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Decomposition of leaf litter in karri (Eucalyptus diversicolor) forest of varying age
Forest Ecology and Management, 24 (1988) 113-125
113
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Decomposition of Leaf Litter in Karri (Eucalyptus diversicolor) Forest of Varying Age A.M. O'CONNELL
Division o[ Forest Research, CSIRO, Private Bag, P.O. Wembley, W.A. 6014 (Australia) (Accepted 17 September 1987)
ABSTRACT O'Connell, A.M., 1988. Decomposition of leaf litter in karri (Eucalyptus diversicolor) forest of varying age. For. Ecol. Manage., 24: 113-125. Rates of weight loss and release of N, P, K, S, Ca, Mg, Na and Cl from decomposing leaf litter were measured in regrowth karri (Eucalyptus diversicolor F. Muell.) forest aged 6, 9 and 40 years and in mature karri forest growing in south-western Australia. Decomposing leaf litter from the four sites lost 47-55% of original dry weight after exposure for 82 weeks on the forest floor. Rates of weight loss from leaf litter at the four sites varied in the order: mature < 40-year-old = 9-yearold = 6-year-old-forest. A composite exponential function, with separate decay functions for labile and resistant litter components, explained 98-99% of the variation in mean dry weight loss in relation to time of exposure. Loss of labile litter components was rapid (half-lives 5-14 weeks). Loss of the resistant components was faster for litter from regrowth stands (half-lives 116-119 weeks) than for litter from mature forest (half-life 189 weeks). Slower rates of leaf litter decomposition together with increasing amounts of leaf and non-leaf (twigs, bark and fruit) litterfall as the stands mature indicate that the rate of accumulation of forest litter will be greater in mature forest than in regrowth stands. The order of element mobility in decomposing karri leaf litter ( Na > C1> K > S > Ca > N > P) was similar to that reported for other eucalypt litters. Nutrient content of fresh leaf litter, and rates of nutrient release during leaf litter decomposition, tended to be greater in the younger regrowth stands than in mature forest. Therefore, although the amounts of leaf litterfall were smaller in the younger stands, the amounts of nutrients released during the initial stages of decomposition were similar to, or greater than, the amounts released from decomposing leaf litter at the mature forest. However, since the proportion of non-leaf components in annual litterfall increases with age of the forest, total release of K, S, Ca, Mg and Na from all litter fractions is likely to be greater on older stands. Nitrogen and P occur at low concentrations in the major non-leaf litters and are imported into these tissues during decomposition. Therefore, in the initial stages of decay, immobilization of N and P will be greater in the litter layer of mature forest than in young regrowth stands.
INTRODUCTION
In south-western Australia, karri (Eucalyptus diversicolor F. Muell.) develops as a tall open forest (Specht, 1970) on the lower valley slopes and in mix-
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114 tures with marri (E. calophylla R. Br.) on mid-slope sites (McArthur and Clifton, 1975 ). Two previous studies, in karri forest ranging in age from 2 years to mature forest, investigated the relationship between stand age and components of the nutrient cycle. Litterfall was shown to increase with increasing age of the forest while the concentrations of many of the nutrients held in freshly fallen litter decreased with stand age (O'Connell and Menage, 1982). Total nutrient accessions to the forest floor ( litterfall ÷ throughfall ÷ stemflow) were 1.5 to 4.6-fold higher in mature forest than in 6year-old regrowth forest (O'Connell, 1985). This was due to the greater amounts of litter and the higher concentrations of nutrients in throughfall in mature forest compared to regrowth stands. The aims of the present investigation were to extend these studies to examine the effect to stand age on rates of breakdown and changes in nutrient content of litter during decomposition. Litter-bag techniques were used to estimate changes in dry weight and nutrient concentrations in karri leaf litter during two wet seasons in four forest stands ranging in age from 6 years to mature forest. EXPERIMENTALDETAILS
Study area The study area is located approximately 250 km south of Perth, between the townships of Manjimup and Pemberton in the main karri forest region of southwestern Australia. This region has a Mediterranean climate with cool wet winters and warm dry summers. Average annual rainfall at Pemberton is 1255 ram. Average monthly maximum temperatures range from 26 °C in summer (January-February) to 15 ° C in winter (July-August) and average monthly minimum temperatures from 13°C in summer to 7°C in winter. Within the study area four sites were selected in even-aged forest where karri was the only tree species present. Three of these stands were in regrowth forest aged, in January 1978, 6, 9 and 40 years, and a fourth site was in mature forest probably several hundred years old. Results from the 40-year-old site, presented elsewhere as part of a separate study ( O'Connell, 1986a), are included here for completeness. Vegetation characteristics of the study area are reported in O'Connell (1985, table I ). Understorey at each of the sites was dense, being dominated by Bossiaea laidlawiana Tovey and Morris and Trymalium spathulatum ( Labill. ) Ostf. Overstorey vegetation was also dense on the youngest stands, but stem density decreased with increasing stand age.
115
Litter bag studies A bulk sample of fresh karri leaf litter from each site was collected by suspending large nets beneath the trees during the peak period of summer litterfall (January-February). Each sample was sorted to remove other litter components, thoroughly mixed, and air dried. Approximately 20 g of litter was placed in 25-cm X 25-cm terylene bags, mesh size 3 mm. Thirty-two bags were positioned at each site in two blocks of 16, located 100-200 m apart, in April 1977. Current-year's litter was removed, bags were pinned to the forest floor and current-year's litter was replaced on the bags. Ten samples of each litter type were retained in the laboratory for determination of initial moisture and nutrient contents. Four bags of litter from each site (two from each block) were returned to the laboratory at each of eight subsequent collection times. Collections were made 5, 11, 18, 30, 42, 51, 68 and 82 weeks after placement on the forest floor. Final collections were made in November 1978. Bags were dried at 70 ° C, litter was brushed free of soil and sample weights were determined.
Soil sampling Soils were sampled (0-5, 5-15 cm) at ten locations at each of the experimental sites. Samples were air-dried and sieved ( < 2 mm) before chemical analysis.
Chemical analysis Soils were analysed for pH, organic C, total N, extractable P and exchangeable cations, using methods outlined by Hingston et al. (1979). Nitrogen content of litter samples was determined autoanalytically following Kjeldahl digestion. Other elements were analysed by X-ray spectrometry using procedures outlined by Norrish and Hutton (1977). Initial lignin and cellulose contents of the litters were measured according to the method of Van Soest (1963 ), and the proportion of readily soluble components in the litters was determined by extraction with aqueous methanol.
Statistical analysis Significance of differences between sites in rates of weight loss and nutrient content of litter were tested by analysis of variance. Composite exponential decay functions were fitted to decomposition weight loss data using non-linear optimization techniques in the general statistical program GENSTAT (Rothhamsted Experimental Station).
116
RESULTS
Soil properties Soil chemical properties at the four experimental sites are listed in Table 1. The concentration of both N and C in soil increased with increasing age of the forest stands. The concentrations of exchangeable Mg, K and Na also tended to be higher in soils from the older stands. The remaining soil properties, pH, extractable P and exchangeable Ca, showed no relationship with age of the forest stands.
Weight loss Weight loss from karri leaf litter contained in mesh bags was rapid during the initial phase of decomposition (Fig. 1 ). About 25% of dry weight was lost after the litter had been exposed for 20 weeks. The proportion of original dry weight that was released from the litter at the four sites during the 82-week exposure period varied in the order mature forest < 40-year-old forest = 9-yearold forest = 6-year-old forest. The relation between dry weight loss and time of exposure was described by a composite exponential function (Bunnell and Tait, 1974; Lousier and Parkinson, 1976) of the form:
W= W1 exp( --kl t) q- W2 e x p ( - k 2 t ) where W is the proportion of original weight remaining after time t, kl and k2 are decay constants for the labile and resistant litter components, W1 is the measured proportion of labile components present initially, and W2 -- 1.0 - W,. TABLE1 Soil chemical properties at the four experimental sites. Overstorey age (years) 6 9 40 Mature SEM
Soil depth (em) 0- 5 5-15 0- 5 5-15 0- 5 5-15 0- 5 5-15 0- 5 5-15
pH
7.3 7.2 6.9 6.8 6.5 6.6 7.0 7.1 0.1 0.1
Organic C (%)
4.5 2.8 4.2 3.2 6.4 4.3 6.6 4.2 0.4 0.2
Analytical methods as for Hingston et al. (1979).
Total N (%)
0.15 0.09 0.17 0.14 0.33 0.20 0.35 0.20 0.02 0.01
BrayP (pgg-1) 9.1 3.3 6.9 3.2 10.0 7.3 10.1 6.6 1.3 0.9
Exchangeable cations (meq per 100 g) Ca
Mg
K
Na
13.5 6.1 9.2 7.4 9.3 4.6 17.6 11.3 1.9 0.7
1.2 0.7 1.4 1.2 3.3 2.1 3.4 2.3 0.2 0.2
0.23 0.15 0.28 0.22 0.28 0.17 0.81 0.77 0.04 0.04
0.19 0.19 0.17 0.18 0.43 0.37 0.39 0.39 0.05 0.06
117 100
# E ~ 80 E m z:
._=
g 6(] u
g.
4C
2r0
410 610 Exposure period (weeks)
810
i
IO0
Fig. 1. Percent of original weight remaining of karri leaf litter decomposing at the 6-year-old ( X ), 9-year-old ( • ) , 40-year-old ( • ) and mature forest ( • ) sites. Bar indicates 95% confidence interval. Fitted curves are derived from the composite exponential decay model.
This function explained 97.9-99.4% of the variation in mean dry weight loss in relation to exposure period (Table 2). The proportion of readily soluble components initially present in litter (24-26%) did not differ significantly between karri leaves from the various sites. This component was lost rapidly from decomposing litter (half-lives 5-14 weeks). Rates of decomposition of the resistant component of litter differed between the regrowth stands and the mature forest. Weight loss of this component was more rapid for leaf litter in the young stands (half-lives 116-119 weeks ) than for litter in the mature forest (half-life 189 weeks).
Nutrient changes The nutrient content of fresh, undecomposed litter differed significantly between the four experimental sites ( Table 3 ). Karri leaf litter from the younger stands tended to have higher concentrations (percent of dry weight) of N, P, K, S, and Mg than litter from the older sites. The concentration of Ca in fresh litter differed between sites, but the differences were not related to the age of the forest stands. During litter decomposition the concentrations of N, P and Ca increased substantially, the concentrations of S and Mg changed little, and the concentrations of K and Na decreased.
118 TABLE 2 Double exponential decay models fitted to mean weight loss from leaf litter for each of the experimental sites Stand age (years)
Readily soluble W1( To)
Decay constant
Half-life
Labile component kl (year -1)
Resistant component k2 (year -1)
Labile component tl(½) (weeks)
Resistant component t2 (½) {weeks)
6
23.9 {0.9)
7.59 (1.13)
0.31 {0.02)
4.8 {0.7)
116.1 (5.8)
9
23.8 {0.6)
5.50 {0.96)
0.31 (0.02)
6.6 (1.1)
116.5 (7.4)
40
26.1 {0.2)
2.66 (0.44)
0.30 {0.02)
13.6 {2.3)
119.3 {9.0)
Mature
24.6 (1.1)
3.53 {0.49)
0.19 (0.01)
10.2 (1.4)
188.9 (14.4)
W1= measured readily soluble component, kl = decay constant for the labile component of litter, k2 = decay constant for the resistant component of litter, tl ( ½) = half life of the labile component, t2 ( ½) = half life of the resistant component. Standard errors in parenthesis.
TABLE3 Element, lignin and cellulose concentrations in karri leaf litter before placement of mesh bags in the field {fresh litter) and element concentrations after exposure for 82 weeks {decomposed litter) at the four experimental sites Overstorey Litter type age {years)
Concentration (To) N
P
K
S
Ca
Mg
Na
Cl
Lignin Cellulose
6
Fresh 0.47 0.015 0.55 0.111 2.15 0.23 0.24 0.38 27.6 Decomposed 0.96 0.037 0.09 0.131 2.95 0.19 0.01 0.07
9.8
9
Fresh 0.51 0.013 0.58 0.115 1.35 0.22 0.21 0.33 31.5 Decomposed 0.94 0.028 0.10 0.116 2.01 0.20 0.01 0.04
8.4
40
Fresh 0.37 0.010 0.25 0.095 1.09 0.29 0.18 0.20 26.9 Decomposed 0.74 0.027 0.09 0.100 1.62 0.29 0.01 0.06
11.1
Mature
Fresh 0.36 0.012 0.25 0.095 1.53 0.28 0.18 0.22 27.9 Decomposed 0.86 0.032 0.10 0.115 2.20 0.28 0.01 0.05
8.6
119 180
180
140
140
,oo
100 80 100
8O 10(3
P
K
75
75
.c 50
50
E
09
" .c
~
25
25
o
o
-~.c_ 100 ~
100
"6 75
75
09
_ 50
5£
25
25
10(3
100
Na
75
75
50
50
CI
25 0 May
L
J
Nov
May
I
May Nov Exposure period
i
i
i
Nov
May
Nov
Fig. 2. Percent of original weight remaining of N, P, K, S, Ca, Mg, Na and C1 in decomposing karri leaf litter at the 6-year-old ( × ), 9-year-old ( • ), 40-year-old ( • ) and mature forest ( • ) sites.
Patterns of nutrient release, expressed as a percentage of the original amount present in litter, differed between elements (Fig. 2). The majority of Na, K and C1 in litter was released rapidly during the initial phase of decomposition while changes in the amounts of S and Mg were more closely related to changes in litter dry weight. In contrast, the amounts of N and P in litter either changed little or increased as decomposition proceeded. The amount of Ca in decomposing litter decreased approximately linearly with time of exposure. Although the amounts of many of the nutrients in decomposing litter differed between sites, the order of nutrient mobility was the same for each site. The proportions of the original nutrients in leaf litter that were released during the 82-week exposure period decreased in the order N a > K > M g > d r y weight >S>Ca>N>P.
120 TABLE 4 Amounts of nutrients in decomposing karri leaf litter, expressed as a percentage of the original amount present, after exposure for 82 weeks on the forest floor at the four experimental sites Overstorey age (years)
Percent oforiginal weight remaining N
P
K
S
Ca
Mg
Na
6 9 40 Mature SEM
97 89 89 127 4
116 103 121 140 6
7 8 16 21 2
55 49 47 64 2
63 73 67 75 3
38 45 45 53 3
1 1 3 3 1
The amounts of each nutrient in decomposing litter after exposure for 82 weeks, expressed as a percentage of the original amount present, are listed in Table 4. For Na and Ca there were no differences between sites. The amounts of each of the other nutrients in decomposed litter differed between sites, and {a) Litter moisture 250r
\ : \\
: oolV'
i\
/
O' 28
(b)
Litter temperature
24 o
20
16
$
Jun
i Sept
~ Dec
MLar
Fig. 3. Litter moisture (a) and mean litter temperature (b) at the 9-year-old ( 0 ) and mature forest ( • ) sites. Data from O'Connell and Grove (1987).
121 TABLE 5 Annual accession of nutrients in karri leaf fall (O'Connell and Menage, 1982) and amounts of nutrients released from karri leaf litter during the 1st year of decomposition Nutrients ( kg h a - 1 y e a r - 1)
Overstorey age (years)
N
P
K
S
Ca
Mg
Na
6
Accession Release
9 0
0.4 - 0.1
7 6
1.6 0.6
33 7
3 2
3 3
9
Accession Release
12 2
0.5 0.0
7 6
2.2 1.1
35 6
5 2
3 3
40
Accession Release
16 - 1
0.6 - 0.2
7 5
2.3 1.0
29 4
8 3
3 3
Mature
Accession Release
11 - 2
0.4 - 0.2
6 3
1.9 0.6
36 5
6 2
3 3
Negative signs indicate uptake of nutrients into decomposing leaf litter.
in general the differences were related to the age of the forest stands. For these elements litter decomposing in regrowth stands lost nutrients more rapidly, or accumulated less nutrients from the environment, than litter decomposing at the mature forest site. DISCUSSION
In this study, decomposition rates at four sites were compared using litter from the karri overstorey. Rates of decomposition of several fractions of karri forest litter at one of the sites are reported elsewhere ( O'Connell, 1987a). Only karri leaf litter was included in litter bags in the present study; however, the bags became intimately associated with other components of the forest floor, particularly during the 2nd year of measurement. In another study ( O'Connell, 1986b ) it was found that mixing legume litter with eucalypt litter in litter bags had no effect on eucalypt litter decomposition rates, suggesting that separation of individual litter components in litter bags provides reasonable estimates of their rates of decay. Weight loss from karri leaf litter (47%-55% after 82 weeks) was similar to that found for leaf litter of other eucalypts growing in south-western Australia, ( O'Connell and Menagd, 1983; O'Connell, 1986b) and for leaf litters of a range of eucalypts growing in eastern Australia (Woods and Raison, 1983). All of these studies were conducted over periods of 1-2 years, during which rates of decay are influenced to a large extent by loss of labile components held in litter. In the longer term, decomposition rates are dependent on breakdown of the
122
resistant components of litter which may differ between the various species. Differences between the rates of decay of litter at the mature and regrowth karri forest sites result mainly from differences in the rates of breakdown of the resistant litter components (Table 2). This may be caused either by environmental factors or variation in substrate composition (Williams and Gray, 1974). In a study of decomposition of needle litter in an age series of slash pine, Gholz et al. (1985) also found decay rates were slower on older sites. This was attributed to chemical properties of the litters rather than environmental differences between the sites. No environmental data were available for the present experiment, but in a parallel study of the seasonal variation in nitrogenase activity in the forest floor ( O'Connell and Grove, 1987), litter temperature and litter moisture were measured at monthly intervals at the mature and 9-year-old regrowth sites (Fig. 3 ). Litter temperatures were similar at the two sites, but litter moisture was greater at the mature forest site than at the regrowth site on 14 of the 15 sampling occasions. During the moist period of the year (May to October) differences in litter moisture content probably had little effect on rates of decomposition because moisture levels ( > 100% dry weight) were not limiting for microbial activity (Flanagan and Bunnell, 1976). In spring (October-December) and autumn (March-May) when the litter layers were drying and re-wetting, respectively, litter moisture was greater at the mature forest site than at the regrowth forest. These conditions favour greater microbial activity in the forest floor of the mature forest during this period. Thus, the slower overall rate of leaf litter decomposition at the mature forest compared to the regrowth stands is unlikely to be caused by environmental effects, but rather is probably due to compositional differences between the litters. Several studies have reported on the relation between decay rate and litter composition (Meentemeyer, 1978; Berg and Staaf, 1980; Melillo et al., 1982; Gholz et al., 1985 ). Among the most important properties influencing decomposition are the intitial lignin and initial N content of the substrate. The ratio of these two parameters has been used to explain variation in the decay rates of a range of North American hardwoods (Melillo et al., 1982 ). Leaf litter from the younger karri stands contains nutrients, including N, at higher concentrations than leaf litter from the mature forest site. Regression of the initial lignin:initial N ratio on the labile (kl) and resistant (k2) decay constants of the various litters explained 76% and 68%, respectively, of the variation in the decay constants. These data suggest that differences in rates of decay of leaf litter in the age sequence of karri stands is partly related to differences in the composition of the litter at the various sites. Slower rates of litter decay, together with greater amounts of litterfall and a higher proportion of more-slowly decaying non-leaf fractions ( O'Connell and Menagd, 1982; O'Connell, 1987a) contribute to greater rates of accumulation of the litter layer as a whole in mature forest compared to regrowth stands.
123
Rate of litter decomposition in recently clearfelled forest is slower than at well-vegetated sites, probably because drier conditions at the soil surface limit microbial activity on the more-exposed clearfelled areas (O'Connell, 1986a). However, this period of reduced decomposition rate does not extend to stands aged 6 years or older. Once a canopy of karri and understorey plants is established, conditions become more favourable for microbial activity and rate of litter decomposition increases. Establishment of a vegetative cover occurs 1-3 years after planting or natural regeneration, and this is probably the period during which conditions less favourable for surface litter decomposition persist. Rate of decomposition of surface litter is one factor which can affect accumulation of soil organic C. In Karri forest, amounts of organic C in soil decline following clearfelling ( O'Connell, 1986a). In the present study, amounts of soil organic carbon were lower in the 6 and 9-year-old stands than in the 40year-old and mature forests (Table 1 ). Total soil N concentrations exhibit a similar pattern in relation to stage of stand development. More intensive sampling is required to define the form of the response of soil organic matter to clearfelling, its dependence on intensity of harvesting and rotation period, and its effect on soil nutrient status. The order of nutrient mobility in decomposing karri leaf litter is similar to that reported for other eucalypt litters (Attiwill, 1968; O'Connell and Menagd, 1983 ). Phosphorus, the most immobile element, is held strongly in decomposing karri litter. In the early stages of decomposition, both concentration and amount of P in each of the litters increased markedly, and no net mineralization of P occurred during the course of the experiment. Initial increases in the amounts of P in litter were related to the concentrations of extractable P in surface soils (R 2_ 0.85 and 0.91 after 5 and 11 weeks, respectively) suggesting that transfer of P in fungal hyphae from soil to litter may have been responsible for net import of P into the litter. During the 2nd year of exposure the amounts of P in each of the litters declined (Fig. 2). The form of the decomposition curves indicates that net mineralization of P from the leaf litter occurs sooner in the regrowth stands than in mature forest. Similarly, there was only limited mineralization of N from litter at the regrowth sites and N accumulation occurred in litter from the mature forest. Higher C:N ratios, and conditions conducive to greater microbial activity in litter at the mature forest, probably favoured greater microbial immobilization of N in litter at this site. Annual inputs of nutrients in leaf fall (O'Connell and Menagd, 1982) and estimates of the amounts of nutrients released from decomposing leaf litter in the 1st year following litterfall are shown in Table 5. On the youngest sites, smaller amounts of leaf litterfall are offset by the generally higher nutrient content of the litter and its greater rates of decomposition and nutrient mineralization. Therefore the amounts of nutrients released from decomposing leaf litter at the regrowth stands tend to be similar to, or greater than, the amounts released from leaf litter at the mature forest. Nutrients released from
124 decomposing karri leaf litter represent only a proportion of the nutrients released from the total litter layer. At the 40-year-old regrowth site, karri leaf litter accounts for about half of the K, S, Ca, Mg and N a released during the first year of litter decomposition (unpublished d a t a ) . Total release of these elements from all litter fractions will be greater on older stands because the contributions of non-leaf fractions (karri twigs, bark, and fruit) to litterfall increases substantially as the forests m at ure (O'Connell and Menage, 1982). However, N and P occur at low concentrations in the major non-leaf litters (O'Connell and Menage, 1982) and are imported into these fractions during decomposition (O'Connell, 1987b). Because of the relatively large contribution of non-leaf litters to litterfall on older stands, the am ount s of N and P immobilized during the initial stages of litter decay will be greater in mature forest t h a n in young regrowth stands. ACKNOWLEDGEMENTS Assistance given by officers of the Manjimup and P e m b e r t o n branches of the Forests D e p a r t m e n t of W es t e r n Australia in choosing experimental sites is gratefully acknowledged. Mr. M. Pal m er (CSIRO Division of Mat hem at i cs and Statistics) advised on statistical calculations and Mr. P. Menagd provided expert technical assistance.
REFERENCES Attiwill, P.M., 1968. Loss of elements from decomposinglitter. Ecology,49: 142-145. Berg, B. and Staaf, H., 1980.Decompositionrate and chemicalchanges of Scots pine needle litter. II. Influence of chemical composition. In: T. Persson (Editors), Structure and Function of Northern ConiferousForests-An Ecosystem Study. Ecol. Bull. Stockholm, 32: 373-390. Bunnell, F.L. and Tait, D.E., 1974. Mathematical simulation models of decompositionprocesses. In: A.J. Holding,O.W. Heal, S.F. MacLean, Jr. and P.W. Flanagan (Editors), Soil Organisms and Decompositionin Tundra. Tundra Biome Steering Committee, Stockholm, pp. 207-225. Flanagan, P.W. and Bunnell, F.L., 1976. Decompositionmodelsbased on climatic variables, substrate variables, microbial respiration and production. In: J.M. Anderson and A. Macfadyen (Editors), The Role of Terrestrial and Aquatic Organisms in DecompositionProcesses.Blackwell, Oxford, pp. 437-457. Gholz, H.L., Perry, C.S., Cropper, W.P. and Hendry, L.C., 1985. Litterfall, decomposition,and nitrogen and phosphorus dynamics in a chronosequenceof slash pine (Pinus eUiotti) plantations. For. Sci., 31: 463-478. Hingston, F.J., Turton, A.G. and Dimock, G.M., 1979.Nutrient distribution in karri (Eucalyptus diversicolor F. Muell.) ecosystems of southwest Western Australia. For. Ecol. Manage., 2: 133-158. Lousier, J.D. and Parkinson, D., 1976. Litter decomposition in a cool deciduous forest. Can. J. Bot., 54: 419-436. McArthur, W.M. and Clifton, A.L., 1975.Forestry and agriculture in relation to soils in the Pemberton area of Western Australia. CSIRO Aust. Div. Soils Land Use Set. 54, 48 pp.
125 Meentemeyer, V., 1978. Macroclimate and lignin control of litter decomposition rates. Ecology, 59: 465-472. Melillo, J.M., Abet, J.D. and Muratore, J.F., 1982. Nitrogen and lignin control of hardwood leaf litter decomposition dynamics. Ecology, 63: 621-626. Norrish, K. and Hutton, J.T., 1977. Plant analysis by x-ray spectrometry. I. Low atomic number elements sodium to calcium. X-Ray Spectrom., 6: 6-11. O'Connell, A.M., 1985. Nutrient accessions to the forest floor in karri (Eucalyptus diversicolor F. Muell.) forests of varying age. For. Ecol. Manage., 10: 283-296. O'Connell, A.M., 1986a. Litter decomposition, soil respiration and soil chemical and biochemical properties at three contrasting sites in karri (Eucalyptus diversicolor F. Muell.) forests of south-western Australia. Aust. J. Ecol., 12: 31-40. O'Connell, A.M., 1986b. Effect of legume understorey on decomposition and nutrient content of eucalypt forest litter. Plant Soil., 92: 235-248. O'Connell, A.M., 1987a. Litter dynamics in karri (Eucalyptus diversicolor F. Muell.) forests of south-western Australia. J. Ecol., 75: 781-796. O'Connell, A.M., 1987b. Nutrient dynamics in decomposing litter in karri (Eucalyptus diversicolorF. Muell.) forests of south-western Australia. J. Ecol. (in press). O'Connell, A.M. and Grove, T.S., 1987. Seasonal variation in C2H2 reduction (N2-fixation) in the litter layer of eucalypt forests of south-western Australia. Soil Biol. Biochem., 19: 135-142. O'Connell, A.M. and Menagd, P.M.A., 1982. Litter fall and nutrient cycling in karri (Eucalyptus diversicolor F. Muell.) forest in relation to stand age. Aust. J. Ecol., 7: 49-62. O'Connell, A.M. and Menagd, P., 1983. Decomposition of litter from three major plant species of jarrah (Eucalyptus marginata Donn ex. Sm.) forest in relation to site fire history and soil type. Aust. J. Ecol., 8: 277-286. Specht, R.L.., 1970. Vegetation. In: G.W. Leeper (Editor), The Australian Environment. CSIRO and Melbourne University Press, Melbourne, Vic., pp. 44-67. Van Soest, P.J., 1963. Use of detergents in the analysis of fibrous feeds. II. Rapid method for determining fibre and lignin. J. Assoc. Off. Anal. Chem., 46: 829-835. Williams, S.T. and Gray, T.R.G., 1974. Decomposition of litter on the soil surface. In: C.H. Dickinson and G.J.F. Pugh (Editors), Biology of Plant Litter Decomposition, Vol. 2. Academic Press, New York, pp. 611-632. Woods, P.V. and Raison, R.J., 1983. Decomposition of litter in sub-alpine forests of Eucalyptus delegatensis, E. Pauciflora and E. dives. Aust. J. Ecol., 8: 287-299.
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Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Decomposition of Leaf Litter in Karri (Eucalyptus diversicolor) Forest of Varying Age A.M. O'CONNELL
Division o[ Forest Research, CSIRO, Private Bag, P.O. Wembley, W.A. 6014 (Australia) (Accepted 17 September 1987)
ABSTRACT O'Connell, A.M., 1988. Decomposition of leaf litter in karri (Eucalyptus diversicolor) forest of varying age. For. Ecol. Manage., 24: 113-125. Rates of weight loss and release of N, P, K, S, Ca, Mg, Na and Cl from decomposing leaf litter were measured in regrowth karri (Eucalyptus diversicolor F. Muell.) forest aged 6, 9 and 40 years and in mature karri forest growing in south-western Australia. Decomposing leaf litter from the four sites lost 47-55% of original dry weight after exposure for 82 weeks on the forest floor. Rates of weight loss from leaf litter at the four sites varied in the order: mature < 40-year-old = 9-yearold = 6-year-old-forest. A composite exponential function, with separate decay functions for labile and resistant litter components, explained 98-99% of the variation in mean dry weight loss in relation to time of exposure. Loss of labile litter components was rapid (half-lives 5-14 weeks). Loss of the resistant components was faster for litter from regrowth stands (half-lives 116-119 weeks) than for litter from mature forest (half-life 189 weeks). Slower rates of leaf litter decomposition together with increasing amounts of leaf and non-leaf (twigs, bark and fruit) litterfall as the stands mature indicate that the rate of accumulation of forest litter will be greater in mature forest than in regrowth stands. The order of element mobility in decomposing karri leaf litter ( Na > C1> K > S > Ca > N > P) was similar to that reported for other eucalypt litters. Nutrient content of fresh leaf litter, and rates of nutrient release during leaf litter decomposition, tended to be greater in the younger regrowth stands than in mature forest. Therefore, although the amounts of leaf litterfall were smaller in the younger stands, the amounts of nutrients released during the initial stages of decomposition were similar to, or greater than, the amounts released from decomposing leaf litter at the mature forest. However, since the proportion of non-leaf components in annual litterfall increases with age of the forest, total release of K, S, Ca, Mg and Na from all litter fractions is likely to be greater on older stands. Nitrogen and P occur at low concentrations in the major non-leaf litters and are imported into these tissues during decomposition. Therefore, in the initial stages of decay, immobilization of N and P will be greater in the litter layer of mature forest than in young regrowth stands.
INTRODUCTION
In south-western Australia, karri (Eucalyptus diversicolor F. Muell.) develops as a tall open forest (Specht, 1970) on the lower valley slopes and in mix-
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114 tures with marri (E. calophylla R. Br.) on mid-slope sites (McArthur and Clifton, 1975 ). Two previous studies, in karri forest ranging in age from 2 years to mature forest, investigated the relationship between stand age and components of the nutrient cycle. Litterfall was shown to increase with increasing age of the forest while the concentrations of many of the nutrients held in freshly fallen litter decreased with stand age (O'Connell and Menage, 1982). Total nutrient accessions to the forest floor ( litterfall ÷ throughfall ÷ stemflow) were 1.5 to 4.6-fold higher in mature forest than in 6year-old regrowth forest (O'Connell, 1985). This was due to the greater amounts of litter and the higher concentrations of nutrients in throughfall in mature forest compared to regrowth stands. The aims of the present investigation were to extend these studies to examine the effect to stand age on rates of breakdown and changes in nutrient content of litter during decomposition. Litter-bag techniques were used to estimate changes in dry weight and nutrient concentrations in karri leaf litter during two wet seasons in four forest stands ranging in age from 6 years to mature forest. EXPERIMENTALDETAILS
Study area The study area is located approximately 250 km south of Perth, between the townships of Manjimup and Pemberton in the main karri forest region of southwestern Australia. This region has a Mediterranean climate with cool wet winters and warm dry summers. Average annual rainfall at Pemberton is 1255 ram. Average monthly maximum temperatures range from 26 °C in summer (January-February) to 15 ° C in winter (July-August) and average monthly minimum temperatures from 13°C in summer to 7°C in winter. Within the study area four sites were selected in even-aged forest where karri was the only tree species present. Three of these stands were in regrowth forest aged, in January 1978, 6, 9 and 40 years, and a fourth site was in mature forest probably several hundred years old. Results from the 40-year-old site, presented elsewhere as part of a separate study ( O'Connell, 1986a), are included here for completeness. Vegetation characteristics of the study area are reported in O'Connell (1985, table I ). Understorey at each of the sites was dense, being dominated by Bossiaea laidlawiana Tovey and Morris and Trymalium spathulatum ( Labill. ) Ostf. Overstorey vegetation was also dense on the youngest stands, but stem density decreased with increasing stand age.
115
Litter bag studies A bulk sample of fresh karri leaf litter from each site was collected by suspending large nets beneath the trees during the peak period of summer litterfall (January-February). Each sample was sorted to remove other litter components, thoroughly mixed, and air dried. Approximately 20 g of litter was placed in 25-cm X 25-cm terylene bags, mesh size 3 mm. Thirty-two bags were positioned at each site in two blocks of 16, located 100-200 m apart, in April 1977. Current-year's litter was removed, bags were pinned to the forest floor and current-year's litter was replaced on the bags. Ten samples of each litter type were retained in the laboratory for determination of initial moisture and nutrient contents. Four bags of litter from each site (two from each block) were returned to the laboratory at each of eight subsequent collection times. Collections were made 5, 11, 18, 30, 42, 51, 68 and 82 weeks after placement on the forest floor. Final collections were made in November 1978. Bags were dried at 70 ° C, litter was brushed free of soil and sample weights were determined.
Soil sampling Soils were sampled (0-5, 5-15 cm) at ten locations at each of the experimental sites. Samples were air-dried and sieved ( < 2 mm) before chemical analysis.
Chemical analysis Soils were analysed for pH, organic C, total N, extractable P and exchangeable cations, using methods outlined by Hingston et al. (1979). Nitrogen content of litter samples was determined autoanalytically following Kjeldahl digestion. Other elements were analysed by X-ray spectrometry using procedures outlined by Norrish and Hutton (1977). Initial lignin and cellulose contents of the litters were measured according to the method of Van Soest (1963 ), and the proportion of readily soluble components in the litters was determined by extraction with aqueous methanol.
Statistical analysis Significance of differences between sites in rates of weight loss and nutrient content of litter were tested by analysis of variance. Composite exponential decay functions were fitted to decomposition weight loss data using non-linear optimization techniques in the general statistical program GENSTAT (Rothhamsted Experimental Station).
116
RESULTS
Soil properties Soil chemical properties at the four experimental sites are listed in Table 1. The concentration of both N and C in soil increased with increasing age of the forest stands. The concentrations of exchangeable Mg, K and Na also tended to be higher in soils from the older stands. The remaining soil properties, pH, extractable P and exchangeable Ca, showed no relationship with age of the forest stands.
Weight loss Weight loss from karri leaf litter contained in mesh bags was rapid during the initial phase of decomposition (Fig. 1 ). About 25% of dry weight was lost after the litter had been exposed for 20 weeks. The proportion of original dry weight that was released from the litter at the four sites during the 82-week exposure period varied in the order mature forest < 40-year-old forest = 9-yearold forest = 6-year-old forest. The relation between dry weight loss and time of exposure was described by a composite exponential function (Bunnell and Tait, 1974; Lousier and Parkinson, 1976) of the form:
W= W1 exp( --kl t) q- W2 e x p ( - k 2 t ) where W is the proportion of original weight remaining after time t, kl and k2 are decay constants for the labile and resistant litter components, W1 is the measured proportion of labile components present initially, and W2 -- 1.0 - W,. TABLE1 Soil chemical properties at the four experimental sites. Overstorey age (years) 6 9 40 Mature SEM
Soil depth (em) 0- 5 5-15 0- 5 5-15 0- 5 5-15 0- 5 5-15 0- 5 5-15
pH
7.3 7.2 6.9 6.8 6.5 6.6 7.0 7.1 0.1 0.1
Organic C (%)
4.5 2.8 4.2 3.2 6.4 4.3 6.6 4.2 0.4 0.2
Analytical methods as for Hingston et al. (1979).
Total N (%)
0.15 0.09 0.17 0.14 0.33 0.20 0.35 0.20 0.02 0.01
BrayP (pgg-1) 9.1 3.3 6.9 3.2 10.0 7.3 10.1 6.6 1.3 0.9
Exchangeable cations (meq per 100 g) Ca
Mg
K
Na
13.5 6.1 9.2 7.4 9.3 4.6 17.6 11.3 1.9 0.7
1.2 0.7 1.4 1.2 3.3 2.1 3.4 2.3 0.2 0.2
0.23 0.15 0.28 0.22 0.28 0.17 0.81 0.77 0.04 0.04
0.19 0.19 0.17 0.18 0.43 0.37 0.39 0.39 0.05 0.06
117 100
# E ~ 80 E m z:
._=
g 6(] u
g.
4C
2r0
410 610 Exposure period (weeks)
810
i
IO0
Fig. 1. Percent of original weight remaining of karri leaf litter decomposing at the 6-year-old ( X ), 9-year-old ( • ) , 40-year-old ( • ) and mature forest ( • ) sites. Bar indicates 95% confidence interval. Fitted curves are derived from the composite exponential decay model.
This function explained 97.9-99.4% of the variation in mean dry weight loss in relation to exposure period (Table 2). The proportion of readily soluble components initially present in litter (24-26%) did not differ significantly between karri leaves from the various sites. This component was lost rapidly from decomposing litter (half-lives 5-14 weeks). Rates of decomposition of the resistant component of litter differed between the regrowth stands and the mature forest. Weight loss of this component was more rapid for leaf litter in the young stands (half-lives 116-119 weeks ) than for litter in the mature forest (half-life 189 weeks).
Nutrient changes The nutrient content of fresh, undecomposed litter differed significantly between the four experimental sites ( Table 3 ). Karri leaf litter from the younger stands tended to have higher concentrations (percent of dry weight) of N, P, K, S, and Mg than litter from the older sites. The concentration of Ca in fresh litter differed between sites, but the differences were not related to the age of the forest stands. During litter decomposition the concentrations of N, P and Ca increased substantially, the concentrations of S and Mg changed little, and the concentrations of K and Na decreased.
118 TABLE 2 Double exponential decay models fitted to mean weight loss from leaf litter for each of the experimental sites Stand age (years)
Readily soluble W1( To)
Decay constant
Half-life
Labile component kl (year -1)
Resistant component k2 (year -1)
Labile component tl(½) (weeks)
Resistant component t2 (½) {weeks)
6
23.9 {0.9)
7.59 (1.13)
0.31 {0.02)
4.8 {0.7)
116.1 (5.8)
9
23.8 {0.6)
5.50 {0.96)
0.31 (0.02)
6.6 (1.1)
116.5 (7.4)
40
26.1 {0.2)
2.66 (0.44)
0.30 {0.02)
13.6 {2.3)
119.3 {9.0)
Mature
24.6 (1.1)
3.53 {0.49)
0.19 (0.01)
10.2 (1.4)
188.9 (14.4)
W1= measured readily soluble component, kl = decay constant for the labile component of litter, k2 = decay constant for the resistant component of litter, tl ( ½) = half life of the labile component, t2 ( ½) = half life of the resistant component. Standard errors in parenthesis.
TABLE3 Element, lignin and cellulose concentrations in karri leaf litter before placement of mesh bags in the field {fresh litter) and element concentrations after exposure for 82 weeks {decomposed litter) at the four experimental sites Overstorey Litter type age {years)
Concentration (To) N
P
K
S
Ca
Mg
Na
Cl
Lignin Cellulose
6
Fresh 0.47 0.015 0.55 0.111 2.15 0.23 0.24 0.38 27.6 Decomposed 0.96 0.037 0.09 0.131 2.95 0.19 0.01 0.07
9.8
9
Fresh 0.51 0.013 0.58 0.115 1.35 0.22 0.21 0.33 31.5 Decomposed 0.94 0.028 0.10 0.116 2.01 0.20 0.01 0.04
8.4
40
Fresh 0.37 0.010 0.25 0.095 1.09 0.29 0.18 0.20 26.9 Decomposed 0.74 0.027 0.09 0.100 1.62 0.29 0.01 0.06
11.1
Mature
Fresh 0.36 0.012 0.25 0.095 1.53 0.28 0.18 0.22 27.9 Decomposed 0.86 0.032 0.10 0.115 2.20 0.28 0.01 0.05
8.6
119 180
180
140
140
,oo
100 80 100
8O 10(3
P
K
75
75
.c 50
50
E
09
" .c
~
25
25
o
o
-~.c_ 100 ~
100
"6 75
75
09
_ 50
5£
25
25
10(3
100
Na
75
75
50
50
CI
25 0 May
L
J
Nov
May
I
May Nov Exposure period
i
i
i
Nov
May
Nov
Fig. 2. Percent of original weight remaining of N, P, K, S, Ca, Mg, Na and C1 in decomposing karri leaf litter at the 6-year-old ( × ), 9-year-old ( • ), 40-year-old ( • ) and mature forest ( • ) sites.
Patterns of nutrient release, expressed as a percentage of the original amount present in litter, differed between elements (Fig. 2). The majority of Na, K and C1 in litter was released rapidly during the initial phase of decomposition while changes in the amounts of S and Mg were more closely related to changes in litter dry weight. In contrast, the amounts of N and P in litter either changed little or increased as decomposition proceeded. The amount of Ca in decomposing litter decreased approximately linearly with time of exposure. Although the amounts of many of the nutrients in decomposing litter differed between sites, the order of nutrient mobility was the same for each site. The proportions of the original nutrients in leaf litter that were released during the 82-week exposure period decreased in the order N a > K > M g > d r y weight >S>Ca>N>P.
120 TABLE 4 Amounts of nutrients in decomposing karri leaf litter, expressed as a percentage of the original amount present, after exposure for 82 weeks on the forest floor at the four experimental sites Overstorey age (years)
Percent oforiginal weight remaining N
P
K
S
Ca
Mg
Na
6 9 40 Mature SEM
97 89 89 127 4
116 103 121 140 6
7 8 16 21 2
55 49 47 64 2
63 73 67 75 3
38 45 45 53 3
1 1 3 3 1
The amounts of each nutrient in decomposing litter after exposure for 82 weeks, expressed as a percentage of the original amount present, are listed in Table 4. For Na and Ca there were no differences between sites. The amounts of each of the other nutrients in decomposed litter differed between sites, and {a) Litter moisture 250r
\ : \\
: oolV'
i\
/
O' 28
(b)
Litter temperature
24 o
20
16
$
Jun
i Sept
~ Dec
MLar
Fig. 3. Litter moisture (a) and mean litter temperature (b) at the 9-year-old ( 0 ) and mature forest ( • ) sites. Data from O'Connell and Grove (1987).
121 TABLE 5 Annual accession of nutrients in karri leaf fall (O'Connell and Menage, 1982) and amounts of nutrients released from karri leaf litter during the 1st year of decomposition Nutrients ( kg h a - 1 y e a r - 1)
Overstorey age (years)
N
P
K
S
Ca
Mg
Na
6
Accession Release
9 0
0.4 - 0.1
7 6
1.6 0.6
33 7
3 2
3 3
9
Accession Release
12 2
0.5 0.0
7 6
2.2 1.1
35 6
5 2
3 3
40
Accession Release
16 - 1
0.6 - 0.2
7 5
2.3 1.0
29 4
8 3
3 3
Mature
Accession Release
11 - 2
0.4 - 0.2
6 3
1.9 0.6
36 5
6 2
3 3
Negative signs indicate uptake of nutrients into decomposing leaf litter.
in general the differences were related to the age of the forest stands. For these elements litter decomposing in regrowth stands lost nutrients more rapidly, or accumulated less nutrients from the environment, than litter decomposing at the mature forest site. DISCUSSION
In this study, decomposition rates at four sites were compared using litter from the karri overstorey. Rates of decomposition of several fractions of karri forest litter at one of the sites are reported elsewhere ( O'Connell, 1987a). Only karri leaf litter was included in litter bags in the present study; however, the bags became intimately associated with other components of the forest floor, particularly during the 2nd year of measurement. In another study ( O'Connell, 1986b ) it was found that mixing legume litter with eucalypt litter in litter bags had no effect on eucalypt litter decomposition rates, suggesting that separation of individual litter components in litter bags provides reasonable estimates of their rates of decay. Weight loss from karri leaf litter (47%-55% after 82 weeks) was similar to that found for leaf litter of other eucalypts growing in south-western Australia, ( O'Connell and Menagd, 1983; O'Connell, 1986b) and for leaf litters of a range of eucalypts growing in eastern Australia (Woods and Raison, 1983). All of these studies were conducted over periods of 1-2 years, during which rates of decay are influenced to a large extent by loss of labile components held in litter. In the longer term, decomposition rates are dependent on breakdown of the
122
resistant components of litter which may differ between the various species. Differences between the rates of decay of litter at the mature and regrowth karri forest sites result mainly from differences in the rates of breakdown of the resistant litter components (Table 2). This may be caused either by environmental factors or variation in substrate composition (Williams and Gray, 1974). In a study of decomposition of needle litter in an age series of slash pine, Gholz et al. (1985) also found decay rates were slower on older sites. This was attributed to chemical properties of the litters rather than environmental differences between the sites. No environmental data were available for the present experiment, but in a parallel study of the seasonal variation in nitrogenase activity in the forest floor ( O'Connell and Grove, 1987), litter temperature and litter moisture were measured at monthly intervals at the mature and 9-year-old regrowth sites (Fig. 3 ). Litter temperatures were similar at the two sites, but litter moisture was greater at the mature forest site than at the regrowth site on 14 of the 15 sampling occasions. During the moist period of the year (May to October) differences in litter moisture content probably had little effect on rates of decomposition because moisture levels ( > 100% dry weight) were not limiting for microbial activity (Flanagan and Bunnell, 1976). In spring (October-December) and autumn (March-May) when the litter layers were drying and re-wetting, respectively, litter moisture was greater at the mature forest site than at the regrowth forest. These conditions favour greater microbial activity in the forest floor of the mature forest during this period. Thus, the slower overall rate of leaf litter decomposition at the mature forest compared to the regrowth stands is unlikely to be caused by environmental effects, but rather is probably due to compositional differences between the litters. Several studies have reported on the relation between decay rate and litter composition (Meentemeyer, 1978; Berg and Staaf, 1980; Melillo et al., 1982; Gholz et al., 1985 ). Among the most important properties influencing decomposition are the intitial lignin and initial N content of the substrate. The ratio of these two parameters has been used to explain variation in the decay rates of a range of North American hardwoods (Melillo et al., 1982 ). Leaf litter from the younger karri stands contains nutrients, including N, at higher concentrations than leaf litter from the mature forest site. Regression of the initial lignin:initial N ratio on the labile (kl) and resistant (k2) decay constants of the various litters explained 76% and 68%, respectively, of the variation in the decay constants. These data suggest that differences in rates of decay of leaf litter in the age sequence of karri stands is partly related to differences in the composition of the litter at the various sites. Slower rates of litter decay, together with greater amounts of litterfall and a higher proportion of more-slowly decaying non-leaf fractions ( O'Connell and Menagd, 1982; O'Connell, 1987a) contribute to greater rates of accumulation of the litter layer as a whole in mature forest compared to regrowth stands.
123
Rate of litter decomposition in recently clearfelled forest is slower than at well-vegetated sites, probably because drier conditions at the soil surface limit microbial activity on the more-exposed clearfelled areas (O'Connell, 1986a). However, this period of reduced decomposition rate does not extend to stands aged 6 years or older. Once a canopy of karri and understorey plants is established, conditions become more favourable for microbial activity and rate of litter decomposition increases. Establishment of a vegetative cover occurs 1-3 years after planting or natural regeneration, and this is probably the period during which conditions less favourable for surface litter decomposition persist. Rate of decomposition of surface litter is one factor which can affect accumulation of soil organic C. In Karri forest, amounts of organic C in soil decline following clearfelling ( O'Connell, 1986a). In the present study, amounts of soil organic carbon were lower in the 6 and 9-year-old stands than in the 40year-old and mature forests (Table 1 ). Total soil N concentrations exhibit a similar pattern in relation to stage of stand development. More intensive sampling is required to define the form of the response of soil organic matter to clearfelling, its dependence on intensity of harvesting and rotation period, and its effect on soil nutrient status. The order of nutrient mobility in decomposing karri leaf litter is similar to that reported for other eucalypt litters (Attiwill, 1968; O'Connell and Menagd, 1983 ). Phosphorus, the most immobile element, is held strongly in decomposing karri litter. In the early stages of decomposition, both concentration and amount of P in each of the litters increased markedly, and no net mineralization of P occurred during the course of the experiment. Initial increases in the amounts of P in litter were related to the concentrations of extractable P in surface soils (R 2_ 0.85 and 0.91 after 5 and 11 weeks, respectively) suggesting that transfer of P in fungal hyphae from soil to litter may have been responsible for net import of P into the litter. During the 2nd year of exposure the amounts of P in each of the litters declined (Fig. 2). The form of the decomposition curves indicates that net mineralization of P from the leaf litter occurs sooner in the regrowth stands than in mature forest. Similarly, there was only limited mineralization of N from litter at the regrowth sites and N accumulation occurred in litter from the mature forest. Higher C:N ratios, and conditions conducive to greater microbial activity in litter at the mature forest, probably favoured greater microbial immobilization of N in litter at this site. Annual inputs of nutrients in leaf fall (O'Connell and Menagd, 1982) and estimates of the amounts of nutrients released from decomposing leaf litter in the 1st year following litterfall are shown in Table 5. On the youngest sites, smaller amounts of leaf litterfall are offset by the generally higher nutrient content of the litter and its greater rates of decomposition and nutrient mineralization. Therefore the amounts of nutrients released from decomposing leaf litter at the regrowth stands tend to be similar to, or greater than, the amounts released from leaf litter at the mature forest. Nutrients released from
124 decomposing karri leaf litter represent only a proportion of the nutrients released from the total litter layer. At the 40-year-old regrowth site, karri leaf litter accounts for about half of the K, S, Ca, Mg and N a released during the first year of litter decomposition (unpublished d a t a ) . Total release of these elements from all litter fractions will be greater on older stands because the contributions of non-leaf fractions (karri twigs, bark, and fruit) to litterfall increases substantially as the forests m at ure (O'Connell and Menage, 1982). However, N and P occur at low concentrations in the major non-leaf litters (O'Connell and Menage, 1982) and are imported into these fractions during decomposition (O'Connell, 1987b). Because of the relatively large contribution of non-leaf litters to litterfall on older stands, the am ount s of N and P immobilized during the initial stages of litter decay will be greater in mature forest t h a n in young regrowth stands. ACKNOWLEDGEMENTS Assistance given by officers of the Manjimup and P e m b e r t o n branches of the Forests D e p a r t m e n t of W es t e r n Australia in choosing experimental sites is gratefully acknowledged. Mr. M. Pal m er (CSIRO Division of Mat hem at i cs and Statistics) advised on statistical calculations and Mr. P. Menagd provided expert technical assistance.
REFERENCES Attiwill, P.M., 1968. Loss of elements from decomposinglitter. Ecology,49: 142-145. Berg, B. and Staaf, H., 1980.Decompositionrate and chemicalchanges of Scots pine needle litter. II. Influence of chemical composition. In: T. Persson (Editors), Structure and Function of Northern ConiferousForests-An Ecosystem Study. Ecol. Bull. Stockholm, 32: 373-390. Bunnell, F.L. and Tait, D.E., 1974. Mathematical simulation models of decompositionprocesses. In: A.J. Holding,O.W. Heal, S.F. MacLean, Jr. and P.W. Flanagan (Editors), Soil Organisms and Decompositionin Tundra. Tundra Biome Steering Committee, Stockholm, pp. 207-225. Flanagan, P.W. and Bunnell, F.L., 1976. Decompositionmodelsbased on climatic variables, substrate variables, microbial respiration and production. In: J.M. Anderson and A. Macfadyen (Editors), The Role of Terrestrial and Aquatic Organisms in DecompositionProcesses.Blackwell, Oxford, pp. 437-457. Gholz, H.L., Perry, C.S., Cropper, W.P. and Hendry, L.C., 1985. Litterfall, decomposition,and nitrogen and phosphorus dynamics in a chronosequenceof slash pine (Pinus eUiotti) plantations. For. Sci., 31: 463-478. Hingston, F.J., Turton, A.G. and Dimock, G.M., 1979.Nutrient distribution in karri (Eucalyptus diversicolor F. Muell.) ecosystems of southwest Western Australia. For. Ecol. Manage., 2: 133-158. Lousier, J.D. and Parkinson, D., 1976. Litter decomposition in a cool deciduous forest. Can. J. Bot., 54: 419-436. McArthur, W.M. and Clifton, A.L., 1975.Forestry and agriculture in relation to soils in the Pemberton area of Western Australia. CSIRO Aust. Div. Soils Land Use Set. 54, 48 pp.
125 Meentemeyer, V., 1978. Macroclimate and lignin control of litter decomposition rates. Ecology, 59: 465-472. Melillo, J.M., Abet, J.D. and Muratore, J.F., 1982. Nitrogen and lignin control of hardwood leaf litter decomposition dynamics. Ecology, 63: 621-626. Norrish, K. and Hutton, J.T., 1977. Plant analysis by x-ray spectrometry. I. Low atomic number elements sodium to calcium. X-Ray Spectrom., 6: 6-11. O'Connell, A.M., 1985. Nutrient accessions to the forest floor in karri (Eucalyptus diversicolor F. Muell.) forests of varying age. For. Ecol. Manage., 10: 283-296. O'Connell, A.M., 1986a. Litter decomposition, soil respiration and soil chemical and biochemical properties at three contrasting sites in karri (Eucalyptus diversicolor F. Muell.) forests of south-western Australia. Aust. J. Ecol., 12: 31-40. O'Connell, A.M., 1986b. Effect of legume understorey on decomposition and nutrient content of eucalypt forest litter. Plant Soil., 92: 235-248. O'Connell, A.M., 1987a. Litter dynamics in karri (Eucalyptus diversicolor F. Muell.) forests of south-western Australia. J. Ecol., 75: 781-796. O'Connell, A.M., 1987b. Nutrient dynamics in decomposing litter in karri (Eucalyptus diversicolorF. Muell.) forests of south-western Australia. J. Ecol. (in press). O'Connell, A.M. and Grove, T.S., 1987. Seasonal variation in C2H2 reduction (N2-fixation) in the litter layer of eucalypt forests of south-western Australia. Soil Biol. Biochem., 19: 135-142. O'Connell, A.M. and Menagd, P.M.A., 1982. Litter fall and nutrient cycling in karri (Eucalyptus diversicolor F. Muell.) forest in relation to stand age. Aust. J. Ecol., 7: 49-62. O'Connell, A.M. and Menagd, P., 1983. Decomposition of litter from three major plant species of jarrah (Eucalyptus marginata Donn ex. Sm.) forest in relation to site fire history and soil type. Aust. J. Ecol., 8: 277-286. Specht, R.L.., 1970. Vegetation. In: G.W. Leeper (Editor), The Australian Environment. CSIRO and Melbourne University Press, Melbourne, Vic., pp. 44-67. Van Soest, P.J., 1963. Use of detergents in the analysis of fibrous feeds. II. Rapid method for determining fibre and lignin. J. Assoc. Off. Anal. Chem., 46: 829-835. Williams, S.T. and Gray, T.R.G., 1974. Decomposition of litter on the soil surface. In: C.H. Dickinson and G.J.F. Pugh (Editors), Biology of Plant Litter Decomposition, Vol. 2. Academic Press, New York, pp. 611-632. Woods, P.V. and Raison, R.J., 1983. Decomposition of litter in sub-alpine forests of Eucalyptus delegatensis, E. Pauciflora and E. dives. Aust. J. Ecol., 8: 287-299.