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Growth responses by Pinus radiata to combinations of superphosphate, urea and thinning type
Forest Ecology and Management, 30 (1990) 313-325
313
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Growth Responses by P i n u s r a d i a t a to Combinations of Superphosphate, Urea and Thinning Type
P. SNOWDON and H.D. WARING
Division o/Forest Research, CSIRO, P.O. Box 4008, Queen Victoria Terrace, Canberra, A.C.T. 2600 (Australia) (Accepted 6 January 1989)
ABSTRACT Snowdon, P. and Waring, H.D., 1990. Growth responses by Pinus radiata to combinations of superphosphate, urea and thinning type. For. Ecol. Manage., 30: 313-325. A 2~ factorial experiment testing the effects of urea (N), superphosphate (P) and two thinning types (diagonal-line thinning and thinning from below) on the basal-area increment of 18-yearold Pinus radiata (D. Don) is described. Although the trees had received localized applications of superphosphate shortly after establishment they were considered at the age of 18 years to be phosphate-deficient because concentrations of phosphorus in the foliage were low (0.065% P). Application of superphosphate (200 kg P ha - 1) increased concentrations in the foliage (0.132 % P) and increased the 7-year basal-area increment by approximately 70%. No growth response was obtained when urea (476 kg N ha-1 ) was applied alone. When both fertilizers were applied there was a further increase in basal-area increment of 81% on line-thinned plots and 26% on plots which had been thinned from below. This interaction reversed the normal trend in which plots thinned from below, by virtue of their greater initial basal area, produced 11-16% more increment than those which had been line-thinned. A model of tree growth using initial basal area and a competition index as independent variables was adequate to describe differences in basal-area growth between thinning types except where urea and superphosphate had both been applied. In the case of N + P-treated plots, the reciprocal of the competition index was required as an additional variable. Analysis of the development of the growth responses over time indicated that application of superphosphate had produced sustained improvement in growth rates but that the response to urea occurred only in some growingseasons. In line-thinned plots the response to urea in addition to phosphate occurred in the 2nd, 3rd and 4th growing-seasons, but the response occurred only in the 2nd and 4th growing-seasons in plots thinned from below. It is argued that this differential response between thinning types can be attributed to differences in stand density and water availability. In order to maximise the growth response to added nitrogen it is important to reduce competition by paying particular attention to both tree spacing and residual basal area.
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© 1990 Elsevier Science Publishers B.V.
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INTRODUCTION On nutrient-deficient sites, fertilization with the deficient element tends to give a growth response irrespective of stocking, but on better sites thinning may be a prerequisite for a response to fertilizer. In view of the dramatic effects that thinning has on stand structure and subsequent development, it is conceivable that timing, intensity or method of thinning will influence growth responses to fertilizer application. Many experiments have compared responses to fertilizer in thinned and unthinned stands, but few have examined different thinning intensities (Steinbrenner, 1968; Wagle and Beasley, 1968; Weetman, 1971; Woollons and Will, 1975; Mead and Gadgil, 1978). Apparently the only study which has examined different types of thinning is that of Wagle and Beasley (1968), who compared the effects of fertilizer on stands which had been thinned from below with those that had been thinned from above. They obtained growth responses to fertilizer when stands were thinned lightly from above or below and to heavy thinning from above, but there was no fertilizer response in stands thinned heavily from below. Increased mechanisation has made line or row-thinning an economically attractive alternative to selective methods of thinning (Kerruish and Shepherd, 1982). Consequently, it is pertinent to consider if the responsiveness of stands to fertilization is dependent upon the type of thinning. This paper examines growth response by Pinus radiata (D. Don) to nitrogen and phosphorus fertilization on two extreme thinning types: diagonal-line thinning (Solomon, 1982 ) which retained trees in a square spacement irrespective of size or form, and thinning strictly from below to retain the largest trees irrespective of spacing and form. METHODS The experiment was located near Moss Vale, N.S.W., in the Belanglo State Forest (34.6 ° S., 150.4 ° E. ) which is 145 km southwest of Sydney. The elevation is ca. 670 m, and the topography is gently undulating. Mean annual rainfall is 880 mm (range 513-1250 mm), distributed fairly evenly throughout the year. The mean daily maximum temperature is 24.5 °C in January while the mean daily minimum temperature is 1.6 ° C in July. The soils are podzolics ca. 1.5 m deep derived from Hawkesbury Sandstone, a Triassic sandstone which includes shale lenses. They are infertile, the primary nutrient deficiency being that of phosphorus. Large growth responses can be obtained to nitrogen once the phosphorus deficiency has been alleviated, but there is no evidence that other nutrients are inadequate to support healthy growth of P. radiata (Waring, 1969). The experimental site had previously carried a full rotation of P. radiata. After clearfelling, it was planted again in 1962 with P. radiata at a stocking of
GROWTH OF PINUS IN RESPONSE TO FERTILIZERS AND THINNING
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1680 stems h a - 1 with square spacement (2.44 m). At that time an experiment was established which tested the effect of various non-factorial combinations of superphosphate (0, 36, 73, 96 kg P ha -1) and urea (0, 58, 116, 232 kg N h a - 1) applied close to the seedling soon after establishment and/or at 1 and 2 years of age. By 18 years of age the basal area of unfertilized trees was 19 m 2 ha -1, compared to 31 m 2 ha -1 for those at the lowest rate of superphosphate and 39 m 2 h a - ~for the two highest rates. At this time there was no significant growth response to urea. In 1980, 32 of the fertilized plots in this experiment were used in a 2 × 2 × 2 factorial testing the effects of two thinning types (T), the application of superphosphate (P), and of urea (N) on subsequent pine growth. There were four replications of the factorial assigned to eight blocks so that the N × P × T interaction was partially confounded. Blocking was balanced according to prior rates of phosphorus application. Each plot was square, and prior to thinning contained 81 trees. The fertilized portion of the plot consisted of the central 49 of these, while growth assessment was made on the central 25 trees. Both thinning treatments reduced the number of stems by half. In the first case, 'thinning from below', the stems with largest diameters at breast height were retained irrespective of tree form or spacing. In the second case, 'diagonalline thinning', all trees in each 2nd diagonal row were removed. Superphosphate was applied at 0 and 200 kg P ha-1 and urea at 0 and 476 kg N ha -1, in both cases, half in September 1980 and the remainder in October 1981. In addition to the main experiment there were eight unthinned plots, of which two had never been fertilized, two had been fertilized at establishment, and four had been fertilized at establishment then treated with superphosphate and urea in the same manner as the thinned plots. Since thinning type influenced initial basal area {Table 1 ), analyses of covariance were made using separate regressions for each thinning type (Woollons, 1985). Comparisons between thinning types were made using a pooled estimate for error. TABLE 1 Characteristics of t h i n n e d a n d n o n - t h i n n e d P. radiata plots at the beginning of the experiment
Stocking (stems h a - 1) Mean diameter (cm) Basal area (m 2 h a - 1) Competition index (m 2 h a -1 ) Reduction Remaining
Non-thinned
Diagonal line-thinning
Thinned from below
1680 16.4 37.2
840 16.3 18.4
840 19.5 25.4
0 31.5
17.7 13.0
11.2 20.2
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A simple competition index was calculated for each tree. The index was a weighted average of the basal area of trees occupying the nearest eight planting positions surrounding the subject tree; trees on the s;des of the square were given a weight of 1.0 while those on the corners had a ":'~!~, ~ 1/x/~. The index was summed within each plot, then used with initial basal area and factors representing the various fertilizer and thinning treatments, to descriptively model the basal-area increment of the plots. At the time of thinning, unweighed composite samples of 1-year-old foliage from six felled trees in each thinned plot were made. Seven years later, individual samples of 1-year-old and 2-year-old foliage were collected during winter from the upper crown of five trees in each plot. Total nitrogen, phosphorus and potassium were determined after digestion with sulphuric acid/hydrogen peroxide (Heffernan, 1985). RESULTS
Initial conditions At age 18, non-fertilized trees had a basal area of 19 m 2 ha -1. Trees at the lowest level of phosphorus had produced 31 m 2 h a - ' while the two highest rates had both produced 39 m 2 ha -1. These growth responses are similar to those obtained 17 years after broadcasting superphosphate onto young naturally regenerated P. radiata elsewhere in the forest (Turner, 1982). Despite the apparent culmination in growth response at or below 73 kg P ha -1, the foliar concentrations of phosphorus (650/lg g- 1) were clearly in the range associated with P-deficiency. At the time of thinning there was no significant effect of urea applied during the establishment phase. Foliar concentrations of nitrogen (1.29%) and potassium (0.59%) were adequate. Diagonal-line thinning resulted in little change in average diameter compared with the unthinned plots, and reduced basal area by about 50% (Table 1). In comparison, trees on plots thinned from below were 3.2 cm larger in diameter, and basal area was reduced by only 31%. The average competition index on plots which had been diagonal-line-thinned was reduced by 58% of pre-thinning values, but on plots thinned from below reduced by only 36%.
Growth 7 years after thinning Cumulative basal-area increment for the 7-year-period following treatments is shown in Table 2. The increment on unthinned plots which had been fertilized only at establishment was slightly larger than that obtained on the thinned plots, but was substantially greater than that obtained on plots which had never been fertilized (6.31 m 2 h a - ' ) . This indicates that there was a marked residual effect of the initial application of phosphate fertilizer. Fertilization of
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TABLE 2 Effect of fertilization with urea (N) and superphosphate (P) on the 7-year basal-area increment (m 2 h a - 1) of t h i n n e d and n o n - t h i n n e d P. radiata
Non-thinned Diagonal line-thinning T h i n n i n g from below
Nil
N
P
N+P
CD1
10.68 9.27 10.27
-8.39 9.25
-15.64 17.85
19.42 23.25 20.50
5.12 3.11 2.68
~Critical difference P < 0.05.
unthinned plots with urea and superphosphate resulted in a large increase in basal area increment but this was smaller than on the thinned plots. On the thinned plots, application of superphosphate resulted in additional growth averaging 7.0 m 2 h a - 1. Application of urea alone resulted in a small but nonsignificant reduction in increment, but when it was added in combination with superphosphate a positive growth response was obtained amounting to 2.65 m 2 h a - 1 on plots thinned from below and 7.61 m e h a - 1 on diagonal-linethinned plots; the magnitudes of these responses being different with probability P < 0.09. As a consequence, total increment for the diagonal-line-thinned N + P plots was greater than those thinned from below ( P < 0.05). This reverses the normal expectation, illustrated by the results obtained with the other fertilizer combinations, that greatest increment is obtained from stands with the greatest initial basal area. In terms of total growing stock, the initial difference between thinning treatments of 7.0 m 2 ha-1 was reduced to 4.25 m 2 h a - ~ 7 years after applying urea and superphosphate.
Development of response The development of basal area over the 7-year growing period is shown in Fig. 1 for both thinning types. There was a significant response in basal-area increment to phosphorus on diagonal-line-thinned plots in the 1st growingseason, but response was delayed until the 2nd growing season on plots thinned from below. Thereafter the magnitude of responses with time and became greater; however, this response was not significant with plots thinned from below, probably because trees on these were initially greater in size. Analyses of annual basal-area increment, using basal area at the beginning of the growing-season as a covariate, showed that growth responses directly attributable to phosphate application, rather than to prior differences in tree size, continued until the end of the period of observation. This suggests that a type-2 response ( Snowdon and Waring, 1984) has occurred by which site productivity has been improved in the long term by the heavy broadcast application of phos-
318
P. SNOWDONANDH.D.WARING I
~ N + P P N+P
Thinning type40 ~_'--~ Fri°gmnballliW e
~
0//~'~"
r
30
....~.XINil
I"
I
I
I
I
I
i
J
1
2
3
4
5
6
7
Years Fig. 1. Effect of diagonal line-thinning and thinning from belowin combination with applications of nitrogen and phosphorus on the growth of P. radiata over a 7-year period. phate to plots which were, for the most part, phosphate-deficient owing to the fact that earlier phosphate dressings had been localised. The response to nitrogen in the presence of phosphorus (N + P ) - became significant after two seasons growth on diagonal-line-thinned plots and after four seasons on plots thinned from below. Analysis of covariance of annual basal area increments, as above, showed that responses to urea occurred in the 2nd, 3rd and 4th growing-seasons on diagonal-line-thinned plots but only in the 2nd and 4th seasons on those thinned from below. This suggests that a type-1 response (Snowdon and Waring, 1984) had occurred, with nitrogen hastening crown development after thinning b u t not improving site quality in the longer term. Stand-growth model
The model used to describe the 7-year basal-area increment on all plots assumed: (1) that there was no growth in the absence of initial basal area, i.e. the model was constrained to pass through the origin; and (2) that increment increased linearly with initial basal area; but that (3) the rate of increase in increment decreased linearly as competition index increased. Separate relationships were allowed for the various combinations of fertilizer. Initial basal area was highly correlated (r=0.94} with competition index.
GROWTH OF PINUS IN RESPONSETO FERTILIZERSAND THINNING
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Consequently, when the former was used as an estimate of competition in the model, 84% of the variance was accounted for compared to 90% when the competition index was used. In both cases, the increment on plots other than those receiving both urea and superphosphate were adequately described. In the case of the exception, the residuals on plots which had been diagonally line-thinned, and to a lesser extent those which had not been thinned, tended to be positive while residuals for plots which had been thinned from below were negative. This suggested that increment on N + P-treatment plots was curvilinearly related to competition index. Subsequently, the reciprocal of competition index was used as an additional predicting variable on N + P-treated plots. This model accounted for 94% of the variance. The final model took the form
A~ =Ab (a+ bCi+c/Ci) where: zlA~ is basal area increment; Ab, initial basal area; Ci, competition index immediately after thinning; and the coefficient c takes the value zero for other than the N + P-treatment. Figure 2 illustrates the manner in which the rate of the 7-year basal-area increment growth per unit initial basal area (A¢/Ab) declines with increasing Ci. For clarity, data for the N-treated plots are omitted. The model obviates the need to use factors to define whether the stands have been thinned, and if so, in what manner. Application of the model to increments in individual growing-seasons showed 1.5
N-P%
U DB Nil • A •
1.0
o. 0.5
A
•
Nil ~
!
I
I
10
20
30
Cornpetition
index
(r',q 2 h a -; )
Fig. 2. The effect of combinations of nitrogen and phosphorus fertilization and competition index on the growth rate (A~/Ah) of u n t h i n n e d (U), diagonal-line-thinned (D) and t h i n n e d from below
( B ) Pinus radiata.
320
P. SNOWDONANDH.D.WARING
TABLE 3 Concentrations of nitrogen, phosphorus and potassium in 1-year-old and 2-year-old foliage of P. radiata at the beginning of the experiment and 7 years after thinning and fertilization with combinations of urea (N) and superphosphate (P) Treatment
Before treatment After 7 years Never fertilized Not thinned Nil N+ P Thinned Nil N P N+P SED 1
Nitrogen ( % )
Phosphorus (#g/g)
Potassium (%)
1-year
2-year
1-year
2-year
1-year
2-year
1.29
--
650
--
0.59
--
1.31
1.31
664
502
0.48
0.36
1.18 1.14
1.14 1.05
755 1178
620 1015
0.45 0.42
0.38 0.44
1.24 1.41 1.23 1.25 0.05
1.20 1.57 1.07 1.05 0.06
764 741 1321 1228 39
584 575 973 956 44
0.49 0.45 0.55 0.43 0.02
0.42 0.34 0.55 0.41 0.02
1Standard error of difference between means for thinned plots only. t h a t , in t h e 1st s e a s o n it a c c o u n t e d for 14% of t h e v a r i a n c e b u t t h e r e a f t e r a c c o u n t e d for 76-91%. T h e c u r v i l i n e a r c o m p o n e n t o n N + P - t r e a t e d plots was significant in t h e 2nd, 3rd a n d 4 t h growing seasons. T h e r e a f t e r , it b e c a m e nonsignificant a n d t h e effect o f c o m p e t i t i o n o n N + P - t r e a t e d plots b e c a m e indist i n g u i s h a b l e f r o m t h a t o n P - t r e a t e d plots.
Foliar analysis T h e c o n c e n t r a t i o n s o f N, P a n d p o t a s s i u m ( K ) in foliage s a m p l e d at the e n d o f t h e 7th g r o w i n g - s e a s o n are s h o w n in T a b l e 3. A p p l i c a t i o n o f s u p e r p h o s p h a t e i n c r e a s e d P c o n c e n t r a t i o n s in 1-year-old foliage to above t h e deficiency limit ( 1000 ttg P g - 1) b u t c o n c e n t r a t i o n s in t h e N + P t r e a t m e n t were significantly lower t h a n t h o s e w h i c h h a d received P alone. A p p l i c a t i o n of u r e a i n c r e a s e d foliar c o n c e n t r a t i o n s o f N w h e n it was applied in t h e absence o f s u p e r p h o s p h a t e , b u t h a d n o effect w h e n P was applied. U r e a also r e d u c e d t h e c o n c e n t r a t i o n s o f K, w h e r e a s a p p l i c a t i o n of s u p e r p h o s p h a t e alone i n c r e a s e d K concentrations. DISCUSSION T h e initially low c o n c e n t r a t i o n s o f P in foliage, t o g e t h e r with t h e increase in c o n c e n t r a t i o n s a n d t h e s u b s t a n t i a l g r o w t h r e s p o n s e a f t e r a p p l i c a t i o n of su-
GROWTH OF P I N U S IN RESPONSE TO FERTILIZERS AND THINNING
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perphosphate, clearly indicate that the trees were P-deficient at the time of thinning. This result suggests that even high rates of P applied to a localised area near to the transplant are insufficient to sustain good growth and to maintain foliar concentrations at sufficiency levels after the stand has closed canopy. In an adjacent experiment, there was no redistribution of P from the site of application to soil in the remainder of the plot (Waring, 1981 ). Under such circumstances, it has been argued that the opportunity for P uptake by P. radiata from localised applications may be limited, particularly when there is rapid depletion of moisture due to root proliferation in the fertilizer-enriched zone, and it is unlikely that the P requirements of an expanding tree-biomass can be met (Flinn and Aeberli, 1982 ). The response to superphosphate alone was 7.0 m 2 h a - 1 when averaged over both thinning types. This is consistent with the results of an adjacent experiment where a growth response was obtained to the reapplication of phosphate (36 kg P h a - 1), when plots which had initially received a localized application of phosphate (36 kg P ha-1), were thinned at 9-years of age (Waring, 1980). The response confirms the diagnosis of P deficiency based on foliar analysis. Application of urea alone resulted in a small but nonsignificant reduction in increments with both thinning types. Such reductions are common when P. radiata transplants are fertilized with N on P-deficient soils (Snowdon and Waring, 1985) and the effect has also been noted in older stands (Hunter et al., 1986). When urea was applied with superphosphate, a further increase in basalarea increment was obtained, parallelling other results obtained in this forest by Waring (1980) after reapplication of fertilizer (36 kg P, 116 kg N ha -1). Combinations of N and P have been applied at the time of thinning to two other stands in Belanglo State Forest, but in neither case was the growth response as substantial as here. In the first case, 128 kg P and 324 kg N ha-1 were added to a third row-thinning in a 16-year-old stand which had earlier received two broadcast applications of 70 kg P h a - 1as superphosphate ( Crane, 1981). In the second case, 135 kg P and 171 kg N ha -1 were added to a selectively thinned, naturally regenerated stand about 20 years old which had previously received broadcast applications of 121 kg P, 47 kg N and 115 kg K h a - 1 (Woollons, 1985 ). It is reasonable to assume in both cases that the component of the response attributable to P had been reduced by the heavy early applications of phosphate and the fact that these had been broadcast, so that the entire root system had access to elevated levels of soil P. However, even if the responses in these experiments were entirely due to N, the magnitude of the responses was much smaller than those observed here. The rates of N application were lower but were substantially more than that required for crown expansion. The response to urea in addition to superphosphate was much greater on plots which had been systematically thinned than those which had been thinned
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P. S N O W D O N A N D H.D. W A R I N G
from below, despite the higher initial basal area of the latter. This occurred because competition between trees markedly affects A~/Ab. The initial basal area of plots thinned from below was 1.38 times that of those diagonally linethinned (Table 1 ). Consequently, the latter must grow at a rate greater than 1.38 times the former in order to achieve a greater increment. Using data from Table 1 for the average levels of competition index, and the corresponding coefficients for A~/Ab used for Fig. 2, the rate of growth for diagonally linethinned plots fertilized with N + P was 1.63 time that of the corresponding plots thinned from below, thus fulfilling the conditions for a greater total increment. By contrast, on P-treated plots the growth rate of those diagonally line-thinned was only 1.25 times that of those thinned from below; thus they produced less total increment. Although competition between neighbouring trees is recognised as an important factor affecting tree growth, there have been few attempts to quantify this in plantations. If tree size is ignored, and only distances are considered in the competition index described above, then a relative measure of competition can be calculated for various methods of systematic thinning. These can be compared with the expected competition index value (Ce) of 0.85 in an unthinned stand. Cremer and Meredith (1976) present data for the growth of P. radiata which had each second row (Ce = 0.25) or every third row (Ce--0.55) removed at thinning. The initial basal area for the latter was 1.35 times that of the former, but A~/Ab for the former was 1.32 times the latter, with the consequence that there was a compensation in total increment over a 6-yearperiod. This confirms with the principle that for closed stands there is no loss in total increment until thinning intensity exceeds 50% (MSller, 1954). Stiell (1982) compared the growth of single trees ofP. resinosawhich had been completely released by thinning (Ce = 0.0 ) with those which had been retained in clumps of four trees (Ce = 0.34 ). Initial basal area was similar but over a 15year period the former produced 19% more total increment, indicating that A~/Ab had been greater. These results illustrate that A~/Ab tends to decline with an increase in Ci, that total increment depends upon both initial basal area and Ci, and Ci is influenced by the method of thinning. In the experiment described here it is also shown that addition of fertilizer can increase A¢/Ab, and that the increases are greater with lower Ci. In particular, fertilization with both N and P can increase A~/Ab sufficiently to overcome the disadvantage of smaller initial basal area and thus to produce greater total increment. This indicates that the response to N is particularly sensitive to the degree of competition in the stand. A number of studies in coniferous forests have shown that growth and degree of response to fertilization depends upon the amount of annual or seasonal precipitation (Butcher, 1977; Turner, 1982; Spiecker, 1987; Turner and Lambert, 1987). It is tempting to argue that the effect of competition on fertilizer response in this trial is predominantly due to differences in water relationships
GROWTH OF P I N U S IN RESPONSE TO FERTILIZERS AND THINNING
323
between the different thinning regimes. Precipitation (July-June) was less than two-thirds of the long-term average during the first three growing-seasons, slightly below average for the next three, and 170 mm greater than average for the last season. Equations provided by Turner and Lambert (1987) for stands in this forest were used to estimate interception by the different stands. Upon the dry seasons, an average of 160 mm would be intercepted by the u n t h i n n e d stands while throughfall (Pt) would increase by 33 mm and 53 m m in stands thinned from below and those diagonally line-thinned, respectively. Based on these figures, the mean annual increment over the first three growing seasons (A:b), and the assumption that all throughfall is utilized by the trees, the average water-use efficiency (A~b/Pt) was 0.0047, 0.0044 and 0.0053 m 2 m m - 1 year- 1 for N + P-treated plots unthinned, thinned from below, and diagonal-line-thinned, respectively. The lower efficiency of trees thinned from below compared to those u n t h i n n e d could be attributed to the failure of the former, due to incomplete occupation of the site, to exploit 20 mm (4.5%) of the available water. Diagonal-line-thinned trees were clearly more efficient. It can be reasonably be assumed that the rate of water-use would be proportional to the basal area of the stands, i.e. the rate of water-use on plots thinned from below would be 1.38 times that on diagonal-line-thinned plots. Under circumstances of low and intermittent rainfall, the periods when soil conditions were favourable to N mineralization and nutrient uptake would be extended on diagonal-line-thinned plots. This could result in a greater utilization of the applied urea. While the extended growing-season in itself would not improve water-use efficiency, the greater uptake of N by the trees leads to the potential for greater N productivity (~,gren, 1982) and/or greater water-use efficiency (Brix and Mitchell, 1986) and hence the greater growth rates observed on the diagonal-line-thinned plots. CONCLUSIONS The results have important implications for forest management. It is apparent that localized application of even heavy rates of P during the establishment phase may be insufficient to maintain the health and growth-rate of plantations at optimum levels. It is also apparent that applications of N at the time of thinning will not result in an increase in growth if there is a dominating deficiency in P. The response to N after addition of P was greatest on plots which had been diagonal-line-thinned. This qualitative difference between thinning types can be resolved by considering the residual competition index. Greatest total increment obtained with added N is obtained when Ci is minimized; to achieve this, particular attention needs to be paid to the spacing of the trees as well as to the residual basal area. The results suggest that, when N is to be applied, the stand might profitably be thinned to a greater extent than
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normal. For commercial thinnings, the extra revenue thus obtained might well cover the costs of fertilizer and its application. ACKNOWLEDGEMENTS
The field trial was established on a site provided by the Forestry Commission of N.S.W. Trial establishment and measurement were ably conducted by A. Eilert and J. Holtzappfel, while J. Harragan performed the chemical analyses. Dr. J. Leech and Mr. R.C. Woollons provided helpful criticism of the manuscript.
REFERENCES
Agren, G.I., 1983. Nitrogen productivity of some conifers. Can. J. For. Res., 13: 494-500. Brix, H. and Mitchell, A.K., 1986. Thinning and nitrogen fertilization effects on soil and tree water stress in a Douglas-fir stand. Can. J. For. Res., 16: 1334-1338. Butcher, T.B., 1977. Impact of moisture relationships on the management of Pinus pinaster Ait. plantations in Western Australia. For. Ecol. Manage., 1: 97-107. Crane, W.J.B., 1981. Growth following fertilization of thinned Pinus radiata stands near Canberra in south-eastern Australia. Aust. For., 44: 14-25. Cremer, K.W. and Meredith, E.M., 1976. Growth of radiata pine after row thinning compared with selective thinning. Aust. For., 39: 193-200. Flinn, D.W. and Aerberli, B.C., 1982. Establishment techniques for radiata pine on poorly drained soils deficient in phosphorus. Aust. For., 45: 164-173. Heffernan, B., 1985. A Handbook of Methods of Inorganic Chemical Analysis for Forest Soils, Foliage and Water. CSIRO Australia, Division of Forest Research, Canberra, 281 pp. Hunter, I.R., Graham, J.D., Prince, J.M. and Nicholson, G.M., 1986. What site factors determine the 4-year basal area response of Pinus radiata to nitrogen fertilizer? N.Z.J. For. Sci., 16: 3040. Kerruish, C.M. and Shepherd, K.R., 1982. Thinning practices in Australia - a review of silvicultural and harvesting trends. N.Z.J. For. Sci., 12: 140-161. Mead, D.J. and Gadgil, Ruth L., 1978. Fertiliser use in established radiata pine stands in New Zealand. N.Z.J. For. Sci., 8: 105-134. MSller, C.M., 1954. The influence of thinning on volume increment. SUNY Coll. For. Tech. Publ., 76: 5-32. Snowdon, P. and Waring, H.D., 1984. Long-term nature of growth responses obtained to fertilizer and weed control applied at planting and their consequences for forest management. In: D.C. Grey, A.P.G. SchSnau and C.J. Schutz (Editors), Proc. IUFRO Symp. Site and Productivity of Fast-Growing Plantations, 30 April-11 May, Pretoria and Pietermaritzburg, South Africa. Forestry Research Institute, Pretoria, pp. 701-711. Snowdon, P. and Waring, H.D., 1985. Effects of factorial combinations of urea, dicalcium phosphate, gypsum and potassium chloride on growth and foliage composition of closely spaced Pinus radiata. Aust. For. Res., 15: 333-352. Solomon, D., 1982. Diagonal line thinning in pine plantations. S. Afr. For. J., 120: 69-72. Spiecker, H., 1987. Dungung, Niederschlag under der jahrliche Volumen-zuwachs einiger Fichtenbestande Sudwestdeutchslands. Allg. Forest. Jagdztg., 158: 70-76. Steinbrenner, E.C., 1967. Research in forest fertilization at Weyerhaeuser Company in the Pacific
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Northwest. In: Forest Fertilization: Theory and Practice. Tennesee Valley Authority, Muscle Shoals, AL, pp. 209-215. Stiell, W.M., 1982. Growth of clumped vs. equally spaced trees. For. Chron., 58: 23-25. Turner, J., 1982. Long-term superphosphate trial in regeneration of Pinus radiata at Belanglo State Forest, N.S.W. Aust. For. Res., 12: 1-9. Turner, J. and Lambert, M.J., 1987. Forest water usage and interactions with nutrition of Pinus radiata. Acta Oecol./Oecol. Plant., 8: 37-43. Wagle, R.F. and Beasley, R.S., 1968. Two year effects of thinning and nutrient treatments on the growth of ponderosa pine. J. Ariz. Acad. Sci., 5: 45-55. Waring, H.D., 1969. The role of nitrogen in the maintenance of productivity in conifer plantations. Commonw. For. Rev., 4: 226-237. Waring, H.D., 1980. Silvicultural and management problems in supplying nitrogen for production forestry. In: R.A. Rummery and F.J. Hingston (Editors), Managing Nitrogen Economics of Natural and Man-Made Forest Ecosystems. CSIRO, Australia, Division of Land Resources, Perth, pp. 83-23. Waring, H.D., 1981. Forest fertilization in Australia: early and late. In: N.D. Turvey (Editor), Proc. Australian Forest Nutrition Workshop, Productivity in Perpetuity, Canberra, 10-14 August 1981. CSIRO Australia, Division of Forest Research, Canberra, pp. 201-217. Weetman, G.F., 1971. Effects of thinning and fertilization on the nutrient uptake, growth and wood quality of upland black spruce. Pulp Pap. Res. Inst. Can. Woodl. Pap., 28:18 pp. Woollons, R.C., 1985. Problems associated with analyses of long-term Pinus fertilizer × thinning experiments. Aust. For. Res., 15: 495-507. Woollons, R.C. and Will, G.M., 1975. Increasing growth in high production radiata pine stands by nitrogen fertilizers. N.Z.J. For., 29: 243-253.
313
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
Growth Responses by P i n u s r a d i a t a to Combinations of Superphosphate, Urea and Thinning Type
P. SNOWDON and H.D. WARING
Division o/Forest Research, CSIRO, P.O. Box 4008, Queen Victoria Terrace, Canberra, A.C.T. 2600 (Australia) (Accepted 6 January 1989)
ABSTRACT Snowdon, P. and Waring, H.D., 1990. Growth responses by Pinus radiata to combinations of superphosphate, urea and thinning type. For. Ecol. Manage., 30: 313-325. A 2~ factorial experiment testing the effects of urea (N), superphosphate (P) and two thinning types (diagonal-line thinning and thinning from below) on the basal-area increment of 18-yearold Pinus radiata (D. Don) is described. Although the trees had received localized applications of superphosphate shortly after establishment they were considered at the age of 18 years to be phosphate-deficient because concentrations of phosphorus in the foliage were low (0.065% P). Application of superphosphate (200 kg P ha - 1) increased concentrations in the foliage (0.132 % P) and increased the 7-year basal-area increment by approximately 70%. No growth response was obtained when urea (476 kg N ha-1 ) was applied alone. When both fertilizers were applied there was a further increase in basal-area increment of 81% on line-thinned plots and 26% on plots which had been thinned from below. This interaction reversed the normal trend in which plots thinned from below, by virtue of their greater initial basal area, produced 11-16% more increment than those which had been line-thinned. A model of tree growth using initial basal area and a competition index as independent variables was adequate to describe differences in basal-area growth between thinning types except where urea and superphosphate had both been applied. In the case of N + P-treated plots, the reciprocal of the competition index was required as an additional variable. Analysis of the development of the growth responses over time indicated that application of superphosphate had produced sustained improvement in growth rates but that the response to urea occurred only in some growingseasons. In line-thinned plots the response to urea in addition to phosphate occurred in the 2nd, 3rd and 4th growing-seasons, but the response occurred only in the 2nd and 4th growing-seasons in plots thinned from below. It is argued that this differential response between thinning types can be attributed to differences in stand density and water availability. In order to maximise the growth response to added nitrogen it is important to reduce competition by paying particular attention to both tree spacing and residual basal area.
0378-1127/90/$03.50
© 1990 Elsevier Science Publishers B.V.
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INTRODUCTION On nutrient-deficient sites, fertilization with the deficient element tends to give a growth response irrespective of stocking, but on better sites thinning may be a prerequisite for a response to fertilizer. In view of the dramatic effects that thinning has on stand structure and subsequent development, it is conceivable that timing, intensity or method of thinning will influence growth responses to fertilizer application. Many experiments have compared responses to fertilizer in thinned and unthinned stands, but few have examined different thinning intensities (Steinbrenner, 1968; Wagle and Beasley, 1968; Weetman, 1971; Woollons and Will, 1975; Mead and Gadgil, 1978). Apparently the only study which has examined different types of thinning is that of Wagle and Beasley (1968), who compared the effects of fertilizer on stands which had been thinned from below with those that had been thinned from above. They obtained growth responses to fertilizer when stands were thinned lightly from above or below and to heavy thinning from above, but there was no fertilizer response in stands thinned heavily from below. Increased mechanisation has made line or row-thinning an economically attractive alternative to selective methods of thinning (Kerruish and Shepherd, 1982). Consequently, it is pertinent to consider if the responsiveness of stands to fertilization is dependent upon the type of thinning. This paper examines growth response by Pinus radiata (D. Don) to nitrogen and phosphorus fertilization on two extreme thinning types: diagonal-line thinning (Solomon, 1982 ) which retained trees in a square spacement irrespective of size or form, and thinning strictly from below to retain the largest trees irrespective of spacing and form. METHODS The experiment was located near Moss Vale, N.S.W., in the Belanglo State Forest (34.6 ° S., 150.4 ° E. ) which is 145 km southwest of Sydney. The elevation is ca. 670 m, and the topography is gently undulating. Mean annual rainfall is 880 mm (range 513-1250 mm), distributed fairly evenly throughout the year. The mean daily maximum temperature is 24.5 °C in January while the mean daily minimum temperature is 1.6 ° C in July. The soils are podzolics ca. 1.5 m deep derived from Hawkesbury Sandstone, a Triassic sandstone which includes shale lenses. They are infertile, the primary nutrient deficiency being that of phosphorus. Large growth responses can be obtained to nitrogen once the phosphorus deficiency has been alleviated, but there is no evidence that other nutrients are inadequate to support healthy growth of P. radiata (Waring, 1969). The experimental site had previously carried a full rotation of P. radiata. After clearfelling, it was planted again in 1962 with P. radiata at a stocking of
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1680 stems h a - 1 with square spacement (2.44 m). At that time an experiment was established which tested the effect of various non-factorial combinations of superphosphate (0, 36, 73, 96 kg P ha -1) and urea (0, 58, 116, 232 kg N h a - 1) applied close to the seedling soon after establishment and/or at 1 and 2 years of age. By 18 years of age the basal area of unfertilized trees was 19 m 2 ha -1, compared to 31 m 2 ha -1 for those at the lowest rate of superphosphate and 39 m 2 h a - ~for the two highest rates. At this time there was no significant growth response to urea. In 1980, 32 of the fertilized plots in this experiment were used in a 2 × 2 × 2 factorial testing the effects of two thinning types (T), the application of superphosphate (P), and of urea (N) on subsequent pine growth. There were four replications of the factorial assigned to eight blocks so that the N × P × T interaction was partially confounded. Blocking was balanced according to prior rates of phosphorus application. Each plot was square, and prior to thinning contained 81 trees. The fertilized portion of the plot consisted of the central 49 of these, while growth assessment was made on the central 25 trees. Both thinning treatments reduced the number of stems by half. In the first case, 'thinning from below', the stems with largest diameters at breast height were retained irrespective of tree form or spacing. In the second case, 'diagonalline thinning', all trees in each 2nd diagonal row were removed. Superphosphate was applied at 0 and 200 kg P ha-1 and urea at 0 and 476 kg N ha -1, in both cases, half in September 1980 and the remainder in October 1981. In addition to the main experiment there were eight unthinned plots, of which two had never been fertilized, two had been fertilized at establishment, and four had been fertilized at establishment then treated with superphosphate and urea in the same manner as the thinned plots. Since thinning type influenced initial basal area {Table 1 ), analyses of covariance were made using separate regressions for each thinning type (Woollons, 1985). Comparisons between thinning types were made using a pooled estimate for error. TABLE 1 Characteristics of t h i n n e d a n d n o n - t h i n n e d P. radiata plots at the beginning of the experiment
Stocking (stems h a - 1) Mean diameter (cm) Basal area (m 2 h a - 1) Competition index (m 2 h a -1 ) Reduction Remaining
Non-thinned
Diagonal line-thinning
Thinned from below
1680 16.4 37.2
840 16.3 18.4
840 19.5 25.4
0 31.5
17.7 13.0
11.2 20.2
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P. SNOWDON AND H,D. WARING
A simple competition index was calculated for each tree. The index was a weighted average of the basal area of trees occupying the nearest eight planting positions surrounding the subject tree; trees on the s;des of the square were given a weight of 1.0 while those on the corners had a ":'~!~, ~ 1/x/~. The index was summed within each plot, then used with initial basal area and factors representing the various fertilizer and thinning treatments, to descriptively model the basal-area increment of the plots. At the time of thinning, unweighed composite samples of 1-year-old foliage from six felled trees in each thinned plot were made. Seven years later, individual samples of 1-year-old and 2-year-old foliage were collected during winter from the upper crown of five trees in each plot. Total nitrogen, phosphorus and potassium were determined after digestion with sulphuric acid/hydrogen peroxide (Heffernan, 1985). RESULTS
Initial conditions At age 18, non-fertilized trees had a basal area of 19 m 2 ha -1. Trees at the lowest level of phosphorus had produced 31 m 2 h a - ' while the two highest rates had both produced 39 m 2 ha -1. These growth responses are similar to those obtained 17 years after broadcasting superphosphate onto young naturally regenerated P. radiata elsewhere in the forest (Turner, 1982). Despite the apparent culmination in growth response at or below 73 kg P ha -1, the foliar concentrations of phosphorus (650/lg g- 1) were clearly in the range associated with P-deficiency. At the time of thinning there was no significant effect of urea applied during the establishment phase. Foliar concentrations of nitrogen (1.29%) and potassium (0.59%) were adequate. Diagonal-line thinning resulted in little change in average diameter compared with the unthinned plots, and reduced basal area by about 50% (Table 1). In comparison, trees on plots thinned from below were 3.2 cm larger in diameter, and basal area was reduced by only 31%. The average competition index on plots which had been diagonal-line-thinned was reduced by 58% of pre-thinning values, but on plots thinned from below reduced by only 36%.
Growth 7 years after thinning Cumulative basal-area increment for the 7-year-period following treatments is shown in Table 2. The increment on unthinned plots which had been fertilized only at establishment was slightly larger than that obtained on the thinned plots, but was substantially greater than that obtained on plots which had never been fertilized (6.31 m 2 h a - ' ) . This indicates that there was a marked residual effect of the initial application of phosphate fertilizer. Fertilization of
GROWTH OF PINUS IN RESPONSETO FERTILIZERS AND THINNING
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TABLE 2 Effect of fertilization with urea (N) and superphosphate (P) on the 7-year basal-area increment (m 2 h a - 1) of t h i n n e d and n o n - t h i n n e d P. radiata
Non-thinned Diagonal line-thinning T h i n n i n g from below
Nil
N
P
N+P
CD1
10.68 9.27 10.27
-8.39 9.25
-15.64 17.85
19.42 23.25 20.50
5.12 3.11 2.68
~Critical difference P < 0.05.
unthinned plots with urea and superphosphate resulted in a large increase in basal area increment but this was smaller than on the thinned plots. On the thinned plots, application of superphosphate resulted in additional growth averaging 7.0 m 2 h a - 1. Application of urea alone resulted in a small but nonsignificant reduction in increment, but when it was added in combination with superphosphate a positive growth response was obtained amounting to 2.65 m 2 h a - 1 on plots thinned from below and 7.61 m e h a - 1 on diagonal-linethinned plots; the magnitudes of these responses being different with probability P < 0.09. As a consequence, total increment for the diagonal-line-thinned N + P plots was greater than those thinned from below ( P < 0.05). This reverses the normal expectation, illustrated by the results obtained with the other fertilizer combinations, that greatest increment is obtained from stands with the greatest initial basal area. In terms of total growing stock, the initial difference between thinning treatments of 7.0 m 2 ha-1 was reduced to 4.25 m 2 h a - ~ 7 years after applying urea and superphosphate.
Development of response The development of basal area over the 7-year growing period is shown in Fig. 1 for both thinning types. There was a significant response in basal-area increment to phosphorus on diagonal-line-thinned plots in the 1st growingseason, but response was delayed until the 2nd growing season on plots thinned from below. Thereafter the magnitude of responses with time and became greater; however, this response was not significant with plots thinned from below, probably because trees on these were initially greater in size. Analyses of annual basal-area increment, using basal area at the beginning of the growing-season as a covariate, showed that growth responses directly attributable to phosphate application, rather than to prior differences in tree size, continued until the end of the period of observation. This suggests that a type-2 response ( Snowdon and Waring, 1984) has occurred by which site productivity has been improved in the long term by the heavy broadcast application of phos-
318
P. SNOWDONANDH.D.WARING I
~ N + P P N+P
Thinning type40 ~_'--~ Fri°gmnballliW e
~
0//~'~"
r
30
....~.XINil
I"
I
I
I
I
I
i
J
1
2
3
4
5
6
7
Years Fig. 1. Effect of diagonal line-thinning and thinning from belowin combination with applications of nitrogen and phosphorus on the growth of P. radiata over a 7-year period. phate to plots which were, for the most part, phosphate-deficient owing to the fact that earlier phosphate dressings had been localised. The response to nitrogen in the presence of phosphorus (N + P ) - became significant after two seasons growth on diagonal-line-thinned plots and after four seasons on plots thinned from below. Analysis of covariance of annual basal area increments, as above, showed that responses to urea occurred in the 2nd, 3rd and 4th growing-seasons on diagonal-line-thinned plots but only in the 2nd and 4th seasons on those thinned from below. This suggests that a type-1 response (Snowdon and Waring, 1984) had occurred, with nitrogen hastening crown development after thinning b u t not improving site quality in the longer term. Stand-growth model
The model used to describe the 7-year basal-area increment on all plots assumed: (1) that there was no growth in the absence of initial basal area, i.e. the model was constrained to pass through the origin; and (2) that increment increased linearly with initial basal area; but that (3) the rate of increase in increment decreased linearly as competition index increased. Separate relationships were allowed for the various combinations of fertilizer. Initial basal area was highly correlated (r=0.94} with competition index.
GROWTH OF PINUS IN RESPONSETO FERTILIZERSAND THINNING
319
Consequently, when the former was used as an estimate of competition in the model, 84% of the variance was accounted for compared to 90% when the competition index was used. In both cases, the increment on plots other than those receiving both urea and superphosphate were adequately described. In the case of the exception, the residuals on plots which had been diagonally line-thinned, and to a lesser extent those which had not been thinned, tended to be positive while residuals for plots which had been thinned from below were negative. This suggested that increment on N + P-treatment plots was curvilinearly related to competition index. Subsequently, the reciprocal of competition index was used as an additional predicting variable on N + P-treated plots. This model accounted for 94% of the variance. The final model took the form
A~ =Ab (a+ bCi+c/Ci) where: zlA~ is basal area increment; Ab, initial basal area; Ci, competition index immediately after thinning; and the coefficient c takes the value zero for other than the N + P-treatment. Figure 2 illustrates the manner in which the rate of the 7-year basal-area increment growth per unit initial basal area (A¢/Ab) declines with increasing Ci. For clarity, data for the N-treated plots are omitted. The model obviates the need to use factors to define whether the stands have been thinned, and if so, in what manner. Application of the model to increments in individual growing-seasons showed 1.5
N-P%
U DB Nil • A •
1.0
o. 0.5
A
•
Nil ~
!
I
I
10
20
30
Cornpetition
index
(r',q 2 h a -; )
Fig. 2. The effect of combinations of nitrogen and phosphorus fertilization and competition index on the growth rate (A~/Ah) of u n t h i n n e d (U), diagonal-line-thinned (D) and t h i n n e d from below
( B ) Pinus radiata.
320
P. SNOWDONANDH.D.WARING
TABLE 3 Concentrations of nitrogen, phosphorus and potassium in 1-year-old and 2-year-old foliage of P. radiata at the beginning of the experiment and 7 years after thinning and fertilization with combinations of urea (N) and superphosphate (P) Treatment
Before treatment After 7 years Never fertilized Not thinned Nil N+ P Thinned Nil N P N+P SED 1
Nitrogen ( % )
Phosphorus (#g/g)
Potassium (%)
1-year
2-year
1-year
2-year
1-year
2-year
1.29
--
650
--
0.59
--
1.31
1.31
664
502
0.48
0.36
1.18 1.14
1.14 1.05
755 1178
620 1015
0.45 0.42
0.38 0.44
1.24 1.41 1.23 1.25 0.05
1.20 1.57 1.07 1.05 0.06
764 741 1321 1228 39
584 575 973 956 44
0.49 0.45 0.55 0.43 0.02
0.42 0.34 0.55 0.41 0.02
1Standard error of difference between means for thinned plots only. t h a t , in t h e 1st s e a s o n it a c c o u n t e d for 14% of t h e v a r i a n c e b u t t h e r e a f t e r a c c o u n t e d for 76-91%. T h e c u r v i l i n e a r c o m p o n e n t o n N + P - t r e a t e d plots was significant in t h e 2nd, 3rd a n d 4 t h growing seasons. T h e r e a f t e r , it b e c a m e nonsignificant a n d t h e effect o f c o m p e t i t i o n o n N + P - t r e a t e d plots b e c a m e indist i n g u i s h a b l e f r o m t h a t o n P - t r e a t e d plots.
Foliar analysis T h e c o n c e n t r a t i o n s o f N, P a n d p o t a s s i u m ( K ) in foliage s a m p l e d at the e n d o f t h e 7th g r o w i n g - s e a s o n are s h o w n in T a b l e 3. A p p l i c a t i o n o f s u p e r p h o s p h a t e i n c r e a s e d P c o n c e n t r a t i o n s in 1-year-old foliage to above t h e deficiency limit ( 1000 ttg P g - 1) b u t c o n c e n t r a t i o n s in t h e N + P t r e a t m e n t were significantly lower t h a n t h o s e w h i c h h a d received P alone. A p p l i c a t i o n of u r e a i n c r e a s e d foliar c o n c e n t r a t i o n s o f N w h e n it was applied in t h e absence o f s u p e r p h o s p h a t e , b u t h a d n o effect w h e n P was applied. U r e a also r e d u c e d t h e c o n c e n t r a t i o n s o f K, w h e r e a s a p p l i c a t i o n of s u p e r p h o s p h a t e alone i n c r e a s e d K concentrations. DISCUSSION T h e initially low c o n c e n t r a t i o n s o f P in foliage, t o g e t h e r with t h e increase in c o n c e n t r a t i o n s a n d t h e s u b s t a n t i a l g r o w t h r e s p o n s e a f t e r a p p l i c a t i o n of su-
GROWTH OF P I N U S IN RESPONSE TO FERTILIZERS AND THINNING
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perphosphate, clearly indicate that the trees were P-deficient at the time of thinning. This result suggests that even high rates of P applied to a localised area near to the transplant are insufficient to sustain good growth and to maintain foliar concentrations at sufficiency levels after the stand has closed canopy. In an adjacent experiment, there was no redistribution of P from the site of application to soil in the remainder of the plot (Waring, 1981 ). Under such circumstances, it has been argued that the opportunity for P uptake by P. radiata from localised applications may be limited, particularly when there is rapid depletion of moisture due to root proliferation in the fertilizer-enriched zone, and it is unlikely that the P requirements of an expanding tree-biomass can be met (Flinn and Aeberli, 1982 ). The response to superphosphate alone was 7.0 m 2 h a - 1 when averaged over both thinning types. This is consistent with the results of an adjacent experiment where a growth response was obtained to the reapplication of phosphate (36 kg P h a - 1), when plots which had initially received a localized application of phosphate (36 kg P ha-1), were thinned at 9-years of age (Waring, 1980). The response confirms the diagnosis of P deficiency based on foliar analysis. Application of urea alone resulted in a small but nonsignificant reduction in increments with both thinning types. Such reductions are common when P. radiata transplants are fertilized with N on P-deficient soils (Snowdon and Waring, 1985) and the effect has also been noted in older stands (Hunter et al., 1986). When urea was applied with superphosphate, a further increase in basalarea increment was obtained, parallelling other results obtained in this forest by Waring (1980) after reapplication of fertilizer (36 kg P, 116 kg N ha -1). Combinations of N and P have been applied at the time of thinning to two other stands in Belanglo State Forest, but in neither case was the growth response as substantial as here. In the first case, 128 kg P and 324 kg N ha-1 were added to a third row-thinning in a 16-year-old stand which had earlier received two broadcast applications of 70 kg P h a - 1as superphosphate ( Crane, 1981). In the second case, 135 kg P and 171 kg N ha -1 were added to a selectively thinned, naturally regenerated stand about 20 years old which had previously received broadcast applications of 121 kg P, 47 kg N and 115 kg K h a - 1 (Woollons, 1985 ). It is reasonable to assume in both cases that the component of the response attributable to P had been reduced by the heavy early applications of phosphate and the fact that these had been broadcast, so that the entire root system had access to elevated levels of soil P. However, even if the responses in these experiments were entirely due to N, the magnitude of the responses was much smaller than those observed here. The rates of N application were lower but were substantially more than that required for crown expansion. The response to urea in addition to superphosphate was much greater on plots which had been systematically thinned than those which had been thinned
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P. S N O W D O N A N D H.D. W A R I N G
from below, despite the higher initial basal area of the latter. This occurred because competition between trees markedly affects A~/Ab. The initial basal area of plots thinned from below was 1.38 times that of those diagonally linethinned (Table 1 ). Consequently, the latter must grow at a rate greater than 1.38 times the former in order to achieve a greater increment. Using data from Table 1 for the average levels of competition index, and the corresponding coefficients for A~/Ab used for Fig. 2, the rate of growth for diagonally linethinned plots fertilized with N + P was 1.63 time that of the corresponding plots thinned from below, thus fulfilling the conditions for a greater total increment. By contrast, on P-treated plots the growth rate of those diagonally line-thinned was only 1.25 times that of those thinned from below; thus they produced less total increment. Although competition between neighbouring trees is recognised as an important factor affecting tree growth, there have been few attempts to quantify this in plantations. If tree size is ignored, and only distances are considered in the competition index described above, then a relative measure of competition can be calculated for various methods of systematic thinning. These can be compared with the expected competition index value (Ce) of 0.85 in an unthinned stand. Cremer and Meredith (1976) present data for the growth of P. radiata which had each second row (Ce = 0.25) or every third row (Ce--0.55) removed at thinning. The initial basal area for the latter was 1.35 times that of the former, but A~/Ab for the former was 1.32 times the latter, with the consequence that there was a compensation in total increment over a 6-yearperiod. This confirms with the principle that for closed stands there is no loss in total increment until thinning intensity exceeds 50% (MSller, 1954). Stiell (1982) compared the growth of single trees ofP. resinosawhich had been completely released by thinning (Ce = 0.0 ) with those which had been retained in clumps of four trees (Ce = 0.34 ). Initial basal area was similar but over a 15year period the former produced 19% more total increment, indicating that A~/Ab had been greater. These results illustrate that A~/Ab tends to decline with an increase in Ci, that total increment depends upon both initial basal area and Ci, and Ci is influenced by the method of thinning. In the experiment described here it is also shown that addition of fertilizer can increase A¢/Ab, and that the increases are greater with lower Ci. In particular, fertilization with both N and P can increase A~/Ab sufficiently to overcome the disadvantage of smaller initial basal area and thus to produce greater total increment. This indicates that the response to N is particularly sensitive to the degree of competition in the stand. A number of studies in coniferous forests have shown that growth and degree of response to fertilization depends upon the amount of annual or seasonal precipitation (Butcher, 1977; Turner, 1982; Spiecker, 1987; Turner and Lambert, 1987). It is tempting to argue that the effect of competition on fertilizer response in this trial is predominantly due to differences in water relationships
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between the different thinning regimes. Precipitation (July-June) was less than two-thirds of the long-term average during the first three growing-seasons, slightly below average for the next three, and 170 mm greater than average for the last season. Equations provided by Turner and Lambert (1987) for stands in this forest were used to estimate interception by the different stands. Upon the dry seasons, an average of 160 mm would be intercepted by the u n t h i n n e d stands while throughfall (Pt) would increase by 33 mm and 53 m m in stands thinned from below and those diagonally line-thinned, respectively. Based on these figures, the mean annual increment over the first three growing seasons (A:b), and the assumption that all throughfall is utilized by the trees, the average water-use efficiency (A~b/Pt) was 0.0047, 0.0044 and 0.0053 m 2 m m - 1 year- 1 for N + P-treated plots unthinned, thinned from below, and diagonal-line-thinned, respectively. The lower efficiency of trees thinned from below compared to those u n t h i n n e d could be attributed to the failure of the former, due to incomplete occupation of the site, to exploit 20 mm (4.5%) of the available water. Diagonal-line-thinned trees were clearly more efficient. It can be reasonably be assumed that the rate of water-use would be proportional to the basal area of the stands, i.e. the rate of water-use on plots thinned from below would be 1.38 times that on diagonal-line-thinned plots. Under circumstances of low and intermittent rainfall, the periods when soil conditions were favourable to N mineralization and nutrient uptake would be extended on diagonal-line-thinned plots. This could result in a greater utilization of the applied urea. While the extended growing-season in itself would not improve water-use efficiency, the greater uptake of N by the trees leads to the potential for greater N productivity (~,gren, 1982) and/or greater water-use efficiency (Brix and Mitchell, 1986) and hence the greater growth rates observed on the diagonal-line-thinned plots. CONCLUSIONS The results have important implications for forest management. It is apparent that localized application of even heavy rates of P during the establishment phase may be insufficient to maintain the health and growth-rate of plantations at optimum levels. It is also apparent that applications of N at the time of thinning will not result in an increase in growth if there is a dominating deficiency in P. The response to N after addition of P was greatest on plots which had been diagonal-line-thinned. This qualitative difference between thinning types can be resolved by considering the residual competition index. Greatest total increment obtained with added N is obtained when Ci is minimized; to achieve this, particular attention needs to be paid to the spacing of the trees as well as to the residual basal area. The results suggest that, when N is to be applied, the stand might profitably be thinned to a greater extent than
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normal. For commercial thinnings, the extra revenue thus obtained might well cover the costs of fertilizer and its application. ACKNOWLEDGEMENTS
The field trial was established on a site provided by the Forestry Commission of N.S.W. Trial establishment and measurement were ably conducted by A. Eilert and J. Holtzappfel, while J. Harragan performed the chemical analyses. Dr. J. Leech and Mr. R.C. Woollons provided helpful criticism of the manuscript.
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