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EFFECT OF PHOSPHORUS (P) AND POTASSIUM (K) FERTILISER ON TREE
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GROWTH AND DRY TIMBER PRODUCTION OF PINUS PATULA ON GABBRO-
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DERIVED SOILS IN SWAZILAND
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J.W. Crous1, 3, A. R. Morris2, and M. C. Scholes3
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1Sappi
Forests (Pty) Ltd, P.O. Box 372, Ngodwana, 1209, South Africa.
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2Shaw
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Research Centre, Sappi Forests (Pty) Ltd, P.O. Box 473, Howick, 3290,
South Africa
3School
of Animal, Plant and Environmental Sciences, University of the Witwatersrand,
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Private Bag X3, WITS 2050, South Africa
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Corresponding author : jacob.crous@sappi.com
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A trial designed to determine the optimum timing and rate of application of PK fertiliser to
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mitigate a growth decline observed in P. patula stands located on the gabbro-derived
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soils (underlying 13% of the plantation area) of the Usutu Forest, was sampled at rotation
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age (15 years). The P and K fertiliser was applied in three quantities, namely: 20/20,
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40/40 and 80/80 kg P and K per hectare. The 40/40 and 80/80 quantities were applied a)
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all at planting; b) all after pruning at the age of five years; and c) as a split application
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where 20 kg of P and K per ha, respectively was applied at planting as a spot application
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and the remaining fertiliser applied after pruning (at age five years) as a broadcast
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application. Tree survival was not affected by either the quantity or time of application of
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the fertiliser. The quantity of fertiliser applied had a greater effect on the increase in tree
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growth and dry timber production than the timing of the application. There were
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indications that the early fertiliser application produced more timber than the late fertiliser
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application. The application of 80/80 kg P and K per hectare significantly increased the
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quadratic mean diameter at breast height (DBH) from 20.0 to 23.0 cm; mean tree height
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from 19.6 to 20.4 m; and volume production by 83 m³ ha-1 (29%). These results support
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the current fertiliser prescription on previously unfertilised stands. Furthermore the
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application of fertiliser resulted in a more uniform DBH distribution. Compared to the
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control, the application of 80/80 fertiliser decreased basic wood density by 3% (from
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0.3718 to 0.3610 g cm-3). The positive volume growth response to the application of
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fertiliser outweighed the disadvantage of a slight decrease in basic wood density as 28 t
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ha-1 (26%) more dry timber was produced as a result of fertilisation. In order to maximise
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dry wood production per unit area, the application of PK fertiliser to previously unfertilised
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gabbro-derived soils at the current recommended rate should continue. The applicability
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of the findings from this study to other P. patula stands on soils originating from basic
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geologies (dolerite, diabase, basalts and other igneous rocks, having similar low P and K
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contents) in southern Africa should be investigated.
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KEYWORDS
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plantations; pulp wood; yield; stand structure; basic wood density; time of fertiliser
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application
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INTRODUCTION
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The 60 000 ha Usutu Forest plantations in Swaziland supplies raw material to a kraft pulp
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mill, mostly from planted Pinus patula trees grown on a 16 year rotation. Long-term
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growth monitoring showed that 95 kg ha-1 phosphorus (P) and potassium (K) fertiliser,
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respectively, had to be applied to 13% of the plantation area, located on gabbro-derived
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soils, in order to maintain or increase volume production from these sites in subsequent
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rotations (Morris 1986, 1987a, 1994a, 2003, Evans 2005). After initial studies showed that
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the application of PK fertiliser improved growth, further trials were established to
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determine the optimum application rate and timing of application to P. patula on these
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gabbro-derived soils (Morris 1987a).
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In exotic species plantations with very rapid growth rates the maintenance of wood quality
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is just as important as the maintenance of timber production (Jagels 2006). Therefore, the
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increase in volume production after fertilisation should be investigated in association with
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wood quality changes, e.g. basic wood density. The quantity of fertiliser, time of
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application and fertiliser type can have an effect on wood properties (Zobel 1992) by
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changing growth patterns of trees, especially the crown-stem relationship (Larson et al.
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2001). A summary of 44 studies with 16 conifer species showed that, in general, fertiliser
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reduced the basic wood density of the timber. The decrease in basic wood density was
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mostly related to an increased period of low density juvenile wood production after
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fertilisation (especially when nitrogen fertiliser was applied), and to an increase in the
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percentage of low density earlywood compared to higher density latewood (Zobel and van
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Buijtene 1989 cited in Zobel, 1992, Beets et al. 2001).
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The rotation age results from a trial planted on a gabbro lithology at Usutu, where various
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quantities of P and K fertiliser were applied to P. patula in a factorial design at different
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stages of stand development, with two levels of weed control are reported here. The
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objectives of this paper are to determine the optimum quantity and time of application of
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fertilizer in order to maximise dry timber production on these sites. Furthermore, the
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effects of fertilisation on stand structure and wood density, which have not been studied
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on these sites previously, are reported.
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MATERIALS AND METHODS
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Trial establishment
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The trial site was situated in compartment A11 (31°4’12”E, 26°27’37”S), at an altitude of
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1250 m
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deep (>1.2 m), red clay soil, derived from gabbro lithology can be classified as an Oxisol
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(Soil Survey Staff 2006). A mean annual temperature of 16.3°C and a mean annual
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precipitation of 1421 mm is reported for this site (Pallett 1990).
above sea level on a gentle upper mid-slope position. The well-drained,
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The previous crop was planted in 1967 and represented the second rotation of P. patula
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planted on this site. Clearfelling of the previous crop was carried out between November
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and December 1991, during which tree lengths were extracted with a cable skidder.
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Harvest residue was left in situ and was not burnt.
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The trial was designed as a 3 x 2 x 2 factorial to test time of application, fertiliser quantity
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and effect of intensive weed control. Two quantities of 0:1:1(17) fertiliser, supplying 40 or
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80 kg ha-1 each of P and K, were applied either all at time of planting (Early), or as a split
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with 20 kg ha-1 each of P and K at time of planting and the rest following pruning (age 5)
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(Split), or all as a post-pruning application (Late) (Table 1). The additional treatments to
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the factorial combinations were a control, where no fertiliser was applied, and a single
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application of 20 kg ha-1 of P and K, respectively, at time of planting. The eight fertiliser
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treatments were tested in combination with either routine operational weeding or intensive
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weeding aimed at removing all competing vegetation. The sixteen treatments were laid
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out as four complete randomised blocks using 7 x 7 row plots at an espacement of 2.74 m
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squared (Morris 1994b).
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The trial was planted on 24th March 1992. Fertiliser was applied on 9th April 1992 as spot
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applications 25 cm from the seedling. The 20/20 fertiliser quantity treatment was applied
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as a single spot, the 40/40 fertiliser quantity treatment was split between two spots on
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opposite sides of the seedling and the 80/80 fertiliser quantity treatment was applied in
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four spots around the seedling. The seedlings were treated with Bexadust (active
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ingredient: Gamma BHC) at the root collar in June, 1992, after a small quantity of
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mortality, associated with the bark beetle, Hylastes angustatus, had been observed.
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However, mortality was minor with less than 5% losses at 17 months after planting, and
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was unrelated to treatment. The trial was not blanked (replacement of dead seedlings).
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Insert Table 1
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When planted, the site was weed free. Weeding of plots receiving the normal operational
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treatment was only confined to cut stump chemical control of bugweed (Solanum
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mauritianum) in June, 1993, and again in April, 1994. This was carried out as part of the
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routine weeding of the surrounding operational stand. The intensively weeded plots
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received, in addition to the operational treatments, two manual hoeing treatments to
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remove all weeds on the plots in February and December 1993. Pruning, to a height of
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two metres, was done during May 1997 and a broadcast application of fertiliser, following
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pruning, was done on 11 September 1997.
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Sampling procedure and chemical analyses
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Tree growth measurements
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All surviving trees of the inner 5 x 5 rows of each plot were assessed for height in June
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1994 and height and DBH (Diameter at Breast Height, 1.3 m above the soil surface) in
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July 1997. In October 1999, May 2001, April 2003, May 2004, April 2005, April 2006 and
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February 2007 the DBH of all 25 trees was measured, but only five trees per plot were
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assessed in terms of height.
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Timber and bark samples
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In order to do a more intensive analysis of basic wood density and nutrient content, four
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trees per plot were selected prior to clearfelling. The four trees included one closest to the
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12.5th percentile tree, two close to the median and one close to the 87.5th percentile tree.
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These were left standing when the trial was felled. Following clearfelling of the unmarked
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trees a 10 cm billet was collected at breast height from each of the remaining trees per
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plot by destructive sampling. As some of the marked trees were felled or damaged during
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harvesting, the mean values of each fertiliser treatment were based on the results from 12
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to 22 trees, covering the DBH range, in stead of the planned 32 trees. The overbark and
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underbark measurements of each disc were taken in field. The dry weight of the bark from
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each disc was determined after drying at 70 °C for 72 hours in a ventilated oven (Kalra
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and Maynard 1991).
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The basic wood density of timber samples was determined as it is one of the cheapest
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and easiest wood properties to measure and provides an indication of both the
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anatomical characteristics and structural properties of timber (Larson et al. 2001, Stanger
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et al. 2002). For wood, different definitions of density apply as a decrease in moisture
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content is associated with a volumetric shrinkage (Simpson 1993). In our study the discs
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used for density analysis were submerged in a dip tank for at least 24 hours until they
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became fully saturated with water. The green volume of each saturated disc was
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determined by volume displacement of water. Thereafter the discs were dried in a
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ventilated oven at 103 °C to a constant weight. The basic timber density was expressed
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as the dry weight to green volume ratio (Simpson 1993).
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Calculations and statistical analysis
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A logarithmic transformation of height values was conducted in order to determine the
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relationship between transformed height and the inverse of DBH in the trial with the least
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squares method of simple linear regression (Bredenkamp 1993). Utilizable volume
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estimation was performed from actual DBH measurements and regression heights using
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a Schumacher and Hall function (Bredenkamp 2000).
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There was clear evidence that taller trees had a lower mean basic wood density than
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shorter trees (Crous et al. 2009), which corresponds with results reported in other trials
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(Morris 1986, Wielinga et al. 2008). Therefore the basic wood density of the samples
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collected at breast height was adjusted using Equation 1 (Crous et al. 2009) to reflect the
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average basic wood density that could be expected if samples were collected along the
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entire tree stem.
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Avg. basic wood density of entire tree = 0.6697(Basic density at 1.3 m) + 0.1016
[1]
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Dry timber production was estimated by multiplying the total stem volume produced by
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the estimated basic wood density per plot. The stem bark mass was estimated per plot by
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the association between DBH, tree height and bark mass (Equation 2), as reported in
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Crous et al. (2009).
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Log10(TBM) = -1.848 + 0.7488 Log10(DBH².HT)
[2]
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Where,
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TBM = Total bark mass per tree (kg)
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DBH = Diameter at breast height (cm)
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HT
= Tree height (m)
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Analysis of Variance procedure (ANOVA) and multiple linear regression of the statistical
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software program, GenStat 10.2, were used for data analyses (Payne, 2007). In order to
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analyse the data of the complete trial simultaneously a dummy variable was used to
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distinguish between the three fertiliser treatment groups, i.e. the control treatment; the
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additional Early 20/20 treatment; and the factorial treatment combinations (two fertiliser
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quantities by three times of application). The initial ANOVA results showed that the weed
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control treatment had no significant effect on any of the growth parameters. Thus, the
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weed control treatment was treated as an additional replication and excluded as a fixed
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effect from the final ANOVA model (Equation 3).
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yijkl =  + Ri + Dj + D(A)j:k +D(T)j:l + D(AT)j:kl + eijkl
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Where,
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yijkl
= value of the plot mean of the lth time of fertiliser application with the kth quantity
of PK fertiliser, within the jth treatment group on the ith replication.
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
= overall mean
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Ri
= random effect of the ith replication
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Dj
= effect of the jth treatment group
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D(A)j:k = effect of the kth quantity of fertiliser within the jth treatment group
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D(T)j:l = effect of the lth time of fertiliser application within the jth treatment group
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D(AT)j:kl= interaction between the kth quantity of fertiliser and the lth time of fertiliser
application within the jth treatment group
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[3]
eijkl
= random error associated with the same subscript identifiers as that for yijkl
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The six plots where all the trees were felled during harvesting were treated as missing
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values in the analyses of bark mass, basic wood density and dry timber production.
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RESULTS
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Tree growth
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The intensive weed control treatment applied during the first two years after planting had
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no significant effect on any of the growth parameters that were assessed in the trial by the
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end of the rotation at the age of 15 years (data not shown).
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When the relationship between logarithmic transformed tree height and the inverse of tree
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diameter was investigated using multiple linear regression the results indicated that from
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the age of 7.6 years the application of fertiliser affected the transformed relationship of
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DBH to height significantly. The application of fertiliser did not change the slope of the
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regression line, but did increase the intercept. Generally, a tree that received the 80/80
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fertiliser quantity was taller than a tree in the control plots with a similar DBH (Figure 1).
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Insert Figure 1
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A summary of the ANOVA results of various tree growth parameters that were measured
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at clearfelling age (15 years) are presented in Table 2. Although the 80/80 fertiliser
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quantity treatments decreased (p < 0.10) the number of live stems between the age of 5
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and 12 years (data not shown), the effect was insignificant at the age of 15 years (Tables
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2 and 3). All other growth parameters were significantly affected by the fertiliser and
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responded similarly to the fertiliser application (Table 2). A significant difference was
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observed for the contrast between the control treatment and fertilised treatments (Table
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2). As could be expected, the time of application of the fertiliser had a greater effect when
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the trial was young, but declined towards clearfelling age (data not shown). Prior to
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harvesting, the time of application only had an effect on quadratic mean diameter (DBHq),
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where the late application of PK fertiliser after pruning produced trees with a smaller
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DBHq than the split application where the fertiliser was split in two applications – at
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planting and after pruning (Table 3). The effect of applying different quantities of fertiliser
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generally increased towards clearfelling age and resulted in significant increases in tree
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diameter, basal area and volume production per unit area. The volume growth increment
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since the age of five years up to clearfelling age illustrates this clearly and shows a
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divergence of the fertiliser treatments from the control with time (Figure 2). In terms of
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basal area and volume the 80/80 treatments performed better than the 40/40 treatments,
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which in turn performed better than the control treatment (Table 3). The 80/80 treatments
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resulted in an 83 m³ha-1 (29%) volume increase at rotation age compared to the control
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treatment.
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Insert Table 2, Table 3 and Figure 2
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Stand structure
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The effect of the fertiliser treatments on the stand structure was also investigated by
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analyzing the coefficient of variation of the DBH data. The ANOVA results showed that
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there was strong evidence (p=0.034) that the coefficient of variation of the DBH data in
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the fertilised plots (20.1% for the factorial 40/40 and 80/80 treatments) were significantly
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lower than the 23.5% of the control treatment plots (Table 4) at clearfelling age. There
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were also weak evidence (p = 0.067) that the split and early fertiliser application had a
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greater effect on the reduction of the coefficient of variation of DBH data than the late
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application of fertiliser (Table 4).
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The application of fertiliser shifted the DBH distribution to the right and the effect
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increased as the quantity of fertiliser increased (Figure 3). This was also evident from the
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summary of minimum, maximum and various percentile values per treatment (Table 4).
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The fertiliser application had very little effect on either the skewness or kurtosis of the
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DBH distributions (Table 4). The DBH distributions were symmetrical for all treatments,
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except for the L80/80 treatment where it was positively skewed. Only the latter treatment
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had a greater kurtosis value, indicating a more peaky distribution, than the rest of the
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treatments, which followed a normal distribution (Table 4).
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Insert Table 4 and Figure 3
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Basic wood density, dry wood production and mass of stem bark
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The ANOVA analysis results indicated that basic wood density was only weakly
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influenced by the quantity of fertiliser applied (Table 2). At the 10% significance level the
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application of the highest quantity of PK fertiliser caused a decrease in basic wood
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density of 0.0161 g cm-3 (Table 3) compared to the control treatment. There was a
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significant increase of 16% in the dry timber production over the control treatment when
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40/40 fertiliser treatments were applied and a further significant increase of 8.6 % when
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80/80 fertiliser treatments were applied (Tables 2 and 3). There was weak evidence
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(Table 3) that the time of application had a significant effect on dry timber production in
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this trial. The early and split application of the 40/40 and 80/80 fertiliser quantity
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treatments produced significantly more dry wood per hectare than the late application of
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the same amounts of fertiliser (Table 3). The stem bark mass was significantly greater
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(14%) on the plots that received 40/40 and 80/80 quantity treatments of P and K fertiliser,
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compared to the control plots (Tables 2 and 3). There was no indication of an interaction
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between fertiliser quantity and time of application in the basic wood density, dry timber
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production or stem bark mass (Table 2).
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DISCUSSION
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Tree growth
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At the age of 17 months the weed control treatment had a small effect on tree growth
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(Morris 1994b). Analysis of measurements from the age of five years up to clearfelling
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age indicated that the weed control treatment had no significant effect on any of the
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growth parameters that were assessed. This could be due to low levels of weed
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development in the trial or due to the fact that the effects of weed control are sometimes
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short-lived and only elicits a temporary response (Nilsson and Allen 2003, Little and
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Rolando 2006). If the Nilssson and Allan (2003) definition is used, the vegetation control
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treatment can be described as a Type C response as the early growth gains were lost
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with time as opposed to their Type B response when growth gains achieved during an
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initial response period early in the rotation are maintained, which is similar to the Type I
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response of Snowdon (2002).
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The logarithmic transformed tree height relationship with DBH indicated that fertilisation
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not only increased the DBH, which resulted in taller trees, but that for a given DBH the
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height of the trees that received the largest quantity of fertiliser was greater than that of
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the control trees. These results suggest that the average taper (describing the degree of
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cylindricity of the stem) was decreased by fertilisation. However, previous studies showed
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that fertilisation of pine increased DBH growth more than height growth, resulting in
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increased stem taper (Schutz 1976, Snowdon et al. 1981, Snowdon and Waring 1984,
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Wielinga et al. 2008). It should be noted that neither the average tree taper (DBH/H) nor
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the stem form (expression of shape in relative terms) of 16 trees sampled in the control
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and S80/80 treatments, respectively, was significantly affected by fertilisation (Crous et al.
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2009). However, the small number of trees of different DBH sizes due to the sampling
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technique might have influenced the results as Snowdon and Waring (1984) cautions that
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valid assessments can only be made if trees of the same size are compared.
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The application of PK fertiliser had no significant impact on the number of live stems at
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rotation age, similar to findings in another PK trial (R128) that have reached rotation age
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(Crous et al. 2008). All other growth parameters were significantly affected by the
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application of fertiliser. In general, the highest application (80/80) increased DBH by 15%,
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mean tree height by 5%, basal area by 21% and volume per hectare by 29% at the age of
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15 years. This volume increase corresponded to estimates of an additional 50 to 70 m³
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ha-1 based on results from a number of trials by Morris (1987a and 1987b), but were
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much greater than the additional 22.9 m³ ha-1 or the 25.2 m³ ha-1 that was reported by
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Crous et al. (2007 and 2008), respectively.
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The results also showed that the fertiliser increased growth over the life of the trees and
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not only temporarily after application. The difference in volume production per unit area
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between trees that received the S80/80 application and the control became progressively
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greater towards clearfelling age. Thus, the volume production showed that the growth of
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the fertilised trees diverged from the control trees with time. This clearly indicated that the
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fertiliser caused a sustained change in the stand productivity – a Type II response as
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defined by Snowdon and Waring (1984) and Snowdon (2002), which is similar to a Type
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A response (Nilsson and Allen 2003). A Type A or Type II response is typically observed
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where fertilisation enhances the capacity of a site to supply a limiting nutrient in the long-
347
term (Snowdon 2002).
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The significance of the time of application of the fertiliser on volume production decreased
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after the post-pruning application at age five years to insignificant levels four years later
351
(data not shown). In contrast, the quantity of fertiliser that was applied had a significant
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impact on volume production from age 7.6 years to the end of the rotation.
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Gabbro is of very limited extent in southern African forestry plantations. However, apart
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from the 13 000 ha of plantations underlain by ultra basic lithology in Swaziland, 13 400
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ha of plantations occurs on ultra basic geology in south-eastern Mpumalanga, eastern
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Mpumalanga and the KwaZulu-Natal Midlands (Smith et al. 2005). Another 49 000 ha of
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plantation are found on basic geology (dolerite, diabase, basalts and other igneous rocks)
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having similar low P and K contents in the aforementioned forestry areas of southern
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Africa (Smith et al. 2005). Currently, the P. patula established on these sites are not
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fertilised and it is likely that P and K deficiencies similar to those observed at Usutu might
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be developing as foliar sampling revealed sub-optimum levels of P, K and magnesium on
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some sites in KwaZulu-Natal (Du Toit and Job 1999) and Mpumalanga (Rolando et al.
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2003). Thus, tree growth on sites underlain by basic and ultra basic geology in other parts
365
of southern Africa should be investigated as there is a strong possibility that remedial P
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and K fertilisation could enhance volume production of subsequent rotations on these
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sites (Carlson 2001).
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Stand structure
371
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Investigation of the effect of fertilisation on the stand structure showed that the DBH
373
distribution was more uniform on the fertilised plots as the coefficient of variation
374
decreased, confirming findings by Schönau (1989) and Snowdon and Waring (1984). In
375
our study the application of fertiliser had very little effect on the skewness of the DBH
376
distribution in contrast to reports by Snowdon and Waring (1984) that fertilisation normally
377
produces a negatively skewed population with a few small trees and many large ones.
378
Our results were similar to the finding by Carlson et. al. (2008) that the distribution shape
379
was not significantly affected by fertiliser application. The L80/80 treatment in our study
380
produced a slightly positively skewed distribution as the most dominant trees with the
381
largest diameters showed the greatest absolute response to the late fertilisation. These
382
results correspond with reports by Wells and Allen (1985), Hynynen et al. (1998) and
383
Carlson et al. (2008).
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385
Basic wood density, dry wood production and mass of stem bark
386
387
The application of the 80/80 PK fertiliser treatments decreased basic wood density by 3%
388
(Table 3) compared to the control treatment. Comparison of basic wood density changes
389
with height in 16 trees sampled on the control plots to 16 trees sampled on S80/80 plots
390
indicated that average basic wood density at breast height (1.3 m) was 7% lower on the
391
fertilised plots (Crous et al., 2009), which was slightly more than the 3% observed in the
392
analysis of the results from the complete trial. McKinnell and Rudman (1973) reported a
393
reduction of 8% after application of 188 kg K ha-1 to P. radiata and Gray and Kyanka
394
(1974) reported a reduction of 5.5% in basic wood density after application of 112 kg K
16
395
ha-1 to P. resinosa. In Australia, the 128% volume increase (Waterworth et al. 2007)
396
reported in the Biology of Forest Growth study with P. radiata, where 400 kg N ha-1,
397
200kg P ha-1, 100 kg K ha-1 and irrigation were applied (Benson, et al. 1992), was
398
associated with a 9 % decrease in basic wood density (Wielinga et al 2008). In the
399
southern USA intensive site preparation with fertilisation and weed control reduced basic
400
wood density of P. taeda by 6 to 10% (Clark et al. 2004; Clark et al. 2006). Cown and
401
McConchie (1981) and Woollons et al. 1995 cited in McGrath et al. (2003) reported a 5%
402
decrease in fertilised P. radiata wood in New Zealand and New South Wales, Australia,
403
respectively. The lowering of wood density can have an influence on the pulp
404
characteristics, for example, higher burst and tensile strengths, but lower tear factor,
405
opacity and bulk (McKinnell and Rudman 1973, Clark et al. 2006). A lower basic wood
406
density will also result in lower yields per digester charge in batch processes, which will
407
have an economic penalty (Clark et al. 2006).
408
409
The reason for the decrease in wood density could not be determined in this trial,
410
because the basic wood density cross each disc was not assessed. In other studies the
411
reduction in mean basic wood density was attributed firstly to an increase in the
412
earlywood content as fertilisation had the greatest effect on spring growth (Gray and
413
Kyankan 1974, Larson et al. 2001, Albaugh et al. 2004); secondly to an increase in the
414
diameter of the juvenile wood core, which is characterised by timber with lower density
415
than mature wood, as a result of increased early tree growth (Clark et al. 2004, Clark et
416
al. 2006, Mora et al. 2007) and thirdly to a reduction in latewood density (Clark et al.
417
2004) associated with increased photosynthetic efficiency (Larson et al. 2001).
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17
419
Despite the negative effect on basic wood density, the total dry wood production
420
increased significantly by 16% where 40/40 fertiliser was applied and 26% where 80/80
421
fertiliser was applied, due to the increase in timber volume production. The early (spot)
422
application of fertiliser produced 8% more dry timber than the late (broadcast) application,
423
but only 1% (not statistically significant) more than the split application. These results
424
confirm that the current commercial prescription to apply a small quantity of PK fertiliser at
425
planting, followed by a larger broadcast application after pruning, is the most effective,
426
because it increases production significantly while the largest proportion of fertiliser cost
427
is incurred later into the rotation and early weed growth is not stimulated too much by a
428
small application of fertiliser at planting (Morris 1994a).
429
430
The 13.7 tons per hectare of bark in the control plots corresponded with values of 14.2 t
431
ha-1 reported by Morris (1992) for 15-year-old P. patula trees at Usutu and 12.9 and 13.7 t
432
ha-1 reported in 19 and 22-year-old P. taeda, respectively (Rubilar, 2003). The bark mass
433
was not affected to the same extent as wood production by the application of fertiliser and
434
the application of the 40/40 and 80/80 quantities increased the bark mass by 14%,
435
compared to the control.
436
437
CONCLUSIONS
438
439
The PK fertiliser increased growth over the life of the trees, unlike the temporary growth
440
increase from weeding, indicating that fertilisation increased the long-term stand
441
productivity. Furthermore the application of fertiliser resulted in a more uniform DBH
442
distribution. The results supported the current prescription to apply 20 kg P and K per ha,
443
respectively, at planting, followed by a broadcast application of 75 kg P and K per ha,
18
444
respectively, after pruning to stands located on gabbro-derived soils that are fertilised for
445
the first time. A split application increased production similarly to an early application,
446
while the largest proportion of fertiliser cost was incurred later into the rotation and early
447
weed growth was not stimulated too much by a small application of fertiliser at planting,
448
before canopy closure occurred.
449
450
Although the application of fertiliser decreased the mean basic wood density of the timber
451
the additional volume production from the application of PK fertiliser far outweighed the
452
disadvantage of a slight (3%) reduction in basic wood density. Therefore, in order to
453
maximise dry wood production per unit area, the application of PK fertiliser to previous
454
unfertilised soil located on gabbro parent material should continue at the current
455
recommended rate.
456
457
The applicability of these results to P. patula stands located on basic and ultra basic
458
lithology in other parts of Southern Africa should be investigated.
459
460
Acknowledgements
461
462
The authors thank Sabelo Khoza, the Usutu Research Team and Milton Nkambule of
463
Super Research Services for assistance with trial measurements and sample collection.
464
The Sappi Forests Research Hardwood Breeding Programme assisted with sample
465
preparation and wood density analysis. Giovanni Sale, Elize Ade and Wayne Jones
466
provided constructive comments regarding this document. Funding was provided by the
467
University of the Witwatersrand, the National Research Foundation of South Africa and
468
Sappi.
469
19
470
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471
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26
Table 1: The sixteen treatment combinations tested in a trial at Usutu where PK fertiliser
was applied to P. patula trees.
Code
E40/40N
E40/40I
E80/80N
E80/80I
S40/40N
S40/40I
S80/40N
S80/80I
L40/40N
L40/40I
L80/80N
L80/80I
E20/20N
E20/20I
CONTRN
CONTRI
Treatments
Fertiliser quantity
Planting
Pruning
40/40
0
40/40
0
80/80
0
80/80
0
20/20
20/20
20/20
20/20
20/20
60/60
20/20
60/60
0
40/40
0
40/40
0
80/80
0
80/80
20/20
0
20/20
0
0
0
0
0
Weed
Control
Normal
Intensive
Normal
Intensive
Normal
Intensive
Normal
Intensive
Normal
Intensive
Normal
Intensive
Normal
Intensive
Normal
Intensive
Treatment code description
Code
Description
Quantity of fertiliser
20/20 20kg elemental P + 20kg elemental K per ha
40/40 40kg elemental P + 40kg elemental K per ha
60/60 60kg elemental P + 60kg elemental K per ha
80/80 80kg elemental P + 80kg elemental K per ha
Time of application
E
All fertiliser applied at planting (Early)
S
Fertiliser applied part at planting (20/20) and
the rest post pruning (Age 5 years) (Split)
L
All fertiliser applied post pruning (Age 5
years) (Late)
Weed control
N
Normal weeding as used operationally
I
Intensive weeding aimed at weed free plots
Table 2: A summary of analysis of variance F-probability values for tree growth parameters, basic wood density and bark mass of
15-year-old P. patula trees in the PK fertiliser trial testing various application rates and time of application at Usutu.
Source of variation
REP stratum
REP.*Units* stratum
Treatment group (D)
D.Quantity
D.Time
D.Quantity.Time
Residual
Total
d.f.
Live
stems
(spha)
Quadratic
mean
diameter
(cm)
3
0.098
0.825
2
1
2
2
53
63
0.543
0.238
0.242
0.547
<.001
0.003
0.026
0.135
Mean tree
height
(m)
Basal
area
(m² ha-1)
Probability > F
0.889
0.217
0.004
0.146
0.241
0.894
<.001
0.024
0.151
0.240
m.v. = missing value
Bolding type indicates statistically significant differences at the 10% level
Volume
per plot
(m³ha-1)
d.f.
(m.v.)
Basic
wood
density
(g cm-3)
Dry timber
production
(t ha-1)
Bark
mass
(t ha-1)
0.219
0.004
0.099
0.345
0.491
0.274
3
0.657
Probability > F
0.732
<.001
0.001
0.098
0.172
2
1
2
2
47 (6)
57 (6)
0.114
0.064
0.195
0.633
<0.001
0.008
0.053
0.117
Table 3: Growth response of P. patula at age 15 years to the application of various
quantities of P and K fertiliser at different times of application in a trial at Usutu,
Swaziland. Different letters indicate a significant difference between means at the 5% or
10% level.
Fertiliser
Time of application
quantity
Control
Early
Split
Late
(kg ha-1)
-1
Live stems (stems ha )
0/0
1173
20/20
1106
40/40
1179
1113
1126
80/80
1113
1019
1140
Quadratic mean DBH (cm)
0/0
20.0
20/20
21.7
40/40
21.1
22.3
21.6
80/80
23.3
23.8
21.7
Average
20.0 c
22.2* ab
23.1 a
21.6 b
Mean tree height (m)
0/0
19.6
20/20
19.4
40/40 & 80/80
20.3
20.7
20.2
Basal area (m² ha-1)
0/0
36.81
20/20
40.43
40/40
41.26
42.88
41.01
80/80
46.89
44.75
41.87
Volume (m³ ha-1)
0/0
286.1
20/20
316.1
40/40
329.7
345.7
329.0
80/80
389.8
373.6
344.0
Basic wood density (g cm -1) (6 missing values)
0/0
0.3718
20/20
0.3754
40/40
0.3723
0.3652
0.3694
80/80
0.3685
0.3585
0.3560
-1
Dry timber production (t ha ) (6 missing values)
0/0
109.3
20/20
123.7
40/40
124.9
131.6
123.3
80/80
147.6
138.2
126.8
Average
109.3 d
136.2* a
134.9 ab
125.1 bc
Stem bark mass (t ha-1) (6 missing values)
0/0
13.71
20/20
14.42
40/40 & 80/80
15.97
15.84
15.18
* Excluding early 20/20 treatment
Average
Statistical
significance
1173
1106
1140
1091
20.0
21.7
21.7
23.0
5% level
C
B
B
A
109.3
123.7
126.6
137.5
5% level
B
B
A
5% level
C
BC
B
A
5% level
C
BC
B
A
10% level
AB
A
B
C
5% level
D
BC
B
A
13.71
14.42
15.66
5% level
B
AB
A
19.6
19.4
20.4
36.81
40.43
41.72
44.51
286.1
316.1
334.8
369.1
0.3718
0.3754
0.3689
0.3610
Table 4. Summary statistics for the DBH distribution by treatment of 15-year-old P. patula
trees in a PK fertiliser trial at Usutu, Swaziland.
Treatment
Control
E20/20
E40/40
E80/80
S40/40
S80/80
L40/40
L80/80
*
Nr.
trees
176
166
177
167
167
153
169
171
Min.
9.2
8.7
8.7
12.0
10.8
12.0
9.5
10.5
25
16.4
18.5
17.5
19.7
18.0
20.2
18.3
18.5
Percentile
50
19.5
21.1
20.6
23.0
22.0
23.0
21.2
20.5
75
22.2
24.2
23.8
25.7
24.9
26.3
24.5
24.0
Max.
CV * (%)
Skewness
Kurtosis †
33.7
31.5
30.6
34.2
32.2
36.6
32.2
36.7
23.5
21.0
20.5
19.1
21.0
19.2
22.1
22.2
0.01
0.05
-0.16
-0.02
-0.10
0.30
-0.13
0.59
-0.04
-0.27
-0.31
-0.27
-0.54
-0.10
-0.46
0.88
CV – Coefficient of variation
from normal distribution, i.e. a value of 3
†Deviation
30
List of figures
Figure 1. The relationship between DBH and tree height of 15-year-old P. patula trees in
the control (no fertiliser applied), across all 40/40 treatments (40 kg ha-1 of P and K
applied) and across all 80/80 treatments (80 kg ha-1 of both P and K applied) in a fertiliser
trial at Usutu.
Figure 2. Volume growth of P. patula trees since the age of five years up to clearfelling
age at 15 years for three selected treatments namely the control (no fertiliser applied),
S40/40 (20 kg ha-1 of both P and K were applied at planting and a further 20 kg ha-1 of
both P and K were applied after pruning at the age of five years) and S80/80 (20 kg ha -1
of both P and K were applied at planting and a further 60 kg ha -1 of both P and K were
applied after pruning) in a fertiliser trial at Usutu. Error bars indicate the least significant
difference between the means at the 5% level.
Figure 3. Histogram of DBH distribution of 15-year-old P. patula trees in the control (no
fertiliser applied), S40/40 (20 kg ha-1 of both P and K were applied at planting and a
further 20 kg ha-1 of both P and K were applied after pruning at the age of five years) and
S80/80 (20 kg ha-1 of both P and K were applied at planting and a further 60 kg ha -1 of
both P and K were applied after pruning) treatments of a fertiliser trial at Usutu.
31
Figure 1
24
Control: ln H = -6.302*(1/DBH ) + 3.2801
40/40: ln H = -6.302*(1/DBH ) + 3.2946
80/80: ln H = -6.302*(1/DBH ) + 3.3122
Tree height (m)
22
Adj. R² = 0.488, n = 312
20
18
16
10
15
20
25
30
35
DBH (cm)
Control
40/40
80/80
32
Figure 2
400
-1
Total plot volume (m³ ha )
350
300
250
200
150
100
50
0
4
6
10
8
12
14
16
Tree age (years)
Control
S40/40
S80/80
33
Figure 3
30
Frequency (%)
25
20
15
10
5
0
8.1-11.0
11.114.0
14.117.0
17.120.0
20.123.0
23.126.0
26.129.0
29.132.0
32.135.0
35.138.0
DBH class (cm)
Control
S40/40
S80/80
34