.18

211.22 51.89 80.11

. 4.17 366.02

1217.88

4S.i6 187.66

72.04 238.44 i 52.69 336.84

1466.04

3.49 12.17 2.88 7.15

17.53 21.52

3.67 3.46

17.34 4.26 6.58

11.84 30.05

3.cK3 12.80 4.91

16.26 i 0.42 22.98

The species listed in this table inchrde only those which contribute tc litter fall significantly (greater than 40 g m-*). The ratio of leaf Iitter to other fractions were 74.25%. 67.21% and 65.42%. respec- tively, for the Ku, Kl and KQ sites.

least disparbed Kp site). T.&ma, Diilmia and Terminalia exhibited slower litter decomposition rates compared with Pterocarpus, Grewia and Xylia.

Decomposition was characterised by an initial faster rate of disappearance followed bv a subsequent slower rate (Figs. 5a, 5h and 5~). For instance the percentages of litter mass remaining at the end of the second month were 52X%, 5 1.2% and 40.8%, respectively, at the Ku, I(1 and Kp sites for Prero- carps. The corresponding figures were 59.4%, 58.2% and 59.1% for Xytia, ancrther labile litter producing species. ~e~?~ii~a~ia had relatively more refrac- tory litter. The leaves of Temna, the paragon commercial timber of the Ori-

B.MOHAN KUM".RANDJ.K.DEEPU

90 ‘L---c iagerstroemta

80 1-1; DIllen,”

70 2----i Termlnallo

60

50

40

30

20

10

0 A M J J A S 0 N D .J FM

Fig. 4. Litter fall patterns of dominant species: (A) Ku site, (B) Kl site, and (C) Kp site.

ent, decomposed at a relatively slower rate. Table 4 contains the mmthly de- cay rate coefficients (k) for different sites and species. The difference in decay rate coefficients were statistically significant for species (F test ratio = 11.4 ),

LITTER DYNAMICS OF A TROPICAL MOIST DECIDUOUS FOREST 191

Fig. 5. Biomass remaining in the litter bags ai various time intervals: (A) KU site. f B) KJ site. and (C) Kp site.

locations (F test ratio= 26.7 ) and their interactions ;F test ratio== 3.3 ). In general, the Kp site recorded higher k values for all species. Rerocaupw ex- hibited significantly higher decay rate at this site compared with the Ku and W sites. Similarly, at the Ku site this species recorded a substantially higher decomposition coefficient. Xytia and Grewia were two other species showing consistently higher decay rate coefficients. Tecsana and Dillenia were inter- mediates. Termindia consistently recorded the lowest decay rate constants.

Monthly decay rate coefficient from different sites were reg:essed on pre- dictor variables such as initial percent lignin, initial percent nitrogen and ini-

I92 El MOIMN KIJM.W 4NDJ.K. DEEPU

TIAbLE 4

Monthh decay rate coefficients (k) corresponding to six species at three sites

Species Location Mean

KU K1 KP

0.44” 0.38’ 0. co;1 0.44 0.34‘d” ();?dcQ 0.3 I*‘@ 0.32 0.3ld’Q 0.3& 0.39b‘ 0.35 0.29”C 0 35Cd 0.35CdC 0.33 0.2Ss o.19fe 0.31dCfg 0.29 0.31d’f* 0.3 IdC” 0.38’ 0.34

Values sharing the same superscript(s) do nc’ differ significantly.

TABLE 5

Initial wncentration of iigmn and nirrogen and the ratio of iignin/nitrogen in the leaves ofsix tropical species

Species Nitrogen (O/o) Lignin (O:tr ) Lignin/nitr’ogen ratio

PlPNICO.Tp4S 1.70 15.81 9.30 Teciona 1.35 17.12 12.6% XjYia 1.61 i&i% 10.05 DiNerri i 1.11 17.1% i 5.47 Tkwirnah 0.94 17.07 I%.SS Grewia 1.60 15.17 9.4% ___-

TABLE 6

Correlation coefficients (I’). intercepts and slopes relating decomposition constants (ii) to iniiirl nitrogen and iignin concentratinns and the ratio of inittal lignin concentration :a the initial nitrcgen concentration for six tropical tree species

SI.NO. iocation Liiter quaiit) veriable

Slope intercept

!I) (2) (31 (4) (5) (6) (7) (8) (9)

Ku Ku KU Ki K! Kl KP KP KP

I

O:o nitraqen & 4’0 iignin ?b lignin/YOnitlogen 9/o nitroge. I lignk o:O lignin/?Yoniriogen 9’0 n:irogen Oi3 iignin ?b lignin/“bnitrc;gen

0.1284 -0.02:f -0.009 )

0.0633 - 0.008% - O.cIOSO

O.l??% -0.0581 -0.013%

0.!525 0.46 0.8045 0.17 0.4545 0.40 0.2471 0.35 0.47%9 0.05 0.3979 0.32 0.1275 0.59 ! .326% 0.44 9.546% 0.51

-___

tiaI percent lignin,/percent nitrogen ratio. Pterocarpus, Grewia and Xylia had relatively higher foliar nitrogen and lower fignin concentrations {Table 5 3. Of the initial structural chemistry attributes, percent nitrogen was a better predictor of decay rate (Table 6). However, the fitted regression mums in general explained less than 60% of the variations. The double logarithmic regression of k on initial percent lignin/percent nitrogen ratio proposed by Melillo et a!. ( I982 ) explained a lesser extent of variations than percent ni- trogen Again, the relatively less disturbed Kp site provided better r2 values.

The total nttrogen content of the biomass remaining increased as the de- composition proceeded, on all three sites (Fig. 6 ). Mean nitrogen concentra- tion of the leaf litter increased from 1.7% to 3.06% for Flerocarpus, 1.35% to 2.78% for Teclona, 0.92 to 2.12% for A’ylia, 1. i i to 2.32% for DiNeniq 0.92 to 2.22% for Terminaiia, and 1.6 to 2.96% for Grewia, over periods ranging from 5 to 8 months.

An zxamination of the macronutrient composition of the domman con- tributors of litter fall revealed that X, P and K contents uf the prkapal oc- cupants of these sites varied considerably ( Fig. 7 ). Foilage nitrogen content of the species examined ranged from 0.65% in Cleistanthw collimrs Benth. to 1.6% for Xyiia and Grewia. Regarding phosphorus content, the range was from 0.034% to 0.077%. The range in potassium content was from 0.25% to 0.62%. C. dinus, an understorey shrub, consistently recorded the lowest folk con-

1 I 1 Stonanrd de,,,ot,on

-~-I_~.L__-i_-l__.-_L_ I

DP RR TG TB CC SO XX GT SN LL HI MP TP

Tree spetse~

centrations for all nutrients among the species examined. Mircamiz~cr .&alla R/3. kg., H&reres isom Linn., T. panicrkm and Ternaindia belkrica Roxb. also recorded relatively lower values for N, P and K contents. L,n,qWro~.W7in imceolatcr Wall.. Bridelia wfusa Spr.. T. grandis, Schlck+cra okma ( Lour. ) Oken., Sq~hnos mrsvomica L. and D. perztug~na were intermediates ( L - 1.5% N, 0.05-0.07*/o P and 0.40~0.60% K). On the other hand, Gwwia and X_~ljc, two important overstorey species consistently recorded high values for N. P and K contents (greater than ! .5Oh N, 0.07% P, and 0.6O/o K).

DISCUSS!*N

‘3ur study in the moist deciduous forest of Wcs~crn Cihats indicated pro- iwnd seasonal variat;ous in detritus fall. Leaf shedding was heavy during the dry periods ranging from November to April at all the three sites and for all dominant species. Tree water stress is a cardinal aspect of the dry seasons, where moisture availability is limited and temperature shoots up. Moore ( 1980 f reported that water stress triggers de mvo synthesis of abscissic acid in the foliage of plants, which in turn, can stimulate senescence of leaves and

other pats. Hence changes in the endogenous hormonal balance can be a plausible explanation for the peak litter fall during summer months. Besides, rises in ambient temperatures owing to fires can also spur an accelerated leaf fall du,ring summer.

Studies on the phenology of litter fall in moist deciduous natural forests are generally rare from the Eastern HerGsphere. Nevertheless, available reports concerning deciduous plantations clearly indicate that deciduous species yield maximum litter during the summer months (Ghosh et al.. 1982; Kikuzawa et al., 1984). Pascal ( 1988 ) working on the wet evergreen forests of Attappadi region of Western Ghats, indicated illat the rhythm of leaf shed was charac- terised by a heavy fall during the dry season. In contrast, in plantations of evergreen species such as Acacia ndotica and Eucaiyptzas t~wi,, ~mxis, he sea- sonal litter yieid peaked during winter months (Gill et al.. I %‘I ?.

The annual addition of detritus through litter fail expressed as I k: ’ year-’ ranged from 12.2 to 14.4 for our sites. This, although comparriibie with the litter production rate (.5.5-I5.3 t ha-’ year-! j reported by ~~~~~~~~~~~ and Gray ( 1974) for equatorial forests, is greater than the litter fall vaiuc predicted from Bray and Gorham’s ( 1054) inverse relationship between total detritus production per year and Jatitude of the region (estimated value for I O”N lat- itude: 9.8 t ha-’ year- i ) and also the values reported for other tropical forests (W&e, 1977; Franken et al., 1979; Luizao and Schubart, 1537 ). The fa- vourable temperature and rainfaii conditions prevailing in this rcguon and the concomittant higher primary productivity can account for this higher amount of litter productiun, because yearly litter yield is primarily a furs&on of the annual synthesis of organic matter as foliage and other ~~11~~~9~~~~~~~1~ (Dray

and Corham. 1964; Tadaki, 1966; Das and Ramakrishnan. 1985 ). iiowvw, despite the higher primary productivity and total litter fall, leafiittcr ft~? ions in the dead biomass were comparable with the values reported in the litera- ture (Klinge, 1977; Franken et al., i979; L.uizao and Schubart, 1987 1.

Surprisingly, the mean annual litter fall rates at the three locations did i.a’r, appear to be directly related to stand basal area and density (Table ? 1. ! ;,i- ferences in the stand basal area of the three stands investigated ranged Cent 18.04 to 25.59 m’ ha- ‘. However, this difference failed to mun~fest itseil lo terms of litter fall rates. Past studies also failed to establish caur:~--~~fi~~~t a’~*la- Czmships between such parameters and litter fall in closed cam .;‘i’i’.jiC:‘atC forest ecosystems ( Bray and Gorham, 1 %‘$I: ~t~~~~~~r~n, 19N:s skshPgren ( 198Xa) suggested that annrrsI Litter fall can be be!.*pLr predict&’ %I function derived from ,the individual tree basal area and I!:,: crown rat!4 iiitter pro- duction was also not related to disturbance intensity. Howeve- 1:~ gange in relative disturbal~~e intensities available in the present st:~“,. *has narrow (Table f ). In order to obtain conclusive information on this ..:.;‘S~~t dctnilcd

i-,Yesttgations ir. natural forest ecosystems w?th a wider magnitude ofdistur- n::nces are required.

Species differences in litter decay rtttcs arc: evident from Figs. 5a, Sb and ?c. Six species studied p*sstntly can be broaddy divided into two groups: i.e. fast and f;low decom;)oscrs. Pterocarj?r6 ,Y~iia 961 d Grersia exhibited rela- tiveIy fa::ter decomposition rates while SY!OV,., LX~~,V~LZ and Tt~mi~“ia were characterised 17~ relatively slower dwca_. PF -r rsies. The ti ne required for complete disannearance of the oragmal biomass ranged from 5 to 8 months. Mass dis- anneara;ice rates in the present stud!; were high on ali the sites and in partic- ular at the Kp site in comparison with the values reported for temperate eco- s:,s:ems. In this context, the available literature on litter decompositiom differena pirie species were compiled by Das and Ramakrishnar ( 1’3% ;< bver 12 months decomposition periods, they reported annual decay rate coeffi- cisnts ranging from 0.307 to 0.46. Melillo et al. I 1982 j/ working on!temperate hardwood spzci~s also reported widely varying a nnual decay rate c$efficients ~0.08-0.47 ). fn a I;-ecent analysis of the ZWO Sierran forest types in Southern Sierra Nevada ofCalifornia, Stchlgren ( I%&3 ) wo::xd c~f armual ctecay rate cc&%irnts of the order of 0.18-0.62 for six species, According lo Stohigen ; 138tia) the time required to 95% decay (using k at 3.6 -years) ran;,:ed from 1 1 to 27 years. The number of published repot?:; concerning litter dccompo- sition dynamics oftropical forests has been verl *ew. In one study on Clito~iri rLtcerM0Sn Benth. from the terra-firma forests of Central Amazonia, Luizao and Schubart ( 1987 ) calculated the time to 95% decay *is 1.9-M years de- pending on the season and area. The large differences observed were attrib- uted to decomposer population dynamics, especially of the macro-arthro- pods. Contrary to the published repcjrts, the decay rare coefficients observed in our study were profoundly greater (see Tabie 4 for monthly decay rate coefficients). This implies that tropical moist deciduous forest species ex- hibit markedly higher decay rates compared with temperate and tera-firma forest species. Furthermore, in tropical forests, usually there is very little or no accurnulatior* of litter, implying I: fast ttrrnover of organic matter in the soil. Changes in temperature and moisture availability have been related to decomposition rates (Woo~~cil and Dykeman, I. 966; Agbim, I987 ). Wil- liam and Gray ( 1974) suggested that differences in temperature and mois- ture supply, their interactions and the higher activity of the decomposer or- ganisms can, to a great extent, explain the large variation5 in litter decomposition rates existing between tronic;.l and temperate regions.

Disturbance can alter the external environmental parameters. Pastor and Post. f 1986 ) postulated that this can increase litter dcca? WCS. However, we found that the less distwbed Kp site (high basal area ) exhibited a faster de-

composition rate, probably because of the favourable changes 112 microcii- matic fa.ctors and decomposer popuiations. The iowef d~~orn~os~~~on rate on a more disturbed site suggests t.hat disturbance can affect the organic matter flow into the soil. Thus. disturbance, quite apart from its possible influer~ce on tree regeneration. soil erosion and the associated ~~~r~e~t drain fom the site. also affects soif organic matter r&lions and thereby seduces site prr)ductiviQ~.

Several author5 have reported that biochemical auality (lignin and nitro- gen and their ratios) of inter exerts a profound ini;uence on ihe decay rate (Singh and Gupta, 1977; Mehlto et al.. 1982: Harmon et al., 1990: and severa! others). The ?hree fast dKX3mpOSf?iS in the present study had a higher nitro- gen content, Iower Sgnin and lower initial Iignin/n~trogen ratios (Table 5 ). However, uniike itilehilo et al.‘s I1982 j suggestion. the iniGai i~gn~n~n~~roge~~ ratio was not closely correlated with the decay rate constant, k. Harmon et al. ; IWO) also observed that some species studied by them (e.g. red alder) de- caj cd more slowXy than predicted by the power-curve relationship of Mehllo et ai. ( 1982 f . In our stud>]. it was %bu~d that of the structural chemistry attri- butes. nitrogen content of ihe detritus appears to be a better predictor of tRe decay rate constant. The r’ values, however. were reIatively ICVV (Table 6 1. lin this connection. several workers failed to find strong dependence of either !&in or iingin/nltrogen ratio on decay rate coefficients !Schfesinger and Hasey. 198 1: ~~C~aughe~~ et al., f 985; Tayhr et al., f 914F f . Howeser. before confirming this position in respect of tropical species. more such species with a wider range of initia! hgnin and nitrogen cantents need be anaiysed. as the lignin contents of the six litter types studied do not cover the entire natural range of tropical leaf litter t; pes.

Chemical composition is an intrinsic property of the iitter which deter- mines the rate of turnover of orga~~~a~~~ bound nutrients. The faster Initia! rate of de~orn~~)s~t~on of leaf litter in ~~~~~~~~u~~~~~. Grewic; and Xyii~ f Figs. 5a, 5b and SC) appears to be dependent on the hi@ initial ni+.rogen concew tration. Species adapted to higher nitrogen ar-:Glrkilities wi!I gcneraily have a faster rate of organic matter turnover. Ater and Mchtlo ( 1982) have earlier suggested that such species exhibit low minera! ~~~~loh~~~sat~o~~ rates also. which in turn, may stimulate quicker decomposition. f-lowe~es, Berg ( I986 1 and Taylor et ai. i 1989) have suggested that ~~tro~~~~s influence over dec@m- position rates decreases as decay rate proceeds. whfk that of hgn!n =ould increase. Hence, the reduction in d~~orn~os~t~o~~ rate at the later stages may be due in part to the quick Joss of1abile coli~nonents and that the subsequent ~rea~do~v~ of the refractory materials is slower ei’en though the nitrogen con-

centration of the residual litter mass increased owing to weight ioss and the associated concentration effect.

Leaf litter from tree species belonging to the upper canopy strata. in gen- eral, registered high values for nutrient ~~~~ce~trati~~~. Interestingiy the top stotey trees, X~t~liu. Grswici, I,u,5’c~r:crroc~nlicc. l~illi~~~i~~ and l’iwr~itdiu also con- tributed most of the litter in these moist deciduous forests (Table 3 )? of which the former two were of the nutrient-rich type, while Ik.yr~~~-troernin and 1X/- /cilia were uf meditrm nutrient contents and Termindia was nutrient-poor. The low elemental concentrations observed in the understorey shrub, C. cok lirlus is particularly signi5cant. One possibility is that their leaves are inher- ently low in nutrient status in view of the low photon flux densities reaching the understorey Many authors havIe shown that under light-limited environ- ments, Leaf nitrogen concentrations are generally low (Field and Mooney, 1983; Hirose, 1988 ). Alternately this could be due to an extremely efficient retransiocation of nutrient from the older pIant parts to meristematic regions. It is not clear which of these two mechanisms operate.

The magnitude of nutrient releaqe i TIXJ the system tnrough htter decay would thus be dependent XI the fioristic composition. Implicit In this would be that the forest types influence tlutrient release pattern in a substantial manner. Further, each forest community has its own litter fall pattern which differ in their composition, chemical and biochemical characteristics and nutrieni transformation rates. Information pertaining to the in_tial macroelement composition and htter decay rate would provide a mechanism for under- standing nutrient release patterns into the ecosystem. The low nutrient litter wouId be characterised by slower turnover or higher immobihsation rates. Other studits also have noted that low initial nutrient concentrations resulted in high litter accumulation rates (Berg and Staaf, i 98 1: Eahey et al., 1985; Stohlgren, 1988b ).

Structural attributes of vegetation such as fioristic composition. basal area and density along with disurbance intensity were not directly related to the quantum oflitter fall in thr: old-growth moist deciduous forest stands of West- ern Ghats. Litter fail followed a monomod~~ seasonA disiribution pattern. Leaf shedding is influenced by ciimatic variations and heavy fall occurred during the dry season, Promkent species, however. exhibited a relay se- quence in the periodicity of leaf faI1, for example Dihia, Grm+ut etc. started leaf fall early in October-Noveznoer followed by A’_rfia. Lag~rroernia, TN- rrkdia. Brideiia and Ckistnnlir~s. The fraction of leaf htter ranged from 65% to 74% of the totat Iitter fall.

Mass disappearance rates of inter in AC litter bags differed significantly among the species. The tropical forest ecosystems are generally characterised

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