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Effect of irrigation with sewage effluent on decomposition of litter in Pinus radiata forests
Forest Ecology and Management, 31 (1990) 205-214
205
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Effect of I r r i g a t i o n w i t h S e w a g e E f f l u e n t on D e c o m p o s i t i o n of Litter in P i n u s r a d i a t a Forests T.G. BAKER', W.J. DYCK, P.G. BARTON, G.R. OLIVER and G. NICHOLSON
Forest Research Institute, Ministry of Forestry, Private Bag, Rotorua (New Zealand) (Accepted 8 March 1989 )
ABSTRACT Baker, T.G., Dyck, W.J., Barton, P.G., Oliver, G.R. and Nicholson, G., 1990. Effect of irrigation with sewage effluent on decomposition of litter in Pinus radiata forests. For. Ecol. Manage., 31: 205-214. Two Pinus radiata D. Don forests (aged 18 and 25 years) were spray-irrigated for 32 months with domestic sewage effluent from a secondary oxidation pond. Effluent was applied at rates of 25 and 50 mm week-~. Irrigation decreased litter organic matter by 43%-57% but there was no change in organic matter in the soil (0-50 mm). Annual loss-constants determined from a litterbag study in one forest approached 1.0 in the irrigated plots, compared to 0.48 in the control. Irrigation increased the pH and the availability of P, Ca and Mg in the soil. Increased concentrations of N, P, Ca and Mg in irrigated litterbags were closely correlated with increases in the rate of decomposition of the organic matter. The effect of irrigation on decomposition rates was due, in part, to the maintenance throughout the year of a more suitable environment in the litter for decomposers, particularly during dry summer periods.
INTRODUCTION
The cycle of nutrients through litterfall and litter decomposition is a distinguishing feature of forest ecosystems, and broadly bears upon forest productivity. Generally, decomposition rates increase where climate and substrate quality favour microbial activity (e.g. Fogel and Cromack, 1977; Meentemeyer, 1978). Litter accumulation in Pinus radiata D. Don plantations can vary widely, even within a region, largely due to differences in decomposition rates rather than differences in litterfall (Florence and Lamb, 1974). In New Zealand, Carey et al. (1982) related the organic matter and nutrient content ofP. radiata litter to a range of climatic, edaphic, biological and silvicultural variables. While coefficients of determination for these relationships were relatively high 'Present address: Dept. of Conservation, Forests and Lands, 378 Cotham Road, Kew, Victoria 3101, Australia.
0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
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T.G. BAKER ET AL.
(R2=0.58-0.82), decomposition rates were not measured and the relationships must be used cautiously to infer any effects on litter decomposition due to management practices. In New Zealand, the possibility of land disposal of waste water, particularly into forests, is receiving increasing attention as environmental concerns are raised regarding conventional disposal and consequent pollution of waterways and lakes. However, the environmental effects of any land-disposal alternative must be determined, including the long-term capacity of forests to receive effluent without significant degradation. This paper reports the effects of irrigation with domestic sewage effluent on the litter layer and litter decomposition rates in two stands of P. radiata. The present study is also part of a series aimed at determining the rates of litter decomposition in P. radiata forests in New Zealand and exploring the relative importance of the factors influencing decomposition. METHODS
Site descriptions The study was located in Waitangi Forest (35 ° 8' S, 174 ° 6' E) in the North Island of New Zealand. During 1981, an experiment of irrigation of P. radiata with domestic sewage effluent from a secondary oxidation pond was established (Barton, 1984) on two contrasting soils: (1) Red Loam soil complex (Ohaewai and Papakauri Silt Loams) formed on Quaternary basalt; and (2) Northern Yellow-Brown Earth (Rangiora Clay series) formed on greywacke and argillite (Anonymous, 1968; Sutherland et al., 1980 ). Three 0.019-ha plots were established in P. radiata (planted 1956, present stocking 220 trees ha-1) on the silt loam soil: a control, and 2 irrigated plots receiving 25 and 50 mm week- 1 of effluent respectively. A 0.10-ha control and an irrigated plot ( 25 mm week -1) were established in P. radiata (planted 1963, present stocking 630 trees ha -1 ) on the clay soil. Mean annual rainfall during the study was 1400 mm and mean summer and winter temperatures were 19°C and 12°C respectively. Irrigation commenced in November 1981 and was continued for 32 months. The effluent was applied weekly to the forest floor using turbo-hammer sprinklers. Mean total concentrations of N, P, K, Ca and Mg in the effluent were 15, 4.6, 7.7, 13 and 3.8 ~g ml -~ respectively. For the 25 mm week -~ irrigation rate, the total amount of N, P, K, Ca, and Mg applied over 32 months was 56, 17, 28, 49 and 14 g m -2 respectively. The pH of the effluent was 8.3.
Litter and soil Litter, excluding wood with a diameter greater than 10 mm, was sampled in June 1984 using twenty 0.25-m 2 quadrats per plot. The soil (0-50 mm) was
D E C O M P O S I T I O N OF P I N U S L I T T E R - E F F E C T OF I R R I G A T I O N W I T H SEWAGE E F F L U E N T
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sampled within the litter quadrats using a volumetric corer. Five cores (50m m diameter) were taken in each quadrat. Litter was sorted into 'fines' and intact twigs, dried (70 °C), weighed and analysed for loss-on-ignition (500 oC, assumed to be equivalent to organic matter) and N, P, K, Ca and Mg (Nicholson, 1984). Data presented here represent the whole litter, and were calculated from the analyses of the two litter fractions. Soil was air-dried, sieved (2 m m ) , and analysed for loss-on-ignition, oxidizable organic C, total N, Bray and Kurtz extractable P {0.03M NH4F, 0.1M HC1), Bray and Kurtz exchangeable K, Ca and Mg, and pH (Nicholson, 1984). Exchangeable cations determined using this reagent are comparable with those from ammonium-acetate leachings (Ballard, 1978). The litter and soil samples from each quadrat were individually weighed and determined for loss-on-ignition. However, bulked samples of soil and of litter for each plot were made for chemical analysis.
Litter-bag study Freshly fallen senescent needles were collected from a 25-year-old P. radiata stand in Kaingaroa Forest (400 km SE from Waitangi) during the peak of litterfall in autumn 1983. Senescent needles from this stand have been used in several studies of litter decomposition in New Zealand P. radiata forests (T.G. Baker and G.M. Will, unpublished data, 1985) and were used in the present study for comparative purposes. In July 1983, 20 litter-bags (250 m m X 3 0 0 mm, 2-mm mesh) containing 15 g of air-dry needle litter were pegged onto the litter layer within 5 randomly located 3-m × 3-m subplots in each of the three silt loam plots. An additional 6 bags were used to determined initial moisture content and nutrient concentrations. In the control plot, throughfall was measured at the centre of each subplot using a 130-mm-diameter rain gauge. One litter-bag was scheduled to be collected from each subplot at 6-monthly intervals. However, decomposition rates in the irrigated plots were unexpectedly high and 5 bags were collected after 6 and 9 months, and 10 bags after 12 months. Foreign matter was discarded and the litter in each bag dried (70 ° C ) and weighed. For the 6-month collection, litter from each bag was analysed separately for loss-on-ignition, N, P, K, Ca and Mg. Because of the small amount of litter remaining after 9 months of decomposition in the irrigated plots, it was necessary to bulk the 5 bags prior to analysis. For the 12-month collection, two bulked samples of 5 bags were made for each subplot.
Statistical analysis Where sampling and analytical replication permitted, the statistical significance of treatment effects were assessed using analysis of variance.
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RESULTS
Litter and soil
Thirty-two months of irrigation with sewage effluent significantly reduced the amounts of organic matter in the litter layers on both silt loam and clay soils, but did not affect the amount of organic matter in the 0-50-mm soil layer (Table 1 ). Concentrations of P and Ca in litter increased markedly as a result of irrigation, but those of N, K and Mg were variably affected between soils (Table 2 ). The decrease in concentration of N due to irrigation of the silt loam resulted from the relatively greater breakdown of the litter 'fines' compared to the Npoor twigs (data not presented). TABLE 1 Organic m a t t e r c o n t e n t (kg m -2) of litter a n d soil (0-50 m m ) Irrigation rate ( m m w e e k - 1) Silt loam soil
Litter Soil Soil + L i t t e r
Clay soil
0
25
50
0
25
1.35 a (+_26) 6.79 a (+-5) 8.15 a (+-7)
0.706 b {_+31) 6.88 a (_+3) 7.58 ab (_+4)
0.583 b (+_15) 6.74 a (_+7) 7.33 b (+-6)
1.81 ~ (_+20) 5.15 b (+_5) 6.96 bc (_+7)
1.03 ab (_+36) 5.47 b (+_6) 6.50 ¢ (_+6)
Values with different superscripts are statistically different ( P < 0.05). T h e confidence interval ( P < 0.05), expressed as a percentage of the mean, is given in parentheses.
TABLE 2 C o n c e n t r a t i o n s (g k g - 1 ) of organic m a t t e r a n d n u t r i e n t s in litter Irrigation rate ( m m week -1) Silt loam soil
Organic m a t t e r N P K Ca Mg
Clay soil
0
25
50
0
25
934 12.1 0.75 0.63 5.7 1.6
941 8.3 0.84 0.77 9.9 1.7
923 9.4 1.15 0.82 12.9 1.8
869 12.7 0.65 1.6 5.1 1.6
825 13.9 1.14 1.5 13.5 2.9
DECOMPOSITIONOF P I N U S LITTER - EFFECT OF IRRIGATIONWITH SEWAGEEFFLUENT
209
TABLE 3 Bulk density and concentrations of nutrients in soil (0-50 mm, < 2 mm ) oven-dry ( 105 ° C ) basis Irrigation rate (mm week -1) Silt loam soil 0 Bulk density 0.29 ( t m -3) pH 5.2 Oxidizable organic 181 (gkg - I ) Total N (g kg -1) 12.6 C:N 14 Bray and Kurtz P 8.0 ( m g k g -1) Exchangeable cations (c mol (p + ) k g - ' ) K 0.26 Ca 8.7 Mg 4.4
Clay soil 25
50 0.31
0 0.30
25 0.78
0.79
6.1 177
6.2 161
4.6 52
5.5 56
12.3 14 83
11.9 14 156
2.7 19 7.8
2.9 19 103
0.68 22.0 9.6
0.55 28.8 10.0
0.22 2.9 1.9
0.08 7.4 4.2
Irrigation increased extractable-P at least 10-fold, exchangeable-Ca and Mg 2-3-fold, and pH by 1 unit (Table 3) in soil. Exchangeable K apparently increased in the silt loam and decreased in the clay as a result of irrigation; however, the method of analysis is subject to interference at high levels of extractable P. There was no effect of irrigation on concentrations of organic C and total N, or bulk density.
Litter-bag study Irrigation with effluent significantly increased the rates of loss of organic matter and nutrients from litter-bags, with increased weight losses (Fig. 1) over-riding increases in concentrations of N, P, Ca and Mg in the litter residue (Table 4). Rates of loss were consistently greater in the 50 mm week- 1 treatment than in the 25 mm week -1 treatment, although the absolute difference between these treatments was usually small relative to losses in the control. Annual loss constants for organic matter (Jenny et al., 1949) in the irrigated treatments were 0.94 and 0.98 compared to 0.48 in the unirrigated control. In the unirrigated control, concentrations of N and Mg in decomposing litter increased markedly with time (Table 4). There was a net accretion of N in bags for at least 9 months (Fig. 1) and accretion of Mg to 118% of initial amount for at least 1 year. Phosphorus and K were lost from the litter at a greater rate than organic matter, indicating leaching, and Ca accumulated relative to organic matter, indicating some immobilization (Fig. 1, Table 4). These
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T.G. BAKER ET AL.
100
ganic matter
80 a p-
60
Z
o 40 <[
~ 20 b
u_ 0
6
0
9
12
Nitrogen ~ 10C Z
~
8c
W
z 6¢ o 4C
a_ o n,-
a_ 2G i
L
i
6
9
12
TIME (MONTHS)
Fig. 1. P r o p o r t i o n o f initial o r g a n i c m a t t e r a n d n i t r o g e n r e m a i n i n g in l i t t e r b a g s for c o n t r o l ( • ), 25 m m w e e k -1 e f f l u e n t ( • ) a n d 50 m m w e e k -~ e f f l u e n t ( I ) t r e a t m e n t s . A t 6 m o n t h s a n d at 12 m o n t h s , p o i n t s m a r k e d w i t h d i f f e r e n t l e t t e r s are s i g n i f i c a n t l y d i f f e r e n t ( P < 0.05), F o r o r g a n i c m a t t e r , t h e vertical b a r s i n d i c a t e t h e c o n f i d e n c e l i m i t ( P < 0.05 ) o f t h e m e a n .
TABLE 4 C o n c e n t r a t i o n s ~ (g kg -~ ) o f o r g a n i c m a t t e r a n d n u t r i e n t s in t h e litter r e s i d u e in litter-bags, o v e n d r y (70 ° C ) b a s i s I r r i g a t i o n rate ( m m w e e k - 1 ) Initial 2
Organic matter N P K Ca Mg
972 5.73 1.07 4.13 4.57 0.74
After 6 months
A f t e r 12 m o n t h s
0
25
5O
0
25
5O
979 a 7.46 ~ 0.82 ~ 0.63 ~ 5.21 a 0.97 ~
952 b 10.5 h 1.13 b 0.98 b 13.35 2.01 b
9485 10.55 1.075 0.925 13.75 2.035
972 z 9.84 ~ 0.75 x 0.79 ~ 5.53 x 1.70"
925 y 13.3 y 1.17 y 0.72 x 17.0 y 2.93 y
937 z 13.2 y 1.21 e 0.83 x 16.6 y 2.79 y
1A t 6 a n d 12 m o n t h s , v a l u e s w i t h d i f f e r e n t s u p e r s c r i p t s are s i g n i f i c a n t l y d i f f e r e n t ( P < 0.05 ). 2Initial o v e n - d r y m a s s o f litter in b a g s -- 13.50 g.
DECOMPOSITION OF P I N U S LITTER - EFFECT OF IRRIGATION WITH SEWAGEEFFLUENT
211
/ /
Ill
/
•
I
z8 0 I.-
~6
z (2) (.9
~ r - - O 97 (n=9)
/ A/'i
"5~I0
/
/o /
2 i
0
i i
.
,
,
,
,
J
~ 40 60 B0 100 ORGANIC MATTER LOSS (°/o)
Fig. 2. Relationship between the concentration of nitrogen in the residue in litterbags (after 6, 9 a n d 12 m o n t h s ) a n d the proportion of organic m a t t e r lost for control ( • ), 25 m m w e e k - 1 effluent ( • ) a n d 50 m m w e e k - ] effluent ( • ) treatments. T h e dotted line indicates the trajectory of Nconcentration if t h e initial nitrogen in the litter was perfectly conserved in the litter residue.
observations are reasonable, given that the needle litter was collected from P. radiata growing on a P- and K-rich but Ca- and Mg-poor rhyolitic pumice soil. Increases of the concentrations of N, P, Ca and Mg in the litter residue due to irrigation (Table 4) were closely associated with increased rates of decomposition (as shown for N, Fig. 2). Correlations (n--9) across all treatments between the concentrations of N, P, Ca and Mg in the residue and the proportion of mass lost were 0.97, 0.85, 0.94 and 0.99 respectively. DISCUSSION
Reductions of the litter mass in P. radiata due to irrigation with sewage effluent for 32 months (43-57% loss) were less than those reported by Richenderfer and Sopper (1979) for red pine (66% loss) and mixed-hardwood (77% loss) stands irrigated for 14 years. However, the litter in irrigated treatments in the present study may not yet represent an equilibrium between litterfall and litter decomposition. In contrast to these reductions, Cromer et al. ( 1984 ) did not find a decrease in litter mass in 15-year-old P. radiata irrigated with waste-water for three years. In their study in a low rainfall area (600 mm) and on a soil with poor moisture storage, litterfall in irrigated plots (400 g m -2 year- 1 ) was significantly greater than that in control plots (320 g m -2 year- 1). However, the annual loss-constant for recently fallen litter increased from 0.33 in the unirrigated treatment to 0.47 with irrigation. In the present study, the litter-bag data show that the response to irrigation was rapid and that the annual loss-constants for litter in the irrigated treatments were comparable to those for tropical forests (approx. 1; Olson, 1963).
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T.G. BAKER ET AL.
The loss of organic matter from the unirrigated litter-bags on the silt loam after 1 year (47%) was approximately equal to losses after 2 years in other studies using litter-bags in P. radiata forests (Will, 1967; Will et al., 1983; Baker and Attiwill, 1985; Baker et al., 1986). Thus, decomposition on the silt loam soil was relatively high and contrasts with that on the clay soil where 1.4 times the mass of litter (Table 1 ) had accumulated in three-quarters of the time in a less-productive stand having fewer thinnings. Both soil type (Florence and Lamb, 1974) and foliar (hence litter) nutrient concentrations (Carey et al., 1982 ) can influence litter accumulation in P. radiata stands, presumably by affecting decomposition rates. While there are clear differences in the concentrations of nutrients in the litter on the silt-loam and clay soils (Table 2) and in the availability of some soil nutrients (Table 3 ), it is unknown if they are sufficient to explain the differences in litter accumulation between the two sites. Baker and Attiwill (1985) and Woods and Raison (1983) found evidence that moisture availability limits decomposition in both P. radiata and Eucalyptus spp. forests in south-eastern Australia. In the present study, irrigation with 25 and 50 m m week-1 of effluent increased the amount of water reaching the forest floor to 2.2 times and 3.5 times respectively that in the control (approx. 1000 mm). However, during the warm summers at Waitangi, rainfall averages only 15 mm week- 1 and the weekly moistening of the litter layer by irrigation was likely to be of greater significance in affecting decomposition rates than was the total amount of effluent applied each year. T h a t is, the greater rates of decomposition of irrigated litter were due, in part, to the maintenance throughout the year of a more suitable environment for decomposers. Generally, increased decomposition rates of litter irrigated with sewage effluent can result from increases in nutrient and moisture availability and from greater solubility at higher pH of some organic fractions of litter. In addition, sewage effluent supplies organic carbon, which may be otherwise limiting to microbial activity in some forest-floor materials (e.g. Foster et al., 1980). These changes may consequently affect the population and activity of decomposers. For example, increased numbers of Amphipoda were observed in the litterbags collected from irrigated treatments in the present study. We cannot quantify the separate or interacting effects on decomposition of the factors which have been changed by irrigation with sewage effluent. Aber and Melillo (1980) used a similar relationship to that in Fig. 2 to determine the proportion of initial litter eventually transferred to the soil. The relationship is a description of the often observed conservation (sometimes net immobilization) of N in decomposing litter (e.g. Staaf, 1980) and provides an estimate of the stage at which there is a net release of N from litter: here, after the loss of 35% of the initial mass or at a concentration of 9 g kg -1. In the present litter-bag study, it is interesting that there was no marked increase in the concentration of N (or P, Ca and Mg) in the residue of irrigated litter
DECOMPOSITIONOFPINUS LITTER- EFFECTOFIRRIGATIONWITHSEWAGEEFFLUENT
213
beyond that explained by a greater decomposition rate than in the control (Fig. 2 ). In contrast, Will et al. (1983) found no effect of N-fertilizer on decomposition rate of P. radiata litter, yet found considerable immobilization of fertilizer-N in the residue. We hypothesize that irrigation with effluent has accelerated the rate of decomposition along the trajectory described in Fig. 2, without changing any particular relationship between nutrient concentration and organic matter. ACKNOWLEDGEMENTS
The studies described in this paper were made while T.G. Baker was a New Zealand N.R.A.C. Postdoctoral Research Fellow at the Forest Research Institute, New Zealand Forest Service, Rotorua. We thank D. Bartram, K. Leslie, D. Pollock, J. Prince and A. Sims for laboratory and field work. We are grateful to Doctors P.M. Attiwill, M.L. Carey, R.N. Cromer, R.L. Edwards, and K.M. Goh for their comments on the manuscript.
REFERENCES
Aber, J.D. and Melillo, J.M., 1980. Litter decomposition: measuring relative contributions of organic matter and nitrogen to forest soils. Can. J. Bot., 58: 416-421. Anonymous, 1968. Soils of New Zealand, Part 3. N.Z. Soil Bur. Bull., 26 (3): 127 pp. Baker, T.G. and Attiwill, P.M., 1985. Loss of organic matter and elements from decomposing litter of Eucalyptus obliqua L'Herit. and P i n u s radiata D. Don. Aust. For. Res., 15: 309-319. Baker, T.G., Oliver, G.R. and Hodgkiss, P.D., 1986. Distribution and cycling of nutrients in P i n u s radiata as affected by past lupin growth and fertiliser. For. Ecol. Manage., 17: 169-187. Ballard, R., 1978. Use of the Bray soil test in forestry. 2. Determination of cation status. N.Z.J. For. Sci., 8: 332-343. Barton, P.G., 1984. Preliminary results from spray irrigating domestic sewage effluent under Pinus radiata at Waitangi Forest. In: R.J. Wilcock (Editor), Proc. Seminar Land Treatment of Wastes. National Water and Soil Conservation Authority, Wellington, New Zealand, Misc. Publ. 69, pp. 63-87. Carey, M.L., Hunter, I.R. and Andrew, I., 1982. P i n u s radiata forest floors: factors affecting organic matter and nutrient dynamics. N.Z.J. For. Sci., 12: 36-48. Cromer, R.N., Tompkins, D., Barr, N.J., Williams, E.R. and Stewart, H.T.L., 1984. Litter-fall in a P i n u s radiata forest: the effect of irrigation and fertilizer treatments. J. Appl. Ecol., 21: 313326. Florence, R.G. and Lamb, D., 1974. Influence of stand and site on radiata pine litter in South Australia. N.Z.J. For. Sci., 4: 502-510. Fogel, R. and Cromack, K., 1977. The effect of habitat and substrate quality on Douglas-fir litter decomposition in western Oregon. Can. J. Bot., 55: 1632-1640. Foster, N.W., Beauchamp, E.G. and Clarke, C.T., 1980. Microbial activity in a P i n u s banksiana Lamb. forest floor amended with nitrogen and carbon. Can. J. Soil. Sci., 60: 199-209. Jenny, H., Gessel, S.P. and Bingham, F.T., 1949. Comparative study of decomposition rates of organic matter in temperate and tropical regions. Soil Sci., 68: 419-432.
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Meentemeyer, V., 1978. Macroclimate and lignin control of litter decomposition rates. Ecology, 59: 465-472. Nicholson, G., 1984. Methods of Soil, Plant and Water Analysis. N.Z. For. Serv. For. Res. Inst., Bull. 70. Olson, J.S., 1963. Energy storage and the balance of producers and decomposers in ecological systems. Ecology, 44: 322-331. Richenderfer, J.L. and Sopper, W.E., 1979. Effect of spray irrigation of municipal sewage effluent on the accumulation and decomposition of the forest floor. In: W.E. Sopper and S.N. Kerr (Editors), Utilization of Municipal Sewage Effluent and Sludge on Forest and Disturbed Land. Pennsylvania State University Press, University Park, PA, pp. 163-177. Staaf, H., 1980. Influence of chemical composition, addition of raspberry leaves, and nitrogen supply on decomposition rate and dynamics of nitrogen and phosphorus in beech leaf litter. Oikos, 35: 55-62. Sutherland, C.F., Cox, J.E., Taylor, N.H. and Wright, A.C.S., 1980. Soil map of WhangaroaKaikohe area (Sheets P04/05), North Island, New Zealand. New Zealand Department of Scientific and Industrial Research, Wellington, N.Z. Soil Bureau Map 183. Will, G.M., 1967. Decomposition ofPinus radiata litter on the forest floor. Part 1, Changes in dry matter and nutrient content. N.Z.J. Sci., 10: 1030-1044. Will, G.M., Hodgkiss, P.D. and Madgwick, H.A.I., 1983. Nutrient losses from litterbags containing Pinus radiata litter: influences of thinning, clearfelling, and urea fertiliser. N.Z.J. For. Sci., 13: 291-304. 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
Effect of I r r i g a t i o n w i t h S e w a g e E f f l u e n t on D e c o m p o s i t i o n of Litter in P i n u s r a d i a t a Forests T.G. BAKER', W.J. DYCK, P.G. BARTON, G.R. OLIVER and G. NICHOLSON
Forest Research Institute, Ministry of Forestry, Private Bag, Rotorua (New Zealand) (Accepted 8 March 1989 )
ABSTRACT Baker, T.G., Dyck, W.J., Barton, P.G., Oliver, G.R. and Nicholson, G., 1990. Effect of irrigation with sewage effluent on decomposition of litter in Pinus radiata forests. For. Ecol. Manage., 31: 205-214. Two Pinus radiata D. Don forests (aged 18 and 25 years) were spray-irrigated for 32 months with domestic sewage effluent from a secondary oxidation pond. Effluent was applied at rates of 25 and 50 mm week-~. Irrigation decreased litter organic matter by 43%-57% but there was no change in organic matter in the soil (0-50 mm). Annual loss-constants determined from a litterbag study in one forest approached 1.0 in the irrigated plots, compared to 0.48 in the control. Irrigation increased the pH and the availability of P, Ca and Mg in the soil. Increased concentrations of N, P, Ca and Mg in irrigated litterbags were closely correlated with increases in the rate of decomposition of the organic matter. The effect of irrigation on decomposition rates was due, in part, to the maintenance throughout the year of a more suitable environment in the litter for decomposers, particularly during dry summer periods.
INTRODUCTION
The cycle of nutrients through litterfall and litter decomposition is a distinguishing feature of forest ecosystems, and broadly bears upon forest productivity. Generally, decomposition rates increase where climate and substrate quality favour microbial activity (e.g. Fogel and Cromack, 1977; Meentemeyer, 1978). Litter accumulation in Pinus radiata D. Don plantations can vary widely, even within a region, largely due to differences in decomposition rates rather than differences in litterfall (Florence and Lamb, 1974). In New Zealand, Carey et al. (1982) related the organic matter and nutrient content ofP. radiata litter to a range of climatic, edaphic, biological and silvicultural variables. While coefficients of determination for these relationships were relatively high 'Present address: Dept. of Conservation, Forests and Lands, 378 Cotham Road, Kew, Victoria 3101, Australia.
0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
206
T.G. BAKER ET AL.
(R2=0.58-0.82), decomposition rates were not measured and the relationships must be used cautiously to infer any effects on litter decomposition due to management practices. In New Zealand, the possibility of land disposal of waste water, particularly into forests, is receiving increasing attention as environmental concerns are raised regarding conventional disposal and consequent pollution of waterways and lakes. However, the environmental effects of any land-disposal alternative must be determined, including the long-term capacity of forests to receive effluent without significant degradation. This paper reports the effects of irrigation with domestic sewage effluent on the litter layer and litter decomposition rates in two stands of P. radiata. The present study is also part of a series aimed at determining the rates of litter decomposition in P. radiata forests in New Zealand and exploring the relative importance of the factors influencing decomposition. METHODS
Site descriptions The study was located in Waitangi Forest (35 ° 8' S, 174 ° 6' E) in the North Island of New Zealand. During 1981, an experiment of irrigation of P. radiata with domestic sewage effluent from a secondary oxidation pond was established (Barton, 1984) on two contrasting soils: (1) Red Loam soil complex (Ohaewai and Papakauri Silt Loams) formed on Quaternary basalt; and (2) Northern Yellow-Brown Earth (Rangiora Clay series) formed on greywacke and argillite (Anonymous, 1968; Sutherland et al., 1980 ). Three 0.019-ha plots were established in P. radiata (planted 1956, present stocking 220 trees ha-1) on the silt loam soil: a control, and 2 irrigated plots receiving 25 and 50 mm week- 1 of effluent respectively. A 0.10-ha control and an irrigated plot ( 25 mm week -1) were established in P. radiata (planted 1963, present stocking 630 trees ha -1 ) on the clay soil. Mean annual rainfall during the study was 1400 mm and mean summer and winter temperatures were 19°C and 12°C respectively. Irrigation commenced in November 1981 and was continued for 32 months. The effluent was applied weekly to the forest floor using turbo-hammer sprinklers. Mean total concentrations of N, P, K, Ca and Mg in the effluent were 15, 4.6, 7.7, 13 and 3.8 ~g ml -~ respectively. For the 25 mm week -~ irrigation rate, the total amount of N, P, K, Ca, and Mg applied over 32 months was 56, 17, 28, 49 and 14 g m -2 respectively. The pH of the effluent was 8.3.
Litter and soil Litter, excluding wood with a diameter greater than 10 mm, was sampled in June 1984 using twenty 0.25-m 2 quadrats per plot. The soil (0-50 mm) was
D E C O M P O S I T I O N OF P I N U S L I T T E R - E F F E C T OF I R R I G A T I O N W I T H SEWAGE E F F L U E N T
207
sampled within the litter quadrats using a volumetric corer. Five cores (50m m diameter) were taken in each quadrat. Litter was sorted into 'fines' and intact twigs, dried (70 °C), weighed and analysed for loss-on-ignition (500 oC, assumed to be equivalent to organic matter) and N, P, K, Ca and Mg (Nicholson, 1984). Data presented here represent the whole litter, and were calculated from the analyses of the two litter fractions. Soil was air-dried, sieved (2 m m ) , and analysed for loss-on-ignition, oxidizable organic C, total N, Bray and Kurtz extractable P {0.03M NH4F, 0.1M HC1), Bray and Kurtz exchangeable K, Ca and Mg, and pH (Nicholson, 1984). Exchangeable cations determined using this reagent are comparable with those from ammonium-acetate leachings (Ballard, 1978). The litter and soil samples from each quadrat were individually weighed and determined for loss-on-ignition. However, bulked samples of soil and of litter for each plot were made for chemical analysis.
Litter-bag study Freshly fallen senescent needles were collected from a 25-year-old P. radiata stand in Kaingaroa Forest (400 km SE from Waitangi) during the peak of litterfall in autumn 1983. Senescent needles from this stand have been used in several studies of litter decomposition in New Zealand P. radiata forests (T.G. Baker and G.M. Will, unpublished data, 1985) and were used in the present study for comparative purposes. In July 1983, 20 litter-bags (250 m m X 3 0 0 mm, 2-mm mesh) containing 15 g of air-dry needle litter were pegged onto the litter layer within 5 randomly located 3-m × 3-m subplots in each of the three silt loam plots. An additional 6 bags were used to determined initial moisture content and nutrient concentrations. In the control plot, throughfall was measured at the centre of each subplot using a 130-mm-diameter rain gauge. One litter-bag was scheduled to be collected from each subplot at 6-monthly intervals. However, decomposition rates in the irrigated plots were unexpectedly high and 5 bags were collected after 6 and 9 months, and 10 bags after 12 months. Foreign matter was discarded and the litter in each bag dried (70 ° C ) and weighed. For the 6-month collection, litter from each bag was analysed separately for loss-on-ignition, N, P, K, Ca and Mg. Because of the small amount of litter remaining after 9 months of decomposition in the irrigated plots, it was necessary to bulk the 5 bags prior to analysis. For the 12-month collection, two bulked samples of 5 bags were made for each subplot.
Statistical analysis Where sampling and analytical replication permitted, the statistical significance of treatment effects were assessed using analysis of variance.
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RESULTS
Litter and soil
Thirty-two months of irrigation with sewage effluent significantly reduced the amounts of organic matter in the litter layers on both silt loam and clay soils, but did not affect the amount of organic matter in the 0-50-mm soil layer (Table 1 ). Concentrations of P and Ca in litter increased markedly as a result of irrigation, but those of N, K and Mg were variably affected between soils (Table 2 ). The decrease in concentration of N due to irrigation of the silt loam resulted from the relatively greater breakdown of the litter 'fines' compared to the Npoor twigs (data not presented). TABLE 1 Organic m a t t e r c o n t e n t (kg m -2) of litter a n d soil (0-50 m m ) Irrigation rate ( m m w e e k - 1) Silt loam soil
Litter Soil Soil + L i t t e r
Clay soil
0
25
50
0
25
1.35 a (+_26) 6.79 a (+-5) 8.15 a (+-7)
0.706 b {_+31) 6.88 a (_+3) 7.58 ab (_+4)
0.583 b (+_15) 6.74 a (_+7) 7.33 b (+-6)
1.81 ~ (_+20) 5.15 b (+_5) 6.96 bc (_+7)
1.03 ab (_+36) 5.47 b (+_6) 6.50 ¢ (_+6)
Values with different superscripts are statistically different ( P < 0.05). T h e confidence interval ( P < 0.05), expressed as a percentage of the mean, is given in parentheses.
TABLE 2 C o n c e n t r a t i o n s (g k g - 1 ) of organic m a t t e r a n d n u t r i e n t s in litter Irrigation rate ( m m week -1) Silt loam soil
Organic m a t t e r N P K Ca Mg
Clay soil
0
25
50
0
25
934 12.1 0.75 0.63 5.7 1.6
941 8.3 0.84 0.77 9.9 1.7
923 9.4 1.15 0.82 12.9 1.8
869 12.7 0.65 1.6 5.1 1.6
825 13.9 1.14 1.5 13.5 2.9
DECOMPOSITIONOF P I N U S LITTER - EFFECT OF IRRIGATIONWITH SEWAGEEFFLUENT
209
TABLE 3 Bulk density and concentrations of nutrients in soil (0-50 mm, < 2 mm ) oven-dry ( 105 ° C ) basis Irrigation rate (mm week -1) Silt loam soil 0 Bulk density 0.29 ( t m -3) pH 5.2 Oxidizable organic 181 (gkg - I ) Total N (g kg -1) 12.6 C:N 14 Bray and Kurtz P 8.0 ( m g k g -1) Exchangeable cations (c mol (p + ) k g - ' ) K 0.26 Ca 8.7 Mg 4.4
Clay soil 25
50 0.31
0 0.30
25 0.78
0.79
6.1 177
6.2 161
4.6 52
5.5 56
12.3 14 83
11.9 14 156
2.7 19 7.8
2.9 19 103
0.68 22.0 9.6
0.55 28.8 10.0
0.22 2.9 1.9
0.08 7.4 4.2
Irrigation increased extractable-P at least 10-fold, exchangeable-Ca and Mg 2-3-fold, and pH by 1 unit (Table 3) in soil. Exchangeable K apparently increased in the silt loam and decreased in the clay as a result of irrigation; however, the method of analysis is subject to interference at high levels of extractable P. There was no effect of irrigation on concentrations of organic C and total N, or bulk density.
Litter-bag study Irrigation with effluent significantly increased the rates of loss of organic matter and nutrients from litter-bags, with increased weight losses (Fig. 1) over-riding increases in concentrations of N, P, Ca and Mg in the litter residue (Table 4). Rates of loss were consistently greater in the 50 mm week- 1 treatment than in the 25 mm week -1 treatment, although the absolute difference between these treatments was usually small relative to losses in the control. Annual loss constants for organic matter (Jenny et al., 1949) in the irrigated treatments were 0.94 and 0.98 compared to 0.48 in the unirrigated control. In the unirrigated control, concentrations of N and Mg in decomposing litter increased markedly with time (Table 4). There was a net accretion of N in bags for at least 9 months (Fig. 1) and accretion of Mg to 118% of initial amount for at least 1 year. Phosphorus and K were lost from the litter at a greater rate than organic matter, indicating leaching, and Ca accumulated relative to organic matter, indicating some immobilization (Fig. 1, Table 4). These
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T.G. BAKER ET AL.
100
ganic matter
80 a p-
60
Z
o 40 <[
~ 20 b
u_ 0
6
0
9
12
Nitrogen ~ 10C Z
~
8c
W
z 6¢ o 4C
a_ o n,-
a_ 2G i
L
i
6
9
12
TIME (MONTHS)
Fig. 1. P r o p o r t i o n o f initial o r g a n i c m a t t e r a n d n i t r o g e n r e m a i n i n g in l i t t e r b a g s for c o n t r o l ( • ), 25 m m w e e k -1 e f f l u e n t ( • ) a n d 50 m m w e e k -~ e f f l u e n t ( I ) t r e a t m e n t s . A t 6 m o n t h s a n d at 12 m o n t h s , p o i n t s m a r k e d w i t h d i f f e r e n t l e t t e r s are s i g n i f i c a n t l y d i f f e r e n t ( P < 0.05), F o r o r g a n i c m a t t e r , t h e vertical b a r s i n d i c a t e t h e c o n f i d e n c e l i m i t ( P < 0.05 ) o f t h e m e a n .
TABLE 4 C o n c e n t r a t i o n s ~ (g kg -~ ) o f o r g a n i c m a t t e r a n d n u t r i e n t s in t h e litter r e s i d u e in litter-bags, o v e n d r y (70 ° C ) b a s i s I r r i g a t i o n rate ( m m w e e k - 1 ) Initial 2
Organic matter N P K Ca Mg
972 5.73 1.07 4.13 4.57 0.74
After 6 months
A f t e r 12 m o n t h s
0
25
5O
0
25
5O
979 a 7.46 ~ 0.82 ~ 0.63 ~ 5.21 a 0.97 ~
952 b 10.5 h 1.13 b 0.98 b 13.35 2.01 b
9485 10.55 1.075 0.925 13.75 2.035
972 z 9.84 ~ 0.75 x 0.79 ~ 5.53 x 1.70"
925 y 13.3 y 1.17 y 0.72 x 17.0 y 2.93 y
937 z 13.2 y 1.21 e 0.83 x 16.6 y 2.79 y
1A t 6 a n d 12 m o n t h s , v a l u e s w i t h d i f f e r e n t s u p e r s c r i p t s are s i g n i f i c a n t l y d i f f e r e n t ( P < 0.05 ). 2Initial o v e n - d r y m a s s o f litter in b a g s -- 13.50 g.
DECOMPOSITION OF P I N U S LITTER - EFFECT OF IRRIGATION WITH SEWAGEEFFLUENT
211
/ /
Ill
/
•
I
z8 0 I.-
~6
z (2) (.9
~ r - - O 97 (n=9)
/ A/'i
"5~I0
/
/o /
2 i
0
i i
.
,
,
,
,
J
~ 40 60 B0 100 ORGANIC MATTER LOSS (°/o)
Fig. 2. Relationship between the concentration of nitrogen in the residue in litterbags (after 6, 9 a n d 12 m o n t h s ) a n d the proportion of organic m a t t e r lost for control ( • ), 25 m m w e e k - 1 effluent ( • ) a n d 50 m m w e e k - ] effluent ( • ) treatments. T h e dotted line indicates the trajectory of Nconcentration if t h e initial nitrogen in the litter was perfectly conserved in the litter residue.
observations are reasonable, given that the needle litter was collected from P. radiata growing on a P- and K-rich but Ca- and Mg-poor rhyolitic pumice soil. Increases of the concentrations of N, P, Ca and Mg in the litter residue due to irrigation (Table 4) were closely associated with increased rates of decomposition (as shown for N, Fig. 2). Correlations (n--9) across all treatments between the concentrations of N, P, Ca and Mg in the residue and the proportion of mass lost were 0.97, 0.85, 0.94 and 0.99 respectively. DISCUSSION
Reductions of the litter mass in P. radiata due to irrigation with sewage effluent for 32 months (43-57% loss) were less than those reported by Richenderfer and Sopper (1979) for red pine (66% loss) and mixed-hardwood (77% loss) stands irrigated for 14 years. However, the litter in irrigated treatments in the present study may not yet represent an equilibrium between litterfall and litter decomposition. In contrast to these reductions, Cromer et al. ( 1984 ) did not find a decrease in litter mass in 15-year-old P. radiata irrigated with waste-water for three years. In their study in a low rainfall area (600 mm) and on a soil with poor moisture storage, litterfall in irrigated plots (400 g m -2 year- 1 ) was significantly greater than that in control plots (320 g m -2 year- 1). However, the annual loss-constant for recently fallen litter increased from 0.33 in the unirrigated treatment to 0.47 with irrigation. In the present study, the litter-bag data show that the response to irrigation was rapid and that the annual loss-constants for litter in the irrigated treatments were comparable to those for tropical forests (approx. 1; Olson, 1963).
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The loss of organic matter from the unirrigated litter-bags on the silt loam after 1 year (47%) was approximately equal to losses after 2 years in other studies using litter-bags in P. radiata forests (Will, 1967; Will et al., 1983; Baker and Attiwill, 1985; Baker et al., 1986). Thus, decomposition on the silt loam soil was relatively high and contrasts with that on the clay soil where 1.4 times the mass of litter (Table 1 ) had accumulated in three-quarters of the time in a less-productive stand having fewer thinnings. Both soil type (Florence and Lamb, 1974) and foliar (hence litter) nutrient concentrations (Carey et al., 1982 ) can influence litter accumulation in P. radiata stands, presumably by affecting decomposition rates. While there are clear differences in the concentrations of nutrients in the litter on the silt-loam and clay soils (Table 2) and in the availability of some soil nutrients (Table 3 ), it is unknown if they are sufficient to explain the differences in litter accumulation between the two sites. Baker and Attiwill (1985) and Woods and Raison (1983) found evidence that moisture availability limits decomposition in both P. radiata and Eucalyptus spp. forests in south-eastern Australia. In the present study, irrigation with 25 and 50 m m week-1 of effluent increased the amount of water reaching the forest floor to 2.2 times and 3.5 times respectively that in the control (approx. 1000 mm). However, during the warm summers at Waitangi, rainfall averages only 15 mm week- 1 and the weekly moistening of the litter layer by irrigation was likely to be of greater significance in affecting decomposition rates than was the total amount of effluent applied each year. T h a t is, the greater rates of decomposition of irrigated litter were due, in part, to the maintenance throughout the year of a more suitable environment for decomposers. Generally, increased decomposition rates of litter irrigated with sewage effluent can result from increases in nutrient and moisture availability and from greater solubility at higher pH of some organic fractions of litter. In addition, sewage effluent supplies organic carbon, which may be otherwise limiting to microbial activity in some forest-floor materials (e.g. Foster et al., 1980). These changes may consequently affect the population and activity of decomposers. For example, increased numbers of Amphipoda were observed in the litterbags collected from irrigated treatments in the present study. We cannot quantify the separate or interacting effects on decomposition of the factors which have been changed by irrigation with sewage effluent. Aber and Melillo (1980) used a similar relationship to that in Fig. 2 to determine the proportion of initial litter eventually transferred to the soil. The relationship is a description of the often observed conservation (sometimes net immobilization) of N in decomposing litter (e.g. Staaf, 1980) and provides an estimate of the stage at which there is a net release of N from litter: here, after the loss of 35% of the initial mass or at a concentration of 9 g kg -1. In the present litter-bag study, it is interesting that there was no marked increase in the concentration of N (or P, Ca and Mg) in the residue of irrigated litter
DECOMPOSITIONOFPINUS LITTER- EFFECTOFIRRIGATIONWITHSEWAGEEFFLUENT
213
beyond that explained by a greater decomposition rate than in the control (Fig. 2 ). In contrast, Will et al. (1983) found no effect of N-fertilizer on decomposition rate of P. radiata litter, yet found considerable immobilization of fertilizer-N in the residue. We hypothesize that irrigation with effluent has accelerated the rate of decomposition along the trajectory described in Fig. 2, without changing any particular relationship between nutrient concentration and organic matter. ACKNOWLEDGEMENTS
The studies described in this paper were made while T.G. Baker was a New Zealand N.R.A.C. Postdoctoral Research Fellow at the Forest Research Institute, New Zealand Forest Service, Rotorua. We thank D. Bartram, K. Leslie, D. Pollock, J. Prince and A. Sims for laboratory and field work. We are grateful to Doctors P.M. Attiwill, M.L. Carey, R.N. Cromer, R.L. Edwards, and K.M. Goh for their comments on the manuscript.
REFERENCES
Aber, J.D. and Melillo, J.M., 1980. Litter decomposition: measuring relative contributions of organic matter and nitrogen to forest soils. Can. J. Bot., 58: 416-421. Anonymous, 1968. Soils of New Zealand, Part 3. N.Z. Soil Bur. Bull., 26 (3): 127 pp. Baker, T.G. and Attiwill, P.M., 1985. Loss of organic matter and elements from decomposing litter of Eucalyptus obliqua L'Herit. and P i n u s radiata D. Don. Aust. For. Res., 15: 309-319. Baker, T.G., Oliver, G.R. and Hodgkiss, P.D., 1986. Distribution and cycling of nutrients in P i n u s radiata as affected by past lupin growth and fertiliser. For. Ecol. Manage., 17: 169-187. Ballard, R., 1978. Use of the Bray soil test in forestry. 2. Determination of cation status. N.Z.J. For. Sci., 8: 332-343. Barton, P.G., 1984. Preliminary results from spray irrigating domestic sewage effluent under Pinus radiata at Waitangi Forest. In: R.J. Wilcock (Editor), Proc. Seminar Land Treatment of Wastes. National Water and Soil Conservation Authority, Wellington, New Zealand, Misc. Publ. 69, pp. 63-87. Carey, M.L., Hunter, I.R. and Andrew, I., 1982. P i n u s radiata forest floors: factors affecting organic matter and nutrient dynamics. N.Z.J. For. Sci., 12: 36-48. Cromer, R.N., Tompkins, D., Barr, N.J., Williams, E.R. and Stewart, H.T.L., 1984. Litter-fall in a P i n u s radiata forest: the effect of irrigation and fertilizer treatments. J. Appl. Ecol., 21: 313326. Florence, R.G. and Lamb, D., 1974. Influence of stand and site on radiata pine litter in South Australia. N.Z.J. For. Sci., 4: 502-510. Fogel, R. and Cromack, K., 1977. The effect of habitat and substrate quality on Douglas-fir litter decomposition in western Oregon. Can. J. Bot., 55: 1632-1640. Foster, N.W., Beauchamp, E.G. and Clarke, C.T., 1980. Microbial activity in a P i n u s banksiana Lamb. forest floor amended with nitrogen and carbon. Can. J. Soil. Sci., 60: 199-209. Jenny, H., Gessel, S.P. and Bingham, F.T., 1949. Comparative study of decomposition rates of organic matter in temperate and tropical regions. Soil Sci., 68: 419-432.
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Meentemeyer, V., 1978. Macroclimate and lignin control of litter decomposition rates. Ecology, 59: 465-472. Nicholson, G., 1984. Methods of Soil, Plant and Water Analysis. N.Z. For. Serv. For. Res. Inst., Bull. 70. Olson, J.S., 1963. Energy storage and the balance of producers and decomposers in ecological systems. Ecology, 44: 322-331. Richenderfer, J.L. and Sopper, W.E., 1979. Effect of spray irrigation of municipal sewage effluent on the accumulation and decomposition of the forest floor. In: W.E. Sopper and S.N. Kerr (Editors), Utilization of Municipal Sewage Effluent and Sludge on Forest and Disturbed Land. Pennsylvania State University Press, University Park, PA, pp. 163-177. Staaf, H., 1980. Influence of chemical composition, addition of raspberry leaves, and nitrogen supply on decomposition rate and dynamics of nitrogen and phosphorus in beech leaf litter. Oikos, 35: 55-62. Sutherland, C.F., Cox, J.E., Taylor, N.H. and Wright, A.C.S., 1980. Soil map of WhangaroaKaikohe area (Sheets P04/05), North Island, New Zealand. New Zealand Department of Scientific and Industrial Research, Wellington, N.Z. Soil Bureau Map 183. Will, G.M., 1967. Decomposition ofPinus radiata litter on the forest floor. Part 1, Changes in dry matter and nutrient content. N.Z.J. Sci., 10: 1030-1044. Will, G.M., Hodgkiss, P.D. and Madgwick, H.A.I., 1983. Nutrient losses from litterbags containing Pinus radiata litter: influences of thinning, clearfelling, and urea fertiliser. N.Z.J. For. Sci., 13: 291-304. 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.