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Impact of moisture relationships on the management of Pinus pinaster ait. Plantations in Western Australia
Forest Ecology and Management, 1 (1977) 97--107 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
97
I M P A C T O F M O I S T U R E R E L A T I O N S H I P S ON T H E M A N A G E M E N T OF P I N U S P I N A S T E R A I T . P L A N T A T I O N S IN W E S T E R N A U S T R A L I A
T.B. BUTCHER
Institute of Forest Research and Protection, Forests Department of Western Australta, Como, W.A. 6152 (Australia) (Received 16 June 1976)
ABSTRACT Butcher, T.B., 1977. Impact of moisture relationships on the management of Pinus pinaster Ait. plantations in Western Australia. Forest Ecol. Manage., 1: 97--107. The major factor determining Pinus pinaster growth potential on the coastal plain sands of south-western Australia, which has a Mediterranean type climate, is soil moisture availability. This in turn is governed by the depth and moisture-holding capacity of the porous sand, which limits the magnitude of moisture storage during the winter, and by the density of the stand, which controls the rate of exhaustion of the stored water during the spring and summer season. Manipulation of the stand density by thinning increases the throughfall and hence the recharge of the soil moisture system. Withdrawal over the long summer drought period is regulated by a lower density of trees. Cambial growth is concentrated by thinning on high value crop trees. In dense stands, cambial growth stops in November, but continues into April in open stands. Effectiveness of fertilizer application is dependent on moisture condition of the soil profile.
INTRODUCTION W e s t e r n A u s t r a l i a has 1.8 m i l l i o n h a u n d e r s e c u r e t e n u r e f o r f o r e s t r y p u r p o s e s . I t is c o n s e r v a t i v e l y e s t i m a t e d t h a t this S t a t e will r e q u i r e an a n n u a l s a w l o g p r o d u c t i o n o f 1.4 m i l l i o n m 3 b y t h e y e a r A . D . 2 0 1 0 . T h i s is t h r e e t i m e s t h e e s t i m a t e d y i e l d f r o m t h e i n d i g e n o u s h a r d w o o d f o r e s t s at t h a t t i m e . To m e e t t h i s r e q u i r e m e n t , a t o t a l p l a n t a t i o n a r e a o f a b o u t 1 4 0 0 0 0 h a will be r e q u i r e d b y A.D. 2 0 0 0 . S o f t w o o d s r a t h e r t h a n e u c a l y p t s will be p l a n t e d , as s a w l o g r o t a t i o n s f o r e u c a l y p t s are o f t h e o r d e r o f 7 0 - - 8 0 y e a r s , c o m p a r e d w i t h t h e s h o r t e r 30 y e a r r o t a t i o n f o r Pinus radiata D. D o n a n d 40 y e a r rot a t i o n f o r Pinus p i n a s t e r Ait. T o t a l p l a n t a t i o n a r e a e s t a b l i s h e d t o d a t e is 37 0 0 0 ha, m a d e u p o f 16 0 0 0 h a o f P. radiata a n d 21 0 0 0 h a o f P. pinaster. O n t h e n o r t h e r n S w a n C o a s t a l Plain, t h e r e is e s t i m a t e d t o b e an a d d i t i o n a l 27 0 0 0 h a s u i t a b l e f o r c o n v e r s i o n
98
to P. pinaster. Growth is slow by comparison, b u t this species is favoured because of its drought resistance and "tolerance to infertile sands. Although the coastal sands are infertile, they are very close to markets. Havel (1968) assessed the potential for plantation establishment on the north Coastal Plain on an ecological basis. He recognized the limitations imposed b y the drought susceptibility of the Spearwood sands, and stressed the need to determine the stand density at which there is adequate replenishment of moisture reserves. This paper describes such a study. METHOD
Location The study was conducted in the " H u n d r e d Acre Block", of the Yanchep Plantation complex, at Longitude 115 ° 42'E and Latitude 31 ° 29'S. This is approximately 55 km north of Perth and 10 km from the ocean. Elevation is 45 m, and depth to groundwater table is approximately 16 m. The geomorphology and climate of this area interact to provide a unique situation. Climate is typically Mediterranean in character. The summer is virtually rainless and very hot, and 70% of the annual rainfall of 780 mm falls during the 4 coolest months of the year. In this period, 25% of rainfall is required for evapotranspiration, and the remainder is potentially available for redistribution and subsequent plant use during the period when evapotranspiration exceeds rainfall. The length of the drought period can be critical. It is estimated by the Commonwealth Bureau of Meteorology (1966) that in 83 years out of 100, drought exceeding 6 months in duration will be experienced at Yanchep. Soils are relatively deep, well-drained sands. They belong to the Spearwood Dune System sands, and are described by McArthur and Bettenay (1960) as consisting of a core of aeolianite, with a hard capping of secondary calcite overlain by varying depths of yellow brown sand. The parent material was calcareous sand to the surface, but continued leaching has removed the carbonate from the upper profile and precipitated it below to form the hard capping. Surface r u n o f f is negligible on this coarse-grained soil. All rainfall reaching the soil surface may be considered to enter and remain in the groundwater body, or to be returned to the atmosphere through evapotranspiration. The " H u n d r e d Acre Block" was planted to Pinus pinaster Leiria strain in 1952, at a stocking equivalent to 2200 seedlings/ha. Seedlings received 57 g of Superphosphate at planting, and a 2.5% zinc sulphate foliar spray at age 4 years.
Procedure A large thinning experiment was established in the study area in 1966,
99 with the objective of relating increment to stand density levels. Stand density, expressed as basal area, was to be maintained by periodic thinning at 25, 17, 11 and 7 m2/ha. Each treatment plot was duplicated and repeated on five different site types. The duplication of plots allowed the 1972 testing of a fertilizer factor. Treatment involved the application of 0.5 tonnes Superphosphate with zinc and copper trace element plus 0.25 tonnes ammonium sulphate/ha. In 1968, detailed hydrological studies were grafted onto this basic design as the importance of soil water to pine growth potential was realized (Havel, 1968). In all, a total of 36 aluminium irrigation tubes were installed as access for a neutron probe, to depths of up to 7 m. Short tubes were used when limestone was encountered above this maximum depth. Two tubes were located on the 0.04 ha study plots, with hole positions randomly allocated. Soil moisture content has been determined through the period April 1968 to July 1975, usually at monthly intervals, by a Troxler neutron probe. Readings were made at 30 cm intervals, and converted to indicate volumetric water content. Interception and stemflow of rainfall was studied in 1969 and 1970, using the technique described b y Wilm (1943). Diameter growth was recorded at 2-weekly intervals on all trees of the four key plots by means of tree ring (dendrometer) bands. All plots were measured for volume every 2 years. RESULTS
Effect of stand density on soil moisture content and diameter growth of trees The average soil moisture contents, recorded in native woodland and three thinned pine plots, are shown in Table I. Soil moisture levels are shown for the end (April--May) and the c o m m e n c e m e n t {August--October) of the drying period in this 4-year study. Rainfall received in the 4 months of winter is shown in Tables I and II. Table II shows that 72% of the mean annual rainfall fails in this period. In the absence of any runoff, rainfall is available for infiltration and redistribution in the soil profile, or is intercepted and lost in evaporation. Canopy interception of rainfall, stemflow and throughfall was studied at the trial area in 1969, and again in 1970. Of the rain falling annually on the Pinus pinaster forest, the portion that is intercepted at leaf and stem surfaces was found to be 10% for the 7 m2/ha density forest, increasing to 26% in the 25 m2/ha forest. The amount of intercepted rainfall replenishing soil moisture reserves as stemflow is relatively insignificant, being 6% of intercepted water at both levels. The inverse relationship between stand density and throughfall, or effective rainfall, has a major bearing on the recharge of the soil moisture reservoir and, through this, the potential for pine growth. Increasing the basal area
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TABLE I Moisture condition of soft profile, for native woodland and three pine stands, over 4 hydrological years. Figures show the rainfall (ram) and quantity of water stored ( m m ) in the surface 6.3 m soil zone
P. pinaster
Native woodland
7 m=/ha
11 m : / h a
25 m2/ha
Measured soil moisture, May 1971
224
267
135
148
Rainfall Measured soil moisture, October 1971 Calculated moisture withdrawal Measured soil moisture, May 1972
505 524 292" 232
505 577 332 245
505 332 177 155
505 299 110 189
Rainfall Measured soil moisture, September ]:972 Calculated moisture withdrawal Measured soil moisture, April 1973
469 488 296 192
469 533 314 219
469 389 240 149
469 380 195 185
Rainfall Measured soil moisture, September 1973 Calculated moisture withdrawal Measured soil moisture, April 1974
620 576 298 278
620 591 326 265
620 507 301 206
620 455 250 205
Rainfall Measured soil moisture, August 1974 Calculated moisture withdrawal Measured soil moisture, May 1975
608 544 281 263
608 552 319 233
608 479 280 199
608 459 256 203
TABLE
II
Rainfall recorded at Yanchep Annual rainfall (mm)
Departure as % o f mean
Winter recharge
Rainfall (ram)
% of annual
1967 1968 1969 1970 1971 1972 1973 1974
945 948 523 712 696 553 760 836
+21 +21 -33 - 9 -11 -29 - 3 + 7
762 679 382 472 505 469 620 608
81 72 73 66 73 85 82 73
Mean
781
560
72
Rainfall year
101
of a stand to 25 m2/ha effectively reduces the average annual rainfall at Yanchep of 780 to 590 mm. In an open stand, throughfall is 708 mm, which is higher than the amount received at the soil surface under the native woodland. Further reductions are caused by the differential build up of raw humus, which is directly related to the density of the forest structure. The soil wetting front under the 25 m2/ha pine stand has not been observed to go beyond the 6 m soil depth in the study period. This results from a considerably lowered intake and a concurrent greater evapotranspirative demand through the greater density of trees. Considerable discharge of moisture below the 7 m sampling depth has been recorded for the native woodland and open pine stands. The completely saturated profile holds approximately 550 mm water. This is available to the wilting point limit, approximating 150 to 200 mm water. The time required to exploit this available water determines the potential cambial growth. Soil water can be rapidly depleted by a large number of trees, or it can be gradually diminished by a small number of well-formed crop trees. Fig.1 shows a plot of data from an open 7 m2/ha pine stand and a dense 25 m2/ha stand. It is a composite figure detailing the moisture stored in the soil profile and its depletion over a summer period, and relates the average tree increases in diameter to these factors.
-700
Soil moisture OiawNIter increment Soil moisture Dtameter increment
-~----
)
7m2ha- I
) 25m2ha-~
,o~ 600
E
E 500
8~
u.l
Fo 400 5
N
\
6} \ \
o
5
300
200
I00
r
f
~
f f 13
18
15
13
25
21
21
SEP
OCT
NOV
DEC
JAN
FEB
MAR
18
APR
Fig. 1. Total soil moisture w i t h d r a w a l and related diameter i n c r e m e n t over the summer drought period. P. pinaster.
102
Moisture storage in the open stand is near field capacity at the end of the rain season. Withdrawal is uniform until mid-January, and then continues at a reduced level over the remaining drought period. Water is not limiting, and diameter growth at the m a x i m u m rate is possible. After January, greater evapotranspirative demands on a diminishing moisture supply slow the rate of diameter growth. In the dense stand, the soil profile is saturated to a 4.5 m depth only. There is a rapid withdrawal of available water, culminating in mid-November. This immediate exhaustion of soil moisture is reflected in the graph of tree diameter growth. Growth ceases at this date.
Patterns o f summer moisture withdrawal and winter recharge Results of cumulative summer moisture withdrawal and winter recharge for the 1 9 7 2 hydrological year are presented in Figs.2 and 3 for native woodland and three pine stands. Any change in the profile during the summer period, due to evapotranspiration or deep drainage, is defined as moisture withdrawal. In each graph, the zero line represents the soil profile moisture MOISTURE I0 20 ,
WITHDRAWAL 40 50 60
30
i
.-"" F
r
,
i I I
(mm) ]0
20
30
i/
/ /" •
50
60
'. ~ \ ~'~, i: t I ;
"\~
.:;
-- ///t~ / /
40
,
",~
,..- f /
J
,
.
~
,'// ....... Native
woodland
P pinaster
7m2ha
-I
~t
c~ 3
"-...~'~. /
;
~'~\\ Sept -Oct
"\ I/~'
P pinaster 11m2ha-I
R pinaster
........ -------- --
Sept- Nov Sept-Dec Sept-Jan Sept- Feb
- -
Sept-
Apt
25m2ha -t
Fig. 2. Patterns of progressive moisture withdrawal with depth for native w o o d l a n d and pine stands.
103
MOISTURE I0
RECHARGE
20
30
40
50
I
i
1
I
,,
1
..
-~
.
160
(ram) 10 1
20
30
40
I
i
I
SO I
60 I
..
2
!, "
....... -
,
...- . - . . "
..-,\\
\ \
"%
oi 7
Native
2
woodland
" .......... \ \
I~ pinaster
7m2ha -}
"~\
,j
"l-
-----....
May-June May -July May-Aug M a y - Sept
- -
May -Oct
~ / S" .J
/~ .....
~
~
O6 u~
Ppinaster
11m2ha-I
Ppnaster
25m2ha -I
Fig. 3. Patterns of progressive moisture replenishment with depth for native woodland and pine stands.
condition at the commencement of the phase. In Fig.2, this line approximates the saturated profile in September. Distance from the zero line of each curve represents the progressive seasonal withdrawal of moisture from the various layers, and, finally, the last curve represents the total water consumption during the drought period. This was 296 mm for the native woodland and 314, 240 and 195 mm for the 7, 11, and 25 m2/ha pine stands respectively. These differences are the result of differentially saturated profiles at the completion of the winter rains. In each graph there is an approximate linear transgression of the soil moisture withdrawal curves with time. They indicate a similar extraction of water through the 7 m profile, and suggest a uniform distribution of plant roots within this depth. A higher concentration of plant roots is indicated at 2 m, under both pine and native woodland, by a more rapid moisture depletion at this depth. For each treatment class, the moisture profile at the end of April, which is the driest part of the season, is similar from year to year. This condition approximates the wilting point, and when it is reached, the deep rooting plants enter a state of dormancy.
104
Fig. 3 i l l u s t r a t e s t h e r e w e t t i n g o f soil p r o f i l e s u n d e r t h e s a m e s t a n d s in t h e 1 9 7 2 w i n t e r . This was a v e r y d r y y e a r , b u t m o s t o f t h e r a i n f a l l was c o n c e n trated into the 4-month winter period. T h e m o v e m e n t o f w e t t i n g f r o n t s d o w n t h e p r o f i l e w i t h t i m e is o b v i o u s on e a c h graph. S i m i l a r w e t t i n g p a t t e r n s in t h e o p e n p i n e a n d n a t i v e w o o d l a n d are also c l e a r l y e v i d e n t . Soil p r o f i l e s t o a d e p t h o f 6 m are c o m p l e t e l y s a t u r a t e d b y m i d - S e p t e m b e r , a n d f l o w b e y o n d this h o r i z o n c o n t i n u e s i n t o N o v e m b e r . In c o n t r a s t t h e w e t t i n g f r o n t r e a c h e s a m a x i m u m d e p t h o f 4.5 m u n d e r t h e 25 m 2 / h a p i n e at t h e s a m e t i m e . F l o w d o e s n o t c o n t i n u e a f t e r c e s s a t i o n of w i n t e r rains b e c a u s e o f t h e r a p i d s p r i n g e v a p o t r a n s p i r a t i o n . F i g . 2 s h o w s t h a t in t h i s d e n s e s t a n d soil m o i s t u r e is e m p t i e d f r o m t h e p r o f i l e b y m i d - N o v e m b e r
Soil moisture -- fertility interaction on pine growth T a b l e I I I ( H a t c h a n d M i t c h e l l , 1971} lists d a t a f o r seven e l e m e n t s e x t r a c t e d f r o m f o l i a g e o f c r o p t r e e s in 1 9 6 7 , 1 9 6 9 , 1 9 7 1 a n d 1 9 7 3 . F o l i a r levels were largely unaffected by thinning treatment. However, there were significant TABLE III Effect of thinning treatment on foliar nutrient levels Basal area (m2/ha)
Year
%
ppm
N
P
K
Ca
Mg
Mn
Zn
7
1967 1969 1971 1973 Fert.
0.77 0.85 0.72 0.73 0.78
0.074 0.076 0.066 0.045 0.099
0.61 0.83 0.86 0.38 0.48
0.17 0.20 0.15 0.13 0.15
0.24 0.23 0.18 0.15 0.17
19.9 17.7 12.7 14.6 17.6
28.1 33.6 30.6 18.6 20.2
11
1967 1969 1971 1973 Fert.
0.78 0.90 0.73 0.77 0.77
0.080 0.085 0.077 0.051 0.082
0.62 0.80 0.81 0.46 0.42
0.18 0.21 0.17 0.12 0.13
0.25 0.25 0.20 0.15 0.15
17.5 16.8 14.4 19.4 21.6
26.2 34.9 31.0 18.8 20.4
17
1967 1969 1971 1973 Fert.
0.78 0.88 0.76 0.87 0.81
0.077 0.077 0.068 0.049 0.084
0.62 0.72 0.78 0.41 0.41
0.16 0.20 0.16 0.13 0.11
0.24 0.23 0.19 0.12 0.13
18.0 19.9 15.9 19.6 20.6
27.8 32.1 29.0 16.4 20.4
25
1967 1969 1971 1973 Fert.
0.76 0.88 0.76 0.78 0.92
0.070 0.076 0.066 0.046 0.080
0.61 0.69 0.74 0.39 0.41
0.16 0.18 0.14 0.11 0.13
0.24 0.24 0.20 0.15 0.17
17.3 17.1 16.2 20.8 25.0
27.7 32.0 29.3 16.8 19.4
105
differences between the sampling years for all elements examined, particularly in the case of the important element phosphorus. Mean phosphorus decreased from 0.078% in 1969 to 0.069% in 1971, and to a deficient 0.048% in 1973. Fertilizer application in September 1 9 7 2 increased the mean phosphorus level to 0.086%. Generally, a level of 0.08% phosphorus is regarded as necessary for the healthy growth of Pinus pinaster, and 0.06% as definitely limiting. Data have been presented in the early sections of this paper showing moisture to be the major factor determining growth potential. This is particularly evident in Fig.4. In 1967 and 1968, rainfall was 18% above the average. This enabled the saturation of the 25 m2/ha dense pine soil profile, and accounted for a 40% higher volume production than in the open stand. 1969 was a drought year, recording 34% below average rainfall, and was followed by a year with 10% below the average. Volume increment in the dense stands was reduced to the level of increment of the open stand, while
EIS F
/ \
/ \
/ \
|12
/
\ 9 "o
"--...,,..
/
/
~o
",\
~
,~ ~'
F
7""
,/
~ ~..r,. ~'''~
F U
qt
u,
I
~,, o
u 6
67'-69
b)
2O
69171
.......... ......
7m 2 ha- I 11m2 ha_l 1 7 m 2 ha-~
. - .
2 5 m 2 h a -s
71[73
73175
F - fertilized U - unfertilized
.............. ~
~
'
................................................
.
..'~ .......
""
. ........ :.:.
"'.'"
~
U
U
b
i
67468
68-69
i
69-70 INCREMENT
i
L
70-73 7) - 7 2 PERIOD
i
i
72-73
73-74
74-75
Fig.4. V o l u m e (a) and diameter (b) increment. P. pinaster.
106
the open stand increment of 8 m3/ha was similar to the previous period. The point to emphasize is that a similar total stand volume was portioned to 210, 360, 715, and 1230 stems/ha for the treatments of 7, 11, 15 and 25 m2/ha basal area. Even in a severe drought year, the soil profile under an open stand was completely recharged and gross volume production was unaffected. Also evident from Fig.4, is the gradual reduction with time of total volume increment (in 7 mS/ha pine}, when soil moisture was n o t limiting. This introduces the second factor affecting pine growth, soil phosphate. The nutrient status of the soil was rectified in September 1972 by the application of 0.5 tonnes Superphosphate with zinc and copper trace elements plus 0.25 tonnes ammonium sulphate/ha, to one half of the plots. The immediate effect of this is seen in Fig.4. Stand density levels are divided to show the fertilizer effect in the 1971--1973 and 1973--1975 increment periods. (Because volume increment is measured at 2-yearly intervals, the increment period 1971--1973 in fact only contains a partial response to the fertilization). Volume increment increased by 30% in the open stand following application of fertilizer. In the dense stand where moisture was limiting, growth increased by 8% in the first year, and a staggering 45% in the next 2 years, when rainfall returned to normal. Similar large differences in total volume increment between dense and open pine stands underline the importance of the soil moisture factor. Fig.4b depicts mean diameter increment over the same period. The key point to note is the halving of diameter increment on smaller stems of the dense stand. The effectiveness of added nutrient depends on the moisture condition of the profile at application. In the open stand, fertilizer which is n o t used immediately or fixed in the soil profile will be leached from the tree rooting zone. The nutrients are retained under a dense stand because of relatively minor water flows through the profile. Figs.4 a and b emphasize this point for the period 1971 to 1975. Improved nutrient status also appears to increase the efficiency of water use by the pine tree. IMPLICATIONS OF RESEARCH FINDINGS
This paper has endeavoured to give some insight into the important soil moisture--soil fertility--pine growth relationship and its formulation into management practice on the Swan Coastal Plain. The single most important factor determining growth potential of Pinus pinaster is soil moisture availability. Sufficient moisture must be stored in the soil profile to allow growth to extend through the long summer drought. This is particularly important in a Mediterranean climate, on deep porous sands with low water retentiveness. This can be achieved by manipulation of stand density. By changing stand density, the infiltration and evapotranspiration can be radically altered. Effective rainfall can be increased by as much as 15% by
107
h e a v y thinning. This increase allows t h e c o m p l e t e s a t u r a t i o n o f t h e soil profile, even in excessively d r y years, and ensures a c o n t i n u i n g t o t a l v o l u m e p r o d u c t i o n o f similar p r o p o r t i o n . F u r t h e r , t h e e v a p o t r a n s p i r a t i v e d e m a n d is r e d u c e d b y t h e sixfold r e d u c t i o n in s t e m n u m b e r s . T h e c o n s e q u e n c e of this is a p r o f o u n d increase in individual t r e e g r o w t h . T h e f o r e g o i n g s e c t i o n revealed t h a t c o n s i d e r a b l e t o t a l v o l u m e i n c r e m e n t w o u l d be sacrificed in c e r t a i n y e a r s if a p o l i c y o f h e a v y t h i n n i n g t o create o p e n stands were a d o p t e d . A l t e r n a t i v e l y , if light t h i n n i n g was t h e goal t o m a x i m i z e t o t a l p r o d u c t i o n , early m o i s t u r e deficit w o u l d m i n i m i z e i n c r e m e n t on final c r o p trees, w o u l d o n l y achieve similar t o t a l p r o d u c t i o n in excessively d r y y e a r s a n d c o u l d cause d r o u g h t d e a t h s in t h e e x t r e m e case. A l t h o u g h n o t e m p h a s i z e d in this p a p e r , d e a d - t o p p i n g and d r o u g h t d e a t h s in y o u n g pines at c a n o p y closure are n o t u n c o m m o n . Western Australia has an a d e q u a t e s u p p l y of small-size case a n d p u l p logs, n o w and in the f u t u r e . H o w e v e r , t h e r e is an e x t r e m e d e f i c i e n c y in t h e larger sawlog size material. T o m e e t this d e f i c i e n c y , m a n a g e m e n t p r a c t i c e s aim to m a x i m i z e t h e p r o d u c t i o n o f sawlogs, b y c o n c e n t r a t i n g i n c r e m e n t on t h e final c r o p s t e m s a n d b y g r o w i n g t h e trees as fast as possible t o r e d u c e the length o f t h e r o t a t i o n t o a c c e p t a b l e e c o n o m i c a l limits. REFERENCES Butcher, T.B. and Havel, J.J., 1976. Influence of moisture relationships on thinning practice. N.Z.J. Forestry Sci., 6(2): 158--170. Commonwealth Bureau of Meteorology, 1966. Climatic Survey of Region 15 -- Metropolitan Western Australia. Bur. Meteorol., Melbourne, Vict. Hatch, A.B. and Mitchell, Y.J., 1971. The effect of forest operations on the availability of nutrients. In: R. Boardman (Editor), The Australian Forest-tree Nutrition Conference. Contributed papers. Section 4. Forestry and Timber Bureau, Canberra, A.C.T., pp. 275--300. Havel, J.J., 1968. The Potential of the Northern Swan Coastal Plain for Pinus pinaster Ait. Plantations. Bull. For. Dep. W. Aust. No. 76. Hopkins, E.R., 1971. Drought Resistance in Seedlings of Pinus pinaster Ait. Bull. For. Dep. W. Aust. No. 82. McArthur, W.M. and Bettenay, E., 1960. The Development and Distribution of the Soils of the Swan Coastal Plain, Western Australia. Soil Publ. No. 16. C.S.I.R.O. Shachori, A., Rosenzweig, D. and Poljakoff-Mayber, A., 1967. Effect of Mediterranean vegetation on the moisture regime. In: W.E. Sopper and H.W. Lull (Editors), Forest Hydrology. Pergamon Press, Oxford, pp. 291-311. Wilm, H.G., 1943. Determining net rainfall under a conifer forest. J. Agric. Res., 67:501 --512.
97
I M P A C T O F M O I S T U R E R E L A T I O N S H I P S ON T H E M A N A G E M E N T OF P I N U S P I N A S T E R A I T . P L A N T A T I O N S IN W E S T E R N A U S T R A L I A
T.B. BUTCHER
Institute of Forest Research and Protection, Forests Department of Western Australta, Como, W.A. 6152 (Australia) (Received 16 June 1976)
ABSTRACT Butcher, T.B., 1977. Impact of moisture relationships on the management of Pinus pinaster Ait. plantations in Western Australia. Forest Ecol. Manage., 1: 97--107. The major factor determining Pinus pinaster growth potential on the coastal plain sands of south-western Australia, which has a Mediterranean type climate, is soil moisture availability. This in turn is governed by the depth and moisture-holding capacity of the porous sand, which limits the magnitude of moisture storage during the winter, and by the density of the stand, which controls the rate of exhaustion of the stored water during the spring and summer season. Manipulation of the stand density by thinning increases the throughfall and hence the recharge of the soil moisture system. Withdrawal over the long summer drought period is regulated by a lower density of trees. Cambial growth is concentrated by thinning on high value crop trees. In dense stands, cambial growth stops in November, but continues into April in open stands. Effectiveness of fertilizer application is dependent on moisture condition of the soil profile.
INTRODUCTION W e s t e r n A u s t r a l i a has 1.8 m i l l i o n h a u n d e r s e c u r e t e n u r e f o r f o r e s t r y p u r p o s e s . I t is c o n s e r v a t i v e l y e s t i m a t e d t h a t this S t a t e will r e q u i r e an a n n u a l s a w l o g p r o d u c t i o n o f 1.4 m i l l i o n m 3 b y t h e y e a r A . D . 2 0 1 0 . T h i s is t h r e e t i m e s t h e e s t i m a t e d y i e l d f r o m t h e i n d i g e n o u s h a r d w o o d f o r e s t s at t h a t t i m e . To m e e t t h i s r e q u i r e m e n t , a t o t a l p l a n t a t i o n a r e a o f a b o u t 1 4 0 0 0 0 h a will be r e q u i r e d b y A.D. 2 0 0 0 . S o f t w o o d s r a t h e r t h a n e u c a l y p t s will be p l a n t e d , as s a w l o g r o t a t i o n s f o r e u c a l y p t s are o f t h e o r d e r o f 7 0 - - 8 0 y e a r s , c o m p a r e d w i t h t h e s h o r t e r 30 y e a r r o t a t i o n f o r Pinus radiata D. D o n a n d 40 y e a r rot a t i o n f o r Pinus p i n a s t e r Ait. T o t a l p l a n t a t i o n a r e a e s t a b l i s h e d t o d a t e is 37 0 0 0 ha, m a d e u p o f 16 0 0 0 h a o f P. radiata a n d 21 0 0 0 h a o f P. pinaster. O n t h e n o r t h e r n S w a n C o a s t a l Plain, t h e r e is e s t i m a t e d t o b e an a d d i t i o n a l 27 0 0 0 h a s u i t a b l e f o r c o n v e r s i o n
98
to P. pinaster. Growth is slow by comparison, b u t this species is favoured because of its drought resistance and "tolerance to infertile sands. Although the coastal sands are infertile, they are very close to markets. Havel (1968) assessed the potential for plantation establishment on the north Coastal Plain on an ecological basis. He recognized the limitations imposed b y the drought susceptibility of the Spearwood sands, and stressed the need to determine the stand density at which there is adequate replenishment of moisture reserves. This paper describes such a study. METHOD
Location The study was conducted in the " H u n d r e d Acre Block", of the Yanchep Plantation complex, at Longitude 115 ° 42'E and Latitude 31 ° 29'S. This is approximately 55 km north of Perth and 10 km from the ocean. Elevation is 45 m, and depth to groundwater table is approximately 16 m. The geomorphology and climate of this area interact to provide a unique situation. Climate is typically Mediterranean in character. The summer is virtually rainless and very hot, and 70% of the annual rainfall of 780 mm falls during the 4 coolest months of the year. In this period, 25% of rainfall is required for evapotranspiration, and the remainder is potentially available for redistribution and subsequent plant use during the period when evapotranspiration exceeds rainfall. The length of the drought period can be critical. It is estimated by the Commonwealth Bureau of Meteorology (1966) that in 83 years out of 100, drought exceeding 6 months in duration will be experienced at Yanchep. Soils are relatively deep, well-drained sands. They belong to the Spearwood Dune System sands, and are described by McArthur and Bettenay (1960) as consisting of a core of aeolianite, with a hard capping of secondary calcite overlain by varying depths of yellow brown sand. The parent material was calcareous sand to the surface, but continued leaching has removed the carbonate from the upper profile and precipitated it below to form the hard capping. Surface r u n o f f is negligible on this coarse-grained soil. All rainfall reaching the soil surface may be considered to enter and remain in the groundwater body, or to be returned to the atmosphere through evapotranspiration. The " H u n d r e d Acre Block" was planted to Pinus pinaster Leiria strain in 1952, at a stocking equivalent to 2200 seedlings/ha. Seedlings received 57 g of Superphosphate at planting, and a 2.5% zinc sulphate foliar spray at age 4 years.
Procedure A large thinning experiment was established in the study area in 1966,
99 with the objective of relating increment to stand density levels. Stand density, expressed as basal area, was to be maintained by periodic thinning at 25, 17, 11 and 7 m2/ha. Each treatment plot was duplicated and repeated on five different site types. The duplication of plots allowed the 1972 testing of a fertilizer factor. Treatment involved the application of 0.5 tonnes Superphosphate with zinc and copper trace element plus 0.25 tonnes ammonium sulphate/ha. In 1968, detailed hydrological studies were grafted onto this basic design as the importance of soil water to pine growth potential was realized (Havel, 1968). In all, a total of 36 aluminium irrigation tubes were installed as access for a neutron probe, to depths of up to 7 m. Short tubes were used when limestone was encountered above this maximum depth. Two tubes were located on the 0.04 ha study plots, with hole positions randomly allocated. Soil moisture content has been determined through the period April 1968 to July 1975, usually at monthly intervals, by a Troxler neutron probe. Readings were made at 30 cm intervals, and converted to indicate volumetric water content. Interception and stemflow of rainfall was studied in 1969 and 1970, using the technique described b y Wilm (1943). Diameter growth was recorded at 2-weekly intervals on all trees of the four key plots by means of tree ring (dendrometer) bands. All plots were measured for volume every 2 years. RESULTS
Effect of stand density on soil moisture content and diameter growth of trees The average soil moisture contents, recorded in native woodland and three thinned pine plots, are shown in Table I. Soil moisture levels are shown for the end (April--May) and the c o m m e n c e m e n t {August--October) of the drying period in this 4-year study. Rainfall received in the 4 months of winter is shown in Tables I and II. Table II shows that 72% of the mean annual rainfall fails in this period. In the absence of any runoff, rainfall is available for infiltration and redistribution in the soil profile, or is intercepted and lost in evaporation. Canopy interception of rainfall, stemflow and throughfall was studied at the trial area in 1969, and again in 1970. Of the rain falling annually on the Pinus pinaster forest, the portion that is intercepted at leaf and stem surfaces was found to be 10% for the 7 m2/ha density forest, increasing to 26% in the 25 m2/ha forest. The amount of intercepted rainfall replenishing soil moisture reserves as stemflow is relatively insignificant, being 6% of intercepted water at both levels. The inverse relationship between stand density and throughfall, or effective rainfall, has a major bearing on the recharge of the soil moisture reservoir and, through this, the potential for pine growth. Increasing the basal area
100
TABLE I Moisture condition of soft profile, for native woodland and three pine stands, over 4 hydrological years. Figures show the rainfall (ram) and quantity of water stored ( m m ) in the surface 6.3 m soil zone
P. pinaster
Native woodland
7 m=/ha
11 m : / h a
25 m2/ha
Measured soil moisture, May 1971
224
267
135
148
Rainfall Measured soil moisture, October 1971 Calculated moisture withdrawal Measured soil moisture, May 1972
505 524 292" 232
505 577 332 245
505 332 177 155
505 299 110 189
Rainfall Measured soil moisture, September ]:972 Calculated moisture withdrawal Measured soil moisture, April 1973
469 488 296 192
469 533 314 219
469 389 240 149
469 380 195 185
Rainfall Measured soil moisture, September 1973 Calculated moisture withdrawal Measured soil moisture, April 1974
620 576 298 278
620 591 326 265
620 507 301 206
620 455 250 205
Rainfall Measured soil moisture, August 1974 Calculated moisture withdrawal Measured soil moisture, May 1975
608 544 281 263
608 552 319 233
608 479 280 199
608 459 256 203
TABLE
II
Rainfall recorded at Yanchep Annual rainfall (mm)
Departure as % o f mean
Winter recharge
Rainfall (ram)
% of annual
1967 1968 1969 1970 1971 1972 1973 1974
945 948 523 712 696 553 760 836
+21 +21 -33 - 9 -11 -29 - 3 + 7
762 679 382 472 505 469 620 608
81 72 73 66 73 85 82 73
Mean
781
560
72
Rainfall year
101
of a stand to 25 m2/ha effectively reduces the average annual rainfall at Yanchep of 780 to 590 mm. In an open stand, throughfall is 708 mm, which is higher than the amount received at the soil surface under the native woodland. Further reductions are caused by the differential build up of raw humus, which is directly related to the density of the forest structure. The soil wetting front under the 25 m2/ha pine stand has not been observed to go beyond the 6 m soil depth in the study period. This results from a considerably lowered intake and a concurrent greater evapotranspirative demand through the greater density of trees. Considerable discharge of moisture below the 7 m sampling depth has been recorded for the native woodland and open pine stands. The completely saturated profile holds approximately 550 mm water. This is available to the wilting point limit, approximating 150 to 200 mm water. The time required to exploit this available water determines the potential cambial growth. Soil water can be rapidly depleted by a large number of trees, or it can be gradually diminished by a small number of well-formed crop trees. Fig.1 shows a plot of data from an open 7 m2/ha pine stand and a dense 25 m2/ha stand. It is a composite figure detailing the moisture stored in the soil profile and its depletion over a summer period, and relates the average tree increases in diameter to these factors.
-700
Soil moisture OiawNIter increment Soil moisture Dtameter increment
-~----
)
7m2ha- I
) 25m2ha-~
,o~ 600
E
E 500
8~
u.l
Fo 400 5
N
\
6} \ \
o
5
300
200
I00
r
f
~
f f 13
18
15
13
25
21
21
SEP
OCT
NOV
DEC
JAN
FEB
MAR
18
APR
Fig. 1. Total soil moisture w i t h d r a w a l and related diameter i n c r e m e n t over the summer drought period. P. pinaster.
102
Moisture storage in the open stand is near field capacity at the end of the rain season. Withdrawal is uniform until mid-January, and then continues at a reduced level over the remaining drought period. Water is not limiting, and diameter growth at the m a x i m u m rate is possible. After January, greater evapotranspirative demands on a diminishing moisture supply slow the rate of diameter growth. In the dense stand, the soil profile is saturated to a 4.5 m depth only. There is a rapid withdrawal of available water, culminating in mid-November. This immediate exhaustion of soil moisture is reflected in the graph of tree diameter growth. Growth ceases at this date.
Patterns o f summer moisture withdrawal and winter recharge Results of cumulative summer moisture withdrawal and winter recharge for the 1 9 7 2 hydrological year are presented in Figs.2 and 3 for native woodland and three pine stands. Any change in the profile during the summer period, due to evapotranspiration or deep drainage, is defined as moisture withdrawal. In each graph, the zero line represents the soil profile moisture MOISTURE I0 20 ,
WITHDRAWAL 40 50 60
30
i
.-"" F
r
,
i I I
(mm) ]0
20
30
i/
/ /" •
50
60
'. ~ \ ~'~, i: t I ;
"\~
.:;
-- ///t~ / /
40
,
",~
,..- f /
J
,
.
~
,'// ....... Native
woodland
P pinaster
7m2ha
-I
~t
c~ 3
"-...~'~. /
;
~'~\\ Sept -Oct
"\ I/~'
P pinaster 11m2ha-I
R pinaster
........ -------- --
Sept- Nov Sept-Dec Sept-Jan Sept- Feb
- -
Sept-
Apt
25m2ha -t
Fig. 2. Patterns of progressive moisture withdrawal with depth for native w o o d l a n d and pine stands.
103
MOISTURE I0
RECHARGE
20
30
40
50
I
i
1
I
,,
1
..
-~
.
160
(ram) 10 1
20
30
40
I
i
I
SO I
60 I
..
2
!, "
....... -
,
...- . - . . "
..-,\\
\ \
"%
oi 7
Native
2
woodland
" .......... \ \
I~ pinaster
7m2ha -}
"~\
,j
"l-
-----....
May-June May -July May-Aug M a y - Sept
- -
May -Oct
~ / S" .J
/~ .....
~
~
O6 u~
Ppinaster
11m2ha-I
Ppnaster
25m2ha -I
Fig. 3. Patterns of progressive moisture replenishment with depth for native woodland and pine stands.
condition at the commencement of the phase. In Fig.2, this line approximates the saturated profile in September. Distance from the zero line of each curve represents the progressive seasonal withdrawal of moisture from the various layers, and, finally, the last curve represents the total water consumption during the drought period. This was 296 mm for the native woodland and 314, 240 and 195 mm for the 7, 11, and 25 m2/ha pine stands respectively. These differences are the result of differentially saturated profiles at the completion of the winter rains. In each graph there is an approximate linear transgression of the soil moisture withdrawal curves with time. They indicate a similar extraction of water through the 7 m profile, and suggest a uniform distribution of plant roots within this depth. A higher concentration of plant roots is indicated at 2 m, under both pine and native woodland, by a more rapid moisture depletion at this depth. For each treatment class, the moisture profile at the end of April, which is the driest part of the season, is similar from year to year. This condition approximates the wilting point, and when it is reached, the deep rooting plants enter a state of dormancy.
104
Fig. 3 i l l u s t r a t e s t h e r e w e t t i n g o f soil p r o f i l e s u n d e r t h e s a m e s t a n d s in t h e 1 9 7 2 w i n t e r . This was a v e r y d r y y e a r , b u t m o s t o f t h e r a i n f a l l was c o n c e n trated into the 4-month winter period. T h e m o v e m e n t o f w e t t i n g f r o n t s d o w n t h e p r o f i l e w i t h t i m e is o b v i o u s on e a c h graph. S i m i l a r w e t t i n g p a t t e r n s in t h e o p e n p i n e a n d n a t i v e w o o d l a n d are also c l e a r l y e v i d e n t . Soil p r o f i l e s t o a d e p t h o f 6 m are c o m p l e t e l y s a t u r a t e d b y m i d - S e p t e m b e r , a n d f l o w b e y o n d this h o r i z o n c o n t i n u e s i n t o N o v e m b e r . In c o n t r a s t t h e w e t t i n g f r o n t r e a c h e s a m a x i m u m d e p t h o f 4.5 m u n d e r t h e 25 m 2 / h a p i n e at t h e s a m e t i m e . F l o w d o e s n o t c o n t i n u e a f t e r c e s s a t i o n of w i n t e r rains b e c a u s e o f t h e r a p i d s p r i n g e v a p o t r a n s p i r a t i o n . F i g . 2 s h o w s t h a t in t h i s d e n s e s t a n d soil m o i s t u r e is e m p t i e d f r o m t h e p r o f i l e b y m i d - N o v e m b e r
Soil moisture -- fertility interaction on pine growth T a b l e I I I ( H a t c h a n d M i t c h e l l , 1971} lists d a t a f o r seven e l e m e n t s e x t r a c t e d f r o m f o l i a g e o f c r o p t r e e s in 1 9 6 7 , 1 9 6 9 , 1 9 7 1 a n d 1 9 7 3 . F o l i a r levels were largely unaffected by thinning treatment. However, there were significant TABLE III Effect of thinning treatment on foliar nutrient levels Basal area (m2/ha)
Year
%
ppm
N
P
K
Ca
Mg
Mn
Zn
7
1967 1969 1971 1973 Fert.
0.77 0.85 0.72 0.73 0.78
0.074 0.076 0.066 0.045 0.099
0.61 0.83 0.86 0.38 0.48
0.17 0.20 0.15 0.13 0.15
0.24 0.23 0.18 0.15 0.17
19.9 17.7 12.7 14.6 17.6
28.1 33.6 30.6 18.6 20.2
11
1967 1969 1971 1973 Fert.
0.78 0.90 0.73 0.77 0.77
0.080 0.085 0.077 0.051 0.082
0.62 0.80 0.81 0.46 0.42
0.18 0.21 0.17 0.12 0.13
0.25 0.25 0.20 0.15 0.15
17.5 16.8 14.4 19.4 21.6
26.2 34.9 31.0 18.8 20.4
17
1967 1969 1971 1973 Fert.
0.78 0.88 0.76 0.87 0.81
0.077 0.077 0.068 0.049 0.084
0.62 0.72 0.78 0.41 0.41
0.16 0.20 0.16 0.13 0.11
0.24 0.23 0.19 0.12 0.13
18.0 19.9 15.9 19.6 20.6
27.8 32.1 29.0 16.4 20.4
25
1967 1969 1971 1973 Fert.
0.76 0.88 0.76 0.78 0.92
0.070 0.076 0.066 0.046 0.080
0.61 0.69 0.74 0.39 0.41
0.16 0.18 0.14 0.11 0.13
0.24 0.24 0.20 0.15 0.17
17.3 17.1 16.2 20.8 25.0
27.7 32.0 29.3 16.8 19.4
105
differences between the sampling years for all elements examined, particularly in the case of the important element phosphorus. Mean phosphorus decreased from 0.078% in 1969 to 0.069% in 1971, and to a deficient 0.048% in 1973. Fertilizer application in September 1 9 7 2 increased the mean phosphorus level to 0.086%. Generally, a level of 0.08% phosphorus is regarded as necessary for the healthy growth of Pinus pinaster, and 0.06% as definitely limiting. Data have been presented in the early sections of this paper showing moisture to be the major factor determining growth potential. This is particularly evident in Fig.4. In 1967 and 1968, rainfall was 18% above the average. This enabled the saturation of the 25 m2/ha dense pine soil profile, and accounted for a 40% higher volume production than in the open stand. 1969 was a drought year, recording 34% below average rainfall, and was followed by a year with 10% below the average. Volume increment in the dense stands was reduced to the level of increment of the open stand, while
EIS F
/ \
/ \
/ \
|12
/
\ 9 "o
"--...,,..
/
/
~o
",\
~
,~ ~'
F
7""
,/
~ ~..r,. ~'''~
F U
qt
u,
I
~,, o
u 6
67'-69
b)
2O
69171
.......... ......
7m 2 ha- I 11m2 ha_l 1 7 m 2 ha-~
. - .
2 5 m 2 h a -s
71[73
73175
F - fertilized U - unfertilized
.............. ~
~
'
................................................
.
..'~ .......
""
. ........ :.:.
"'.'"
~
U
U
b
i
67468
68-69
i
69-70 INCREMENT
i
L
70-73 7) - 7 2 PERIOD
i
i
72-73
73-74
74-75
Fig.4. V o l u m e (a) and diameter (b) increment. P. pinaster.
106
the open stand increment of 8 m3/ha was similar to the previous period. The point to emphasize is that a similar total stand volume was portioned to 210, 360, 715, and 1230 stems/ha for the treatments of 7, 11, 15 and 25 m2/ha basal area. Even in a severe drought year, the soil profile under an open stand was completely recharged and gross volume production was unaffected. Also evident from Fig.4, is the gradual reduction with time of total volume increment (in 7 mS/ha pine}, when soil moisture was n o t limiting. This introduces the second factor affecting pine growth, soil phosphate. The nutrient status of the soil was rectified in September 1972 by the application of 0.5 tonnes Superphosphate with zinc and copper trace elements plus 0.25 tonnes ammonium sulphate/ha, to one half of the plots. The immediate effect of this is seen in Fig.4. Stand density levels are divided to show the fertilizer effect in the 1971--1973 and 1973--1975 increment periods. (Because volume increment is measured at 2-yearly intervals, the increment period 1971--1973 in fact only contains a partial response to the fertilization). Volume increment increased by 30% in the open stand following application of fertilizer. In the dense stand where moisture was limiting, growth increased by 8% in the first year, and a staggering 45% in the next 2 years, when rainfall returned to normal. Similar large differences in total volume increment between dense and open pine stands underline the importance of the soil moisture factor. Fig.4b depicts mean diameter increment over the same period. The key point to note is the halving of diameter increment on smaller stems of the dense stand. The effectiveness of added nutrient depends on the moisture condition of the profile at application. In the open stand, fertilizer which is n o t used immediately or fixed in the soil profile will be leached from the tree rooting zone. The nutrients are retained under a dense stand because of relatively minor water flows through the profile. Figs.4 a and b emphasize this point for the period 1971 to 1975. Improved nutrient status also appears to increase the efficiency of water use by the pine tree. IMPLICATIONS OF RESEARCH FINDINGS
This paper has endeavoured to give some insight into the important soil moisture--soil fertility--pine growth relationship and its formulation into management practice on the Swan Coastal Plain. The single most important factor determining growth potential of Pinus pinaster is soil moisture availability. Sufficient moisture must be stored in the soil profile to allow growth to extend through the long summer drought. This is particularly important in a Mediterranean climate, on deep porous sands with low water retentiveness. This can be achieved by manipulation of stand density. By changing stand density, the infiltration and evapotranspiration can be radically altered. Effective rainfall can be increased by as much as 15% by
107
h e a v y thinning. This increase allows t h e c o m p l e t e s a t u r a t i o n o f t h e soil profile, even in excessively d r y years, and ensures a c o n t i n u i n g t o t a l v o l u m e p r o d u c t i o n o f similar p r o p o r t i o n . F u r t h e r , t h e e v a p o t r a n s p i r a t i v e d e m a n d is r e d u c e d b y t h e sixfold r e d u c t i o n in s t e m n u m b e r s . T h e c o n s e q u e n c e of this is a p r o f o u n d increase in individual t r e e g r o w t h . T h e f o r e g o i n g s e c t i o n revealed t h a t c o n s i d e r a b l e t o t a l v o l u m e i n c r e m e n t w o u l d be sacrificed in c e r t a i n y e a r s if a p o l i c y o f h e a v y t h i n n i n g t o create o p e n stands were a d o p t e d . A l t e r n a t i v e l y , if light t h i n n i n g was t h e goal t o m a x i m i z e t o t a l p r o d u c t i o n , early m o i s t u r e deficit w o u l d m i n i m i z e i n c r e m e n t on final c r o p trees, w o u l d o n l y achieve similar t o t a l p r o d u c t i o n in excessively d r y y e a r s a n d c o u l d cause d r o u g h t d e a t h s in t h e e x t r e m e case. A l t h o u g h n o t e m p h a s i z e d in this p a p e r , d e a d - t o p p i n g and d r o u g h t d e a t h s in y o u n g pines at c a n o p y closure are n o t u n c o m m o n . Western Australia has an a d e q u a t e s u p p l y of small-size case a n d p u l p logs, n o w and in the f u t u r e . H o w e v e r , t h e r e is an e x t r e m e d e f i c i e n c y in t h e larger sawlog size material. T o m e e t this d e f i c i e n c y , m a n a g e m e n t p r a c t i c e s aim to m a x i m i z e t h e p r o d u c t i o n o f sawlogs, b y c o n c e n t r a t i n g i n c r e m e n t on t h e final c r o p s t e m s a n d b y g r o w i n g t h e trees as fast as possible t o r e d u c e the length o f t h e r o t a t i o n t o a c c e p t a b l e e c o n o m i c a l limits. REFERENCES Butcher, T.B. and Havel, J.J., 1976. Influence of moisture relationships on thinning practice. N.Z.J. Forestry Sci., 6(2): 158--170. Commonwealth Bureau of Meteorology, 1966. Climatic Survey of Region 15 -- Metropolitan Western Australia. Bur. Meteorol., Melbourne, Vict. Hatch, A.B. and Mitchell, Y.J., 1971. The effect of forest operations on the availability of nutrients. In: R. Boardman (Editor), The Australian Forest-tree Nutrition Conference. Contributed papers. Section 4. Forestry and Timber Bureau, Canberra, A.C.T., pp. 275--300. Havel, J.J., 1968. The Potential of the Northern Swan Coastal Plain for Pinus pinaster Ait. Plantations. Bull. For. Dep. W. Aust. No. 76. Hopkins, E.R., 1971. Drought Resistance in Seedlings of Pinus pinaster Ait. Bull. For. Dep. W. Aust. No. 82. McArthur, W.M. and Bettenay, E., 1960. The Development and Distribution of the Soils of the Swan Coastal Plain, Western Australia. Soil Publ. No. 16. C.S.I.R.O. Shachori, A., Rosenzweig, D. and Poljakoff-Mayber, A., 1967. Effect of Mediterranean vegetation on the moisture regime. In: W.E. Sopper and H.W. Lull (Editors), Forest Hydrology. Pergamon Press, Oxford, pp. 291-311. Wilm, H.G., 1943. Determining net rainfall under a conifer forest. J. Agric. Res., 67:501 --512.