Effects of an exotic conifer (Pinus radiata) plantation on forest nutrient cycling in southeastern Australia

Forest Ecology and Management, 7 (1983/1984) 77--102 Elsevier Science Publishers B.V., Amsterdam --Printed in The Netherlands

77

EFFECTS OF AN EXOTIC CONIFER (PINUS RADIA TA) PLANTATION ON F O R E S T N U T R I E N T CYCLING IN S O U T H E A S T E R N A U S T R A L I A

M.C. FELLER

Faculty of Agriculture and Forestry, University of Melbourne, Vic. (Australia) Present address: Faculty of Forestry, University of British Columbia, Vancouver, B.C. V6T lW5 (Canada) (Accepted 17 March 1983)

ABSTRACT Feller, M.C., 1983. Effects of an exotic conifer (Pinus radiata) plantation on forest nutrient cycling in southeastern Australia. For. Ecol. Manage., 7: 77--102. The distribution and movement of N, P, K, Na, Mg, and Ca were studied in southeastern Australia in a 37--year~ld Pinus radiata plantation and in a nearby Eucalyptus obliqua -- Eucalyptus dives forest of the same age and of the same type as that which had been replaced by the P. radiata plantation. The soil beneath the P. radiata plantation contained significantly less total N and exchangeable K, Mg, and Ca than that beneath the eucalypt forest. No large accumulation of nutrients was found in either the litter or the trees in the P. radiata plantation relative to that in the eucalypt forest. However, there was a slightly greater accumulation of N and K in the P. radiata biomass than in the eucalypt biomass. The annual soil nutrient balance obtained by subtracting outputs (mineral soil leachate + biomass incorporation) from inputs (precipitation + mineral weathering) indicated a more favourable balance for each nutrient in the soil beneath the eucalypts than in the soil beneath the pines. Calculations suggested that these balances could only partially account for the differences in soil nutrient quantifies between eucalypt and pine ecosystems. It was hypothesized that these differences are also partially explainable in terms of the nutrient losses accompanying two fires which had occurred in the pine plantation area. Nitrogen balances in this study were incomplete because several potentially important fluxes were not measured.

INTRODUCTION P l a n t a t i o n s o f e x o t i c conifers, primarily P i n u s radiata D. D o n , have b e e n and are being established t h r o u g h o u t Australia t o s u p p l y s o f t w o o d p r o d u c t s . R e p o r t s o f d e c r e a s e d p r o d u c t i v i t y in s e c o n d r o t a t i o n P. radiata stands g r o w ing on p o o r s a n d y soils in S o u t h Australia a p p e a r e d in t h e 1 9 6 0 ' s ( L e w i s and Harding, 1 9 6 3 ; Keeves, 1 9 6 6 ) . E v i d e n c e o f similar declines was also r e p o r t e d f r o m N e w Zealand ( S t o n e a n d Will, 1 9 6 5 ; W h y t e , 1 9 7 3 ) . S e c o n d r o t a t i o n declines in p r o d u c t i v i t y m a y be c a u s e d b y one o r m o r e o f several c o m p l e x , interacting processes. F l o r e n c e ( 1 9 6 7 ) reviewed t h o s e pro-

0378-1127/83/$03.00

© 1983 Elsevier Science Publishers B.V.

78 cesses associated with the effect of tree species on soil biology and chemistry. Following studies of litter decomposition in P. radiata plantations in South Australia, Florence and Lamb (1975) proposed that the second rotation decline in productivity in those plantations was due to a lack of ecological diversity within the system, causing a lack in numbers or diversity of decomposer organisms. This results in a rate of mineralization which is insufficient to maintain high rates of photosynthesis and biomass production. Until 1975, little attention in Australia was paid to the possible significance of nutrient losses associated with P. radiata management. In England, Rennie (1955) had already drawn attention to the significance of nutrient losses through tree removal. North American workers (e.g. Cole and Gessel, 1965, Likens et al., 1970) first reported that forest clearing could increase the loss of nutrients in solution from the soil, while Allen (1964) in England, and Isaac and Hopkins (1937) and Klemmedson et al. (1962) in the United States had drawn attention to the significance of nutrient losses to the atmosphere caused by burning slash or forest floor materials. Slashburning has been an intimate part ofP. radiata management in Australia. In view of the uncertain applicability of Florence and Lamb's (1975) hypothesis and the uncertain significance, in relation to second rotation productivity declines, of components of the nutrient cycle other than litter decomposition, a study was initiated in 1975 to try to resolve some of these uncertainties. Previous papers have described various aspects of the study (Feller, 1978; 1980; 1981a, b). The present paper discusses the effects of P. radiata establishment and growth on nutrient cycles and soil nutrient status. STUDY AREA The study area was located within the Melbourne and Metropolitan Board of Works Maroondah catchment which lies on the southerly slopes of the Great Dividing Range approximately 60 km northeast of Melbourne (Latitude 37 °38'S, Longitude 145°35'E). The area had a westerly aspect, slopes of 15 ° (the eucalypt plot) and 5 ° (the pine plot), and an elevation of 170-180 m. The area has a Cfb climate (Kfppen, 1936) which is a mesothermal or warm temperate rainy climate. The average annual precipitation is 1200 mm, although during the 2 years of the study (April 1976 - - M a r c h 1978) the annual precipitation was 1020 rnm. The mean annual temperature is 15.5°C. Both plots were underlain by quartz-dacite bedrock which has weathered to produce deep (10--15 m) soils which were both classified as Gn 4.54 using Northcote's method (Northcote, 1974). The soils had clay contents of 40--60%, which increased with depth, and were acidic throughout, with the soft beneath Pinus radiata being more acidic than the soil beneath the eucalypts. The physical and chemical properties of the soils are given in greater detail in Table I.

A B1 B2

A B1 B2

Eucalypt

Pine

A B1 B2

A B1 B2

Eu calypt

Pine

38 (3) 32 (5) 32 (3)

% Sand

0--30 31 (3) 30.--90 25 (4) 90+ 24 (7)

0--25 25--80 80+

Depth (cm)

1:2.5,

CaCI 2

4.3(0.1)

4.4(0.1)

4.2(0.1)

4.0(0.1)

4.1(0.1)

4.1(0.1)

I:I,

H 20

5.7(0.1)

6.0(0.3)

5.8(0.3)

5.2(0.2)

5.7(0.2)

5.5(0.1)

(b) Chemical properties Plot Horizon pH

Horizon

Plot

(a) Physical properties

0.04(0.00)

0.05(0.01)

0.08(0.01)

0.04(0.01)

0.06(0.01)

0.25(0.06)

1 (1) 1 (1) 1 (1)

1 (1) 1 (1) 1 (1)

0.010(0.002)

0.009(0.001)

0.011(0.001)

0.010(0.001)

0.011(0.001)

0.015(0.003)

0.004(0.001)

0.007(0.001)

0.007(0.001)

0.004(0.001)

0.006(0.000)

0.008(0.001)

Available (%)

P

% Coarse fragments ( ~ 2 m m )

Total (%)

P

38 (2) 45 (3) 58 (6)

38 (1) 47 (9) 55 (4)

% Clay

Total (%)

N

31 (1) 30 (4) 19 (4)

24 (2) 21 (5) 13 (2)

% Silt

0.2(0.1)

0.1(0.1)

0.2(0.0)

0.2(0.1)

0.2(0.1)

0.6(0.1)

K

0.2(0.1)

0.2(0.0)

0.2(0.1)

0.3(0.0)

0.2(0.1)

0.2(0.1)

Na

(me q per 100 g soil)

1.6(0.3)

0.6(0.3)

0.5(0.1)

2.0(0.4)

0.7(0.2)

1.3(0.5)

Mg

Exc ha nge a bl e cations

1.2 (.1) 1.6 (.1) 1.6 (.2)

1.2 (.2) 1.4 (.2) 1.7 (.1)

Bulk de ns i t y (g/cm 3)

0.1(0.1)

0.2(0.1)

0.3(0.1)

0.3(0.1)

0.3(0.1)

0.7(0.1)

Ca

16.1(3.6)

9.8(3.9)

11.5(0.8)

14.6(6.1)

9.8(2.2)

15.5(1.6)

CEC (meq per I00 g soft)

14(3)

13(6)

10(3)

21(6)

15(3)

17(2)

Saturation

% Base

Selected physical and chemical properties of the softs in the s t udy plots; da t a pre s e nt e d are t he me a ns of six samples (standaxd errors are given in parentheses)

TABLE I

¢,0

80 Some of the native dry sclerophyU (Beadle and Costin, 1952) eucalypt forest was cleared around 1930 and planted to P. radiata. The plantation and the surrounding eucalypt forest were burned by a forest fire in 1939. The P. radiata plantation was re-established by planting and the eucalypt forest regenerated naturally although some eucalypt trees survived the fire. During the early 1960's the P. radiata plantation was thinned by removing suppressed and co-dominant trees. Both plantation and forest were about 37 years old half way through the study. One plot was established in the P. radiata plantation and the other about 150 m away in the adjacent eucalypt forest. The eucalypt forest was dominated by Eucalyptus obliqua L' Herit but contained a small amount of Eucalyptus dives Schauer. The understory was dominated by shrubs and ferns. The pine plantation had a fairly open canopy and a moderately dense understory of tree ferns, shrubs, and grasses. Some characteristics of the trees in each of the two plots are given in Table II. Further information about the study area is given in Feller (1980). T A B L E II S e l e c t e d characteristics of the tree species in t h e s t u d y p l o t s Plot

Species

Number of living s t e m s (per h a )

N u m b e r of s t a n d i n g dead stems (per ha)

22 (20--25) 16 (15--20)

830

250

90

30

26 (25--30)

670

15

Mean diameter outside b a r k dbhob

Stand basal area of living trees

Mean canopy height

(cm)

(m2/ha)

(m)

Eucalyptus obliqua Eucalyptus dives

29.1 (13.7)*

64.9 (12.7)

(4.0)

0.9

(1.2)

Plnus mdiata

29.0 (11.7)

51.2

(3.9)

(range) Eucalypt (EO)

Pine (PR)

10.8

* S t a n d a r d el~rors are g i v e n i n p a r e n t h e s e s . D a t a are b a s e d o n m e a s u r e m e n t s o b t a i n e d f r o m t w o b i o m a s s p l o t s p e r S t u d y plot. The plots in study plot E O m e a s u r e d 2 0 X 2 0 m w h e r e a s those in s t u d y plot P R m e a s u r e d 2 0 × 1 5 m a n d 2 5 X 1 5 m .

METHODS

Field measurement o f forest biomass and ecosystem nutrient pools Nutrients in soil and biomass components were obtained by standard sampling techniques as described in Feller (1980). Six soil pits were dug to a depth of 1.5 m in each stand and samples were taken from each of the three horizons present during autumn 1976. These were air dried, and then stored frozen prior to analysis. Above ground tree biomass was estimated using a dimensional analysis approach (Whittaker and Woodwell, 1968). The techniques used in this study are given in greater detail in Feller (1980). Regression equations were developed relating the mass of a tree component to its breast height dia-

81

meter (D) or to D2H where H is tree height. Trees covering a range of sizes were felled (six Eucalyptus obliqua, five E. dives and five Pinus radiata) and sampled to provide data for the regression equations. Foliage, small branches (basal diameter <( 15 cm) and cones in the case of P. radiata, were weighed fresh in the field. Subsamples were taken to the laboratory and dried to determine moisture content. This allowed estimation of dry weights of these components. Field measurements were made to determine the volumes of stemwood, stembark, and large branches (basal diameter ~> 15 cm). These volumes were converted to dry weights using specific gravity data. Specific gravities of the eucalypt stemwoods and large branches were obtained from Kingston and Risdon (1961). Specific gravities of eucalypt stembark, P. radiata stemwood, and P. radiata stembark were determined using a water immersion technique. The regression equations developed for the two eucalypt species have already been given (Feller, 1980), those developed for P. radiata are given in Table III. Stand biomass was determined by using the regression equations to estimate the biomass of each tree within the two biomass plots per stand (Table II). T A B L E III B i o m a s s regression e q u a t i o n s f o r Pinus radiata relating b i o m a s s (kg) o f a component ( Y ) t o f u n c t i o n s o f t r e e d i a m e t e r a t b r e a s t h e i g h t ( D ) a n d t r e e h e i g h t (H); f o r l o g a r i t h m i c f u n c t i o n s , D is in c m ; f o r f u n c t i o n s involving D2H, b o t h D a n d H are in m

Component

Equation

r2

s.e

Significance

No. o f observations

Stemwood Stembark Living b r a n c h e s Dead b r a n c h e s Needles Cones

Y= 5.0 + 1 7 1 . 3 D2H Y= 0.8 + 2 7 . 4 D2H Y = -2.4 + 9.3 D2H Y = -16.7 + 7.5 In D Y = -1.2 + 3.2 D2H Y= 2.3 + 2.8 D~H

0.96 0.96 0.98 0.82 0.99 0.99

91.4 14.6 2.6 18.2 0.6 0.7

* * * * * **

5 5 4 4 4 4

s.e. is t h e s t a n d a r d e r r o r o f t h e e s t i m a t e . * * significant a t P < 0 . 0 0 1 . * significant at P < 0 . 0 1 .

For each felled tree, replicate (three to six) samples of each component {live branches, dead branches, leaves, cones (in the case of P. radiata) stembark, and stemwood) were dried at 70°C, ground to pass a 1-mm sieve, then stored awaiting chemical analysis. For each of live branches, dead branches, leaves, and cones (P. radiata), all the analyses for a particular nutrient were averaged for each tree species to obtain the average nutrient composition of each tree component by species. For each of stembark and stemwood (for which samples were analyzed from the base-, the top-, and the mid-sections of the stem) weighted average nutrient composition per tree was obtained

82

by weighting the measured concentrations according to the relative volume of bark or wood to which each analysis applied. The weighted mean nutrient composition of bark and w o o d for a tree species was obtained by averaging the individual tree values for all trees of the one species. Average nutrient concentrations in tree biomass components are given in Table IV. Nutrient quantities in the stand of trees were obtained by multiplying the total stand biomass of each component of the trees by the average nutrient concentrations in that component. For dead standing trees, the nutrient composition of stembark was assumed to be the same as that of stembark on living trees but the nutrient composition of stemwood and branches was assumed to be the same as that of dead branches on living trees. Feller (unpublished data, 1982) found that bark on livingDouglas-fir trees in coastal British Columbia had almost identical nutrient concentrations to bark on dead Douglas-fir trees. Schlesinger (1978) recorded similar nutrient concentrations in dead stems and dead branches in a cypress swamp-forest in the southeastern U.S. These results support the two assumptions just made in the present study. Even if the nutrient concentrations estimated for dead standing trees were 100% in error, the resulting error in the calculated total biomass nutrient quantities would be less than 3% for all nutrients in the P. radiata ecosystem and 6% or less for all nutrients in the eucalypt ecosystem except for P (24%) and M g (11%). Understory aboveground biomass was determined by removing all the aboveground understory vegetation from five 2 m × 2 m square plots per stand. For each understory plot, the vegetation was separated into major components (shrub stems, shrub leaves, ferns, grasses, mosses) which were dried at 70°C and then weighed. Duplicate samples of each component of each understory plot were ground and later analyzed for nutrient content. The resulting mean nutrient concentrations in each component were multiplied by the mass of each component to obtain the nutrient content of each understory plot. Forest floor (litter) biomass was determined by removing the litter from ten 0.5 m 2 circular plots per stand. After drying at 70°C and weighing, duplicate samples from each litter plot were ground for chemical analysis. The nutrient content of each litterplot was determined by multiplying the littermass by its mean nutrient concentrations. Root biomass was determined by excavation of three 1-m 2 plots per stand to a depth of 50 cm. Roots were collected from two depths (0--20 c m and 20--50 cm) and t w o size fractions ( < 1 cm diameter and > 1 cm diameter) yielding four samples per root plot. Each sample was dried at 70°C, weighed, and t w o subsamples ground for chemical analysis. The nutrient c o n t e n t of the roots in each root plot was determined by multiplying the r o o t mass of each o f the four samples by its mean nutrient concentrations, then summing the f o u r values for each nutrient. Tree roots were not separated from understory roots.

0.1O (0.05) 0.21 (0.16) 0.16 (0.05) 0.14 (0.05) 0.99 (0.47) 0.21(0.06) 0.07 (0.02)

stemwood stembark living branches dead branches needles cones roots

P. radiata

(0.00) (0.29) (0.05) (0.06) (0.36)

0.15 0.46 0.16 0.16 0.96

stemwood stembark living branches dead branches leaves

E. dives

(0.0O0) (0.006) (0.002) (0.001) (0.020)

(0.002) (0.002) (0.006) (0.002) (0.021) (0.014)

0.003 (0.000) 0.008 (0.003) 0.011(0.003) 0.004 (0.001) 0.080 (0.021) 0.011 (0.002) 0.013 (0.002)

0.003 0.016 0.007 0.005 0.060

P (0.03) 0.002 (0.06) 0.007 (0.04) 0.013 (0.05) 0.008 (0.29) 0.053 (0.06) 0.023

N 0.08 0.16 0.13 0.16 0.69 0.12

Component E. obliqua stemwood stembark living branches dead branches leaves roots

Tree

0.03 0.05 0.12 0.04 0.65 0.03 0.27

0.03 0.13 0.07 0.02 0.44

K 0.01 0.05 0.07 0.03 0.41 0.30

(0.01) (0.01) (0.03) (0.01) (0.30) (0.01) (0.07)

(0.00) (0.02) (0.01) (0.00) (0.09)

(0.01) (0.04) (0.03) (0.01) (0.15) (0.17)

0.01 0.01 0.02 0.02 0.02 0.02 0.07

0.02 0.06 0.03 0.01 0.12

Na 0.01 0.06 0.04 0.04 0.16 0.06

(0.01) (0.01) (0.00) (0.00) (0.01) (0.00) (0.02)

(O.OO) (0.00) (0.01) (0.00) (0.01)

(0.01) (0.01) (0.01) (0.01) (0.03) (0.01)

0.02 0.02 0.06 0.05 0.16 0.00 0.06

0.Ol 0.15 0.06 0.02 0.21

Mg 0.01 0.02 0.08 0.09 0.25 0.11

(O.O0) (0.01) (0.01) (0.01) (0.07) (0.00) (0.02)

(O.0O) (0.09) (0.01) (0.00) (0.03)

(0.00) (0.01) (0.02) (0.02) (0.10) (0.03)

(0.01) (o.o9) (o.o6) (o.o4) (0.26) (0.04) 0.03 (0.02) 0.48 (0.18) 0.16 (0.08) 0.I0 (0.03) 0.58 (0.22) 0.05 (o.o2) 0.10 (o.o7) 0.16 (o.o6) 0.21 (o.o8) 0.20 (0.12) 0.00 (0.00) 0.08 (0.05)

Ca 0.03 0.15 0.13 0.10 0.57 0.06

Mean concentrations of nutrients (% by weight) in tree biomass components; stemwood and stembark concentrations are weighted according to the volume of wood they represent; due to root identification problems root nutrient data are incomplete (standard errors are given in parentheses)

TABLE IV

O0

84

Field measurement o f nutrient fluxes Nutrient fluxes in solution in precipitation, throughfaU, stemflow, and mineral soil leachate were calculated from estimates of water fluxes (as described in Feller (1981b)) and measurements of nutrient concentrations. Precipitation quantities were measured by a standard rain gauge placed in a clearing 1 km from the study plots. Precipitation samples for chemical analysis (bulk precipitation) were obtained from two simple polyethylene collectors located near the rain gauge. Samples were collected every 2 weeks or after precipitation events during drier periods. Nutrient fluxes were obtained by multiplying monthly quantities of precipitation by weighted mean monthly nutrient concentrations in the precipitation. Concentrations were weighted by volume of precipitation. Throughfall collectors (seven per plot) consisted of 17 cm diameter polyethylene funnels leading into 4.9-1 polyethylene storage containers. Each funnel contained a cotton wool plug to minimize sample contamination. The collectors were emptied monthly when volumes were recorded and samples taken for chemical analysis. Each collector was randomly relocated after each monthly sampling. Monthly nutrient fluxes were determined for each collector on the basis of the volume of water collected and its nutrient concentrations. The nutrient flux values of the seven collectors in a stand were averaged to obtain the monthly nutrient flux for that stand. Stemflow was collected by attaching polyethylene collars to five E. obliqua and two E. dives trees in the eucalypt plot and to five P. radiata trees in the pine plot. The collars led to 23-1 polyethylene storage containers. Stemflow was sampled over a range of tree size classes for each species. Measured stemflow volumes were converted to stand quantities according to methods described by Feller (1981b). Stemflow volumes were recorded monthly when samples were taken for chemical analysis. Stemflow nutrient fluxes were determined on a monthly basis by multiplying total stand stemflow quantities for each tree species by weighted mean monthly nutrient concentrations. The concentrations were weighted by stemflow volume. Mineral soil leachate was collected monthly using tensionless lysimeters, each consisting of a 17 cm diameter polyethylene funnel in which was inserted a perforated disc covered by a layer of cotton to prevent soil particles moving into the system. A polyethylene tube led from the bottom of the funnel into a 4.9-1 storage container. The lysimeters (six per plot) were inserted in the soil at a depth of 50 cm on the upslope side of soil pits. Mineral soil leachate volumes could not be estimated from the amount of water collected by the lysimeters due to the great variability between lysimeters in the amount collected. This suggested non-uniform flow of water through the soft. This was attributed to the observed distribution patterns of large channels (macrochannels) in the soil which conducted much water during and after storm events. In place of measurement, the volume of water flowing through the soil was assumed to be the difference between gross

85 precipitation and evapotranspiration calculated on an annual basis. Evapotranspiration for each plot was estimated by methods described by Feller (1981b). These methods produced only a range of values. Consequently, only a range of values, rather than one specific value, could be estimated for mineral soil leachate volumes. Annual mineral soil leachate nutrient fluxes were then calculated by multiplying the estimated leachate volumes by the weighted mean annual nutrient concentrations. These concentrations were obtained by weighting the mean monthly concentrations for each plot according to the monthly precipitation quantity. Litterfall was collected in ten 37-cm square plastic trays per plot (18 cm tall, with perforated bases). Litterfall was collected monthly when all litterfall collectors were randomly relocated. LitterfaU was dried at 70°C and separated into under- and over-story components for weighing. Then each of these components was bulked by plot and a sample was taken and ground for chemical analysis. Litterfall nutrient fluxes were calculated on a monthly basis for both over- and under-story components, by multiplying the average litterfall mass by its nutrient concentrations. Current net annual nutrient accumulation in the tree biomass was calculated from estimates of the nutrient content of the living biomass at two points in time, 2 years apart. This was accomplished for the aboveground biomass as described above using biomass regression equations and measurements of the diameters of all the trees in the biomass plots (Table II) in May 1978 and again in June 1980. It was assumed that nutrient concentrations in biomass components did not change during this 2-year period. Nutrient accumulation in roots was calculated by assuming that: (1) the percentage increase in the root biomass over the 2-year period was the same as the percentage increase in the aboveground tree biomass during this period; and (2) nutrient concentrations in roots did not change during the 2-year period. No significant difference was found in the aboveground understory biomass between May 1976 and May 1978 and it was assumed that there was no increase in understory root biomass during the 1978--1980 period. Since tree roots could not be separated from understory roots and the percentage increase in aboveground tree biomass was applied to the total (i.e. tree + understory) root biomass, the increase in root biomass, and hence the nutrient accumulation in roots, has probably been overestimated. However, in view of the relative insignificance of the aboveground understory biomass compared to the aboveground tree biomass, it is likely that the understory only accounts for a minor amount of the total root biomass s o that the overestimate would be relatively small. Nutrient inputs to the soil from mineral weathering were estimated assuming a bedrock weathering rate of 700 kg ha-~ year-~ as determined by Feller (1981a) for nearby areas. Nitrogen weathering rates were not given previously by Feller (1981a) due to the absence of analytical data for N in the bedrock of the study area. However, it is assumed that the bedrock had a N content of 0.005% which appears to be typical of most igneous rocks

86 (Mason, 1958). It is also assumed that weathering rates were the same for both eucalypt and pine stands.

Chemical analyses Analytical methods for soils and plant materials have been described previously (Feller, 1980). Soil analyses followed standard procedures described in Black et al. {1965). Total N (Kjeldahl), total (perchloric and nitric acid) and extractable (0.5 M NaHCO3, pH 8.5) P, and exchangeable (1 M NH4OAc, pH 7) K, Na, Mg, and Ca were determined on each soil sample, and converted to kg/ha from measurements of bulk density and percent coarse fragments. Soil pH was determined in both 1:1 (soil:water) and 1:2.5 (soih0.1 M CaC12 solution) using a pH meter. All plant material samples were analyzed in triplicate for N by Kjeldahl digestion and for P, K, Na, Mg, and Ca by a dry ashing procedure. Ecosystem solution samples were analyzed for K, Na, Mg, Ca, NH4, NO3, and total dissolved P, the latter following an acid-persulphate digestion (Taras et al., 1971). All K, Na, Mg, and Ca analyses were carried out by atomic absorption spectrophotometry, with Mg and Ca being determined in the presence of lanthanum chloride. All NH4, NO3, and P (PO4) analyses were carried out using automated colorimetric methods on a Technicon Autoanalyzer. RESULTS AND DISCUSSION Nutrient contents of the softs in the two ecosystems are given in Table V. There were apparently no major differences between eucalypt and pine soils in the distribution of any nutrient between the surface (0--50 cm) and the lower (50--100 cm) soil layers, with most of the N, extractable P, and exchangeable K and Ca in the upper layer, and most of the exchangeable Na and Mg in the lower layer. Total P was almost evenly distributed between the two layers. The soil in the eucalypt ecosystem contained greater quantities of all nutrients, the differences between eucalypt and pine soils being highly significant (t test) in the case of N, K, Mg, and Ca, but not significant in the case of P and Na. Similar results were found in another study near Narbethong, 13 km northeast of the Maroondah study area (Feller, 1978). In that study of Knasnozem soft (Stace et al., 1968) -- similar to those in the present study -greater quantities of exchangeable K, Na, Mg, and Ca, but smaller quantities of total N and P were found in the surface 20 cm of soil beneath a Eucalyptus obliqua forest compared to the soft beneath and adjacent Pinus radiata plantation. The statistical significance of the differences, however~ could n o t be assessed due to the soil sampling procedures. Both E. obliqua forests and both P. radiata plantations were established soon after the 1939 fires.

Depth (cm)

0--50 50--100 0--100

Pine

4897 3300 8197

(519) (324) (612)*

9512 (1781) 3931 (436) 13443 (1834)

N

487 (63) 370 (108) 857 (125) Ns

490 (137) 365 (44) 855 (144)

P extractable

787 (58) 819 (185) 1606 (194) Ns

834 (177) 870 (87) 1704 (146)

P total

60

Pine

57

57 49

49

.34 45

54

294 (81) 364 (99) 658 (128) NS

233 (73) 452 (79) 685 (108)

Na

61

488(129) 413 (160) 901 (206)*

938 (294) 603(248) 1541 (385)

K

Values are the means of six values. Standard deviations are given in parentheses. * Significantly different from the eucalypt value at P <0.01. NS, Not significantly different from the eucalypt value.

71

Eucalypt

Proportions o f nutrients in the upper half (0---50 cm) o f the soil

0-- 50 50--100 0--100

Eucalypt

Nutrient content

Plot

38

37

439(116) 722 (158) 1161 (197)*

775 (221) 1329(486) 2104 (534)

Mg

62

56

507 (133) 308 (107) 815(171)*

889 (97) 693(146) 1582(175)

Ca

Nutrient content (kg/ha) of the soils in the eucalypt and pine ecosystems and the proportions of nutrients found in the upper half (0--50 cm) of the soft. Proportions are expressed as percentages of the total

TABLE V

O0

88 The results from both Maroondah and Narbethong suggest that either the establishment, grow.th, or some other aspect of P. radiata plantation forestry in E. obliqua forest areas in the region has affected the nutrient status of the soils. A comparison of nutrient distribution and cycling in the adjacent eucalypt and pine ecosystems at Maroondah will now be used to l~elp explain why these effects on soil nutrient status have occurred. The results of the study at Maroondah are summarized in Figs. 1--6. All annual fluxes are averaged from 2 years data. In these figures nutrient quantities given for roots are for the sum of overstory plus understory roots, and nutrient quantities given for trees are for the sum of living and standing dead trees. The aboveground return to the soil includes the sum of overstory and understory fluxes in which the return in throughfall was calculated as the quantity measured in throughfall minus that measured in precipitation. On the basis of Na:K ratios in precipitation, throughfall, and stemflow, aerosol impaction was not considered to be important. The total uptake refers to the sum of current net annual accumulation in biomass and the aboveground return to the soil. It does not include the belowground return to the soil and, consequently, is an underestimate of the total amount of nutrients taken up by the vegetation. However, this does not affect the calculation of the current annual soil nutrient balance which was defined as the sum of the inputs (precipitation + aboveground return to the soil + mineral weathering) minus the sum of the outputs (total uptake + soil leaching). Although there are greater quantities of N and K stored in the biomass in the pine ecosystem than in the eucalypt ecosystem (Table VI), these quantities are insufficient to explain the large differences between eucalypt and pine ecosystems in soil N and K quantities. Phosphorus, Na, Mg, and Ca storage in biomass are all greater in the eucalypt ecosystem than in the pine ecosystem but the differences between ecosystems are slight. The pine ecosystem was thinned in the early 1960's. Although the precise intensity of the thinning is unknown, it is likely to have been heavy in view of the current relatively low stand density (Table II). At the time of the study, the pine stand had an aboveground dry weight biomass of 337.5 t/ha, of which 310.2 t/ha was in stembark and stemwood. By comparison with data in Fig. 9 of Madgwick et al. (1977) it can be estimated that the upper limit of stem biomass removal in the thinning would have been approximately 100 t/ha, which would have included approximately 120 kg/ha N, 5 kg/ha P, 30 kg/ha K, 10 kg/ha Na, 20 kg/ha Mg and 60 kg/ha Ca assuming the pine stand in the 1960's had a similar stemwood/stembark ratio, and similar stemwood and stembark nutrient concentrations to the corresponding values found at the time of this study. Addition of these values to those given for the pine ecosystem in Table VI does not substantially alter the statements made in the preceding paragraph. The current annual soil nutrient balance was more favourable (i.e. greater net input or a lesser net output) for the eucalypt ecosystem for each of the nutrients studied except Na where the ranges in values were too wide to

, j',,-

- 0.3 ,

-(7.1 to 7.3)

/ 0.1

,

1~_.z :',,/~+,++ '++

PINE

'

ill''

;,I,

't, I I

,,i,, I

t

i

~.~

(kg ha -1)

- mineral soil (0-100 cm)

. roots

-

- aboveground understory forest floor

• a b o v e g r o u n d overstory

-(25.0 to 26.4)

'i ~

II

',I t i ~+,, t i i i

Fig. 1. Distribution and m o v e m e n t of nitrogen in the eucalypt and pine ecosystems.

Current annual soil balance (inputs • outputs)

\ ..~-~

~+~S

@i

EUCALYPT

NITROGEN

-

- mineral w e a t h e r i n g soil leaching

• total u p t a k e

• aboveground.return to the soil (litterfall & throughfall & s t e m f l o w )

- precipitation

(kg ha -1 year -1)

Oo ~D

"

I

balance

( i n p u t s ,,o,, - oulpuls) ..... , .....

":~~"'15 "~. ~

'~'~-

' ; ';'~....

'J;:?'~"~'"/'I':M~"~~'~

'

EUCALYPT

0 5•

II, ii i', ,I,,111i

II;'":I,,,

PINE

l

0 3•

PHOSPHORUS

- mineralsoil (0-100cm)

• abovegroundunderstory - forest floor - roots

- abovegroundoverstory

(kg ha -1)

- mineralweathering • soil leaching

• total uptake

- aboveground-retumto the soil (litterfall & throughfall& stemflow)

- precipitation

(kg ha -1 y e a r "-1 )

¢,0

./~

g

-

2

t

,',

i

1.9 )

14.6 to 15.9

/0.6-

i

~,, i~ i=

I,,',III il

PINE ii i r

understory

• aboveground • forest floor

3.7 to 4.8

• m i n e r a l s o i l (0-100 c m )

• roots

overstory

- aboveground

(kg ha -1)

Fig. 3. Distribution and movement of potassium in the eucalypt and pine ecosystems.

~:,:::'.~,:~u;','O'o'o,ou,s,

,,

,I

EUCALYPT

POTASSIUM

• mineral weathering - soil l e a c h i n g

- total uptake

- a b o v e g r o u n d - r e t u m t o t h e soil (litterfall & t h r o u g h f a l l & s t e m f i o w )

- precipitation

(kg ha -1 y e o r -1 )

¢D

..j'.

i&6 .....

-10.7 to 10.2

7/~,

@

• m i n e r a l s o i l (0.100 c m )

- roots

- aboveground understory - forest floor

-13.6 to 9.8

. 9 . 1 • 32.52

.J.t.-I~ . ; ~

(kgha -1)

- aboveground overstory

Fig. 4. Distribution and m o v e m e n t of sodium i n t h e eucalypt and pine ecosystems.

Cucrent annual soil balance (inputs - OUtputs)

~17.7

/'"N

,'

,it

i : : ; :1;; ; ',',

ii~4:

PINE I ' ,I' ',',', ; ' ',i' ~-&,, " '|/,g ~I '" ',~I ', I

EUCALYPT

SODIUM

- mineral weathering - soil leaching

. total uptake

• aboveground-retum to the soil (litterfall & throughfall & stemflow)

- precipitation

(kg ha -1 y e a r --1 )

~O l'O

Z

,

.,.,,,,

~

,13.5t~1~, ' ....

1.2

..~ .

to

3.9

PINE

"

.I

Br: 2 ~ 2 2 ] ~

~1

','' ........' ~ ,II ii i i ~ IIi I I

II

'"

-(0.1 to 5.4)

understory

• aboveground - forest floor

• m i n e r a l s o i l (0.100 c m )

. roots

overstory

(kg ha -1)

- aboveground

FP~

Fig. 5. Distribution and movement of magnesium in the euealypt and p!ne ecosystems.

hal.... (input. . . . ,puts)

Current a n n u a l soil

~

.,~

ii

',:',',r,',',:l ,I'~)I,[ if t i , i

EUCALYPT

MAGNESIUM

uptake

• mineral weathering - soil l e a c h i n g

-total

. aboveground-retum to the soil (litterfall & throughfall & stemflow)

. precipitation

(kg ha-1 yeor -1)

CD CO

,J.,,.... ,,.,.,

i:I ~

11.3 to 12.9

i

~

~

/

PINE

....

_

,ik

ii

" ',• i ;f ,~,i t ii

~

1.5 to 4.3

~

o mineral soil (0-100 cm)

• roots

- aboveground understory - forest floor

ovemtory

(kg ha -1)

- aboveground

Fig. 6. Distribution and movement o f calcium in the eucalypt and pine ecosystems.

., ....

Current annual soil

7 .4

;:i

EUCALYPT

CALCIUM y e o r -1 )

- soil

weathering leaching

- mineral

• total uptake

• aboveground-mtum to the soil (litterfall & throughfall & stemflow)

- precipitation

Oig h a -1

¢.0

95 TABLE VI Total quantities of nutrients stored in the biomass of the eucalypt and pine ecosystems. Discrepancies in totals are due to rounding errors (kg/ha) Ecosystem/component

N

P

K

Na

Mg

Ca

Eucalypt Abovegroundvegetation 457 28 118 110 86 287 Belowground vegetation 64 9 113 25 43 43 Forest floor 132 3 9 4 14 40 Total 653 40 240 139 142 370 Pine

Aboveground vegetation 462 17 173 Belowground vegetation 54 7 134 Forest floor 182 4 12 Total 699 28 319

43 82 228 18 35 39 2 12 24 63 129 290

show a clear separation (Figs. 1--6). The annual soil nutrient balance is likely to change with time due to changes in the various fluxes which determine it. During the early years of life of a forest, the magnitude of the difference b e t w e e n plant uptake and return to the soil (i.e. accumulation in the biomass) will b e relatively low, b u t will increase with time, peaking around the time o f canopy closure, then will slowly decline as the forest ages (Miller, 1981). Mineral weathering (Likens et al., 1970, Bormann and Likens, 1979,) and soil leachate (Cole and Gessel, 1965; Tamm et al., 1974; Bormann and Likens, 1979; Vitousek et al., 1979; Hart et al., 1981; Sollins and McCorison, 1981) nutrient fluxes are likely to be relatively high during the early years of life of a forest, declining over a period of several years to levels which undergo some fluctuations b u t which remain moderately stable during the remainder of the life of the forest. The significance of any differences between the eucalypt and pine ecosystems in the long term balance between plant uptake and return to the soil should be reflected in the measurements of nutrients stored in biomass. These measurements suggested that some differences in these nutrient balances have occurred b u t that they have been insufficient to account for most o f the observed differences in soil nutrient quantities between the eucalypt and pine ecosystems, as discussed above. Differences between the t w o ecosystems in mineral weathering have not been quantified. Neither have changes in mineral weathering rates with time. However, in view of the limited range of weathering rates reported for a variety of ecosystems (Feller, 1981a) and the initial less than 3--fold increase in weathering rates following deforestation at H u b b a r d B r o o k (Likens et al., 1970), it is unlikely that differences between the t w o ecosystems in weathering rates could fully account for the observed differences in soil nutrient

96 quantities. This is certainly the case for N in view of the low N content in igneous rock. Soil leachate nutrient fluxes are likely to have been greater in the pine ecosystem following the clearing of the original eucalypt forest and for the first few years of life of the pine plantation (Cole and Gessel, 1965; Tamm et al., 1974; Bormann and Likens, 1979; Vitousek et al., 1979; Hart et al., 1981; Sollins and McCorison, 1981). However, the differences between the two ecosystems are likely to have been less than 10 kgha-' year -1 for each nutrient, as found by most other workers. This is supported by Langford and O'Shaughnessy (1980) who found that clearcutting Eucalyptus regnans and mixed eucalypt forests resulted in only small increases in nutrient fluxes in streams draining these forests. Langford and O'Shaughnessy's study area lay approximately 3 km from the present study area and their mixed eucalypt forest was very similar to the eucalypt forest in the present study. Consequently, differences between the eucalypt and pine ecosystems in total soil leachate nutrient fluxes since the time of the initial clearing of the eucalypt forest are also unlikely to fully account for the observed differences in soil nutrient quantities. There are some uncertainties in the N fluxes, however. The N balance is incomplete because neither N-fixation inputs, denitrification losses, nor fluxes of organic N in solution were measured. Data from Baker and Attiwill (1981) and Lawrie (1981) indicate that N-fixation in eucalypt forests may be around 1--10 kgha-' year-', with values up to 50 kg ha-' year-' for the first few years of life of the forest following a disturbance such as fire. If one assumes that N-fixation in the eucalypt ecosystem has exceeded N-fixation in the pine ecosystem by 50 k g h a - ' year-' since the time of establishment of the forests -- an extremely unlikely situation -- then differences in N balances would still account for only approximately half of the observed difference in soil N contents. Denitrification losses have not been quantified for Australian forests but may be of the same order of magnitude as nonsymbiotic N fixation inputs (Todd et al., 1975). In this case they may be less than 10 kgha-' year-1 by comparison with nonsymbiotic N-fixation rates of 1.5 k g h a - ' year-' reported by Baker and Attiwill (1981) for E. obliqua forest in southeastern Australia. Such small fluxes, combined with the fixation rates discussed above, are unable to explain the large differences in soil N status between the eucalypt and pine ecosystems. Of the fluxes making up the soil nutrient balance in Figs. 1--6, only precipitation, aboveground return to the soil, and soil leaching would be affected by inclusion of dissolved organic N data. Precipitation is the same for both ecosystems. Of t h e 26.6 kgha -1 year-1 aboveground N return to the soil in the eucaiypt ecosystem, the contribution from throughfall and stemflow was negligible (approximately 0.0 k g h a - ' year-'). For the pine ecosystem, the contribution from throughfall and stemflow represented 1.0 of

97 the 21.9 kg ha- 1 year- 1. It is unlikely that the inclusion of organic N fluxes would have resulted in a large difference between pine and eucalypt ecosystems in aboveground N return to the soil, in view of the fact that most studies which have included organic N measurements report annual throughfall plus stemflow N fluxes of <15 kg/ha (e.g. Tarrant et al., 1968; Johnson and Risser, 1974; Bormann et al., 1977; Verry and Timmons, 1977; Sollins et al., 1980). In Australia, O'Connell et al. (1979) reported low quantities (2.6--3.2 kg ha -I year -I) of organic N plus ammonium N in throughfall in Eucalyptus signata-dominated forest. This rather large N flux was tentaquantities (34 kg ha -1 year -I ) of total N in throughfall plus stemflow in a Eucalyptus signata -- dominated forest. This rather large N flux was tentatively attributed to the location of the forest within 1.5 km of the sea (Westman, 1978). Organic N in sea foam could be deposited on the forest canopy to be later washed off by rain. This form of N is unlikely to be important at Maroondah since the sea is over 60 km away. Soil leachates are unlikely to contain high organic N concentrations (Sollins et al., 1980; M.C. Feller, unplublished data, 1982). Consequently differences between the eucalypt and pine ecosystems in organic N fluxes in soil leachate are also unlikely to explain the differences in soil N quantities. Although each flux, by itself, could not fully account for the observed differences in soil nutrient quantities, the net effect of all the fluxes considered together might do so. The results of such a consideration are presented in Table VII. In this table each individual flux difference calculation was done in such a way as to maximize its likelihood of accounting for the difference in soil nutrient quantities. Even with such an extreme biasing of the calculations, differences in nutrient fluxes between the eucalypt and pine ecosystems are still unlikely to fully account for the differences in soil N, Mg, and Ca quantities. The unexplained difference in soil N quantities is particularly large. Another possible cause of the differences in soil nutrient quantities between the eucalypt and pine ecosystems is loss of nutrients during the process of converting the eucalypt forest to a pine plantation. This conversion process involved clearing of the eucalypt forest with burning of the residues. Such a process can cause substantial nutrient losses from the site, most of which occur as losses in fly ash or due to volatilization to the atmosphere as a result of burning (Feller, 1982). Burning of forest residues (slashburning) has already been shown to cause substantial loss of nutrients, particularly N, in Australia (Harwood and Jackson, 1975; Flinn et al., 1979) and elsewhere (Isaac and Hopkins, 1937; Klemmedson et al., 1962; Knight, 1966; Debell and Ralston, 1970; Grief, 1975; Klemmedson, 1976; Wells et al., 1979; Tiedemann and Anderson, 1980; Feller et al., 1983). Hingston and Raison (1982) reported that N losses to the atmosphere during fires in Australia have ranged from 40 to 2000 kg/ha. In 1939, approximately 9 years after the initial pine plantation had been established, both the plantation and adjacent eucalypt forest were burnt by

98 TABLE VII Estimated maximum differences between the eucalypt and pine ecosystems in nutrient fluxes which might contribute to the difference in soil nutrient quantities. Nutrient flux differences are for the 48-year period since the initial clearing of part of the eucalypt forest. All numbers are rounded to the nearest ten (kg/ha) N

(A) Balance between plant uptake and return to the soil (B) Mineral weathering (C) Soilleachate (D) Balance between N-fxation and

K

40 0 120 880

Mg

Ca

-10

-80

450 80

140 300

360 180

--

--

--

--

--

--

30 640 640

20 450 940

60 520 770

80

denitrification

(E) Dissolved organic N flux in through- 340 f a l l , stemflow, and soil leachate (F) Thinning ofP. radiata 120 Total(A+B+C+D+E+F) 1500 Difference in soil quantities 5250

The quantities were estimated as follows: (A) This was calculated as the difference between eucalypt and pine ecosystems in the total quantities of nutrients stored in the biomass at the time of this study, obtained from Table VI. (B) The mineral weathering rate in the pine ecosystem was assumed to be half that in the eucalypt ecosystem throughout the 48-year period. Weathering rates were assumed to have remained constant throughout the 48-year period. (C) Leachate nutrient quantities in the pine ecosystem were assumed to have exceeded those in the eucalypt ecosystem by 10 kgha -I year -1 for the first 3 years, by 5 kg ha -~ year -~ for the next 3 years, and then by the maximum possible currently (maximum pine v a l u e - minimum eucalypt value from Figs. 1,3,5,6) for the next 42 years. (D) Nitrogen fixation less denitrification in the eucalypt ecosystem was assumed to have exceeded that in the pine ecosystem by 50 kgha -I year -~ for 10 years, then by 10 kg ha -1 year -1 for the remaining 38 years. (E) Organic N concentrations in throughfall and stemfow were assumed to be twice the inorganic concentrations and the throughfall + stemflow N flux was assumed to have remained constant for 48 years. Inclusion of organic N concentrations in soil leachates was assumed to have doubled the estimated difference in soil leachate inorganic N fluxes (120 kg/ha from C). (F) Thinning loss calculations have been described in the text. a f o r e s t fire. N o p i n e s in t h e s t u d y p l o t area survived this fire a l t h o u g h s o m e e u c a l y p t s did. This c o m p l i c a t e s the situation since b o t h the e u c a l y p t and the pine e c o s y s t e m s have been a f f e c t e d b y fire since t h e initial clearing o f p a r t o f the e u c a l y p t forest. H o w e v e r , since: (a) the pine p l a n t a t i o n area has been a f f e c t e d b y t w o fires a n d the e u c a l y p t f o r e s t b y o n l y one fire; a n d (b) the e u c a l y p t , b u t n o t t h e pine, e c o s y s t e m has h a d living b i o m a s s t h r o u g h o u t the 48 y e a r p e r i o d since initial clearing f o r the p l a n t a t i o n , it is likely t h a t fire has c a u s e d g r e a t e r n u t r i e n t losses f r o m t h e pine p l a n t a t i o n area t h a n f r o m t h e e u c a l y p t f o r e s t area.

99

Recent studies in southern Australia have suggested that the major cause of second rotation declines in productivity has been loss of ecosystem N and organic matter caused by slashburning (Squire et al., 1979; Flinn et al., 1980, Woods, 1980; Farrell et al., 1981; Squire and Flinn, 1981). Raison (1980) has also drawn attention to the possible consequences of nutrient loss accompanying slashburning in Australian forests. In view of the results of the present study, and those discussed above concerning slashburning, it is hypothesized that the observed differences in soil nutrient quantities between the eucalypt and pine ecosystems are associated to some extent with nutrient losses occurring during the 2 fires that have occurred in the pine plantation area since 1930. The effects of these fires on soil nutrient status have been reinforced and accentuated with time as the pine soil has exhibited less favourable annual nutrient balances than the eucalypt soil. It should be stressed that it is unlikely that the two fires could account for all of the differences in soil nutrient contents. Changes in nutrient fluxes with time, as given in Table VII, are likely to account for a certain proportion of these differences. To date, second rotation declines in productivity in pine plantations in Australia have only been found on sites with nutritionally poor, sandy soils. The results of the present study suggest that the potential for such productivity declines also exists on richer soils. ACKNOWLEDGEMENTS

The research was sponsored by the Rural Credits Development Fund of the Reserve Bank of Australia, the University of Melbourne, and the CSIRO, through an extramural grant. The assistance of Danielle Schoen and Lesley Botterill with field work is gratefully acknowledged. I am particularly grateful to P.J. O'Shaughnessy, K.J. Langford, R.J. Moran, and other members of the Melbourne and Metropolitan Board of Works, not only for permission to work within a M.M.B.W. catchment, but also for supplying me with much hydrological data without which the research could n o t have been completed. I am also grateful to J.P. Kimmins for reviewing the manuscript. The research was carried out while the author was the Australian Paper Manufacturer's Lecturer in Forestry at the University of Melbourne.

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