Biomass and morphology of Pinus radiata coarse root components in a sub-humid temperate silvopastoral system

Forest Ecology and Management 177 (2003) 387±397

Biomass and morphology of Pinus radiata coarse root components in a sub-humid temperate silvopastoral system Madan K. Gautama, Donald J. Meadb,*, Peter W. Clintonc, Scott X. Changd a

Forestry Programme, School of Resources, Environment and Society, The Australian National University, Canberra, Australia b Silviculture Adviser, Milnthorpe, Golden Bay, New Zealand c Forest Research Institute, P.O. Box 29237, Fendalton, Christchurch, New Zealand d Department of Renewable Resources, Centre for Enhanced Forest Management, 4-42 ESB, University of Alberta, Edmonton, Canada T6G2E3 Received 2 January 2002; accepted 20 July 2002

Abstract Understanding the dynamics and distribution of root system components and how they are affected by pasture±tree interactions in silvopastoral systems are important for better management of agroforestry systems. The biomass and morphology of coarse root components were studied for 3- and 4-year-old Pinus radiata clonal and seedling trees growing with or without lucerne (Medicago sativa). Root:shoot ratio and lateral and vertical root biomass were greater by 1.5, 2.3 and 6.1 times, respectively, in clonal than in seedling trees, particularly in the no understory treatment compared to the lucerne treatment. Fractional allocation of root biomass to lateral and vertical roots was higher in clonal than in seedling trees by 1.1 and 2.6 times, respectively, while allocation to the root core was 1.4 times higher in the seedlings than the clone. Competition from lucerne reduced fractional allocation of root biomass to lateral roots by 40% in the seedling tree in 1993 and increased allocation to the root core. Competition was more intense at age 3 than at 4 years. No competition effect on lateral roots was observed with the clonal trees. Radiata pine root systems showed strong morphological plasticity to respond to changing soil conditions. Ripping coupled with thinning increased lateral root growth in the ripped zone so that by age 4 years 60% of lateral roots was in this zone. This was probably due to alterations in soil structure and reduced competition, particularly for moisture. Similarly higher soil moisture led to more lateral root biomass growth in the no understory compared to the lucerne treatment and on the south side of the trees compared to the north side. These results illustrated that selection of genotypes and planting material as well as management techniques such as soil cultivation, selection and placement of pasture understory, and thinning can all be used to manipulate rooting patterns and tree productivity in agroforestry systems. Furthermore, selection of planting material can also be used to reduce tree toppling in radiata pine. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Root architecture; Root:shoot ratio; Tree±pasture interaction; Root biomass allocation; Tree toppling

1. Introduction * Corresponding author. Present address: c/-39 Waitapu Rd, Takaka, New Zealand. Tel.: ‡64-3-524-8553. E-mail addresses: [email protected] (M.K. Gautam), [email protected] (D.J. Mead).

Growing trees with understory crops such as in silvopastoral systems, creates a range of interactions between the trees and the understory, especially during

0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 2 ) 0 0 4 1 1 - 5

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the early stages of a rotation (Ong, 1991; Mead and Mansur, 1993). Mead et al. (1993) found that the presence of lucerne (Medicago sativa L.) as an understory pasture considerably decreased height and DBH growth of 2-year-old radiata pine (Pinus radiata D. Don) trees. In addition, moisture competition resulted in differences in aboveground biomass allocation, crown architecture and water use ef®ciency (Kolb and Steiner, 1990; Miller et al., 1998; Bandara et al., 1999). Understory competition can also reduce N uptake in radiata pine (Nambiar and Zed, 1980; Mead and Mansur, 1993; Clinton and Mead, 1994) and ®ne root production of trees (Jonsson et al., 1988; Gautam et al., in press) and result in changes to root:shoot ratio, coarse root morphology, biomass production and allocation among the coarse root system components (Clinton, 1990; Squire et al., 1987; Warren et al., 1987). Coarse root morphology re¯ects the vigour of tree growth (Nambiar, 1984; Theodorou and Bowen, 1993), and is affected by the spatial and temporal variation in water and nutrient supply (Coutts, 1987), which can be brought about by competition between trees and understory species (Kolb and Steiner, 1990; Warren et al., 1987; Smethurst and Nambiar, 1989). On the other hand, as trees grow and expand their canopies they will eventually come to dominate the site thereby reducing pasture production (Pollock et al., 1994, 1997). Overtime this could lead to a reduction in the impact of understory pasture on tree root morphology and biomass production. Inadequate root development and poor vertical and lateral root quality (distribution) may result in juvenile tree instability (Mason, 1985; Gautam et al., 1999). Instability can be greater where weeds or pasture is controlled (Chavasse, 1969) and with seedlings compared to clones of radiata pine (Mead et al., 1993; Gautam et al., 1999). However, it is not clear if understory competition and the use of clones may improve tree stability by altering root:shoot ratio, biomass production and allocation among the coarse root components, or through coarse root morphology. A better understanding of the impact of understory competition on biomass allocation and morphology of coarse root system is needed to ensure good tree establishment and growth during the early phase of tree growth. In addition, selection of tree genotypes

with greater plasticity in coarse root biomass allocation or its morphology, may be a possibility for agroforestry systems, or forestry in general, where understory competition is severe. The objectives of this study were to (a) examine the interaction between understory competition and planting material type on the production and allocation of biomass to the coarse root systems and morphology and (b) investigate short-term temporal changes in the above traits as trees expand their canopies and as pastures age. 2. Materials and methods The site and experimental design have been described in detail by Mead et al. (1993) and Gautam et al. (in press). Brie¯y, the soil is classed as a Templeton silt loam in the New Zealand soil classi®cation system (Udic Haplustepts in the US Soil Taxonomy system). It is medium draining with a moderate capacity to hold moisture. The climate is temperate and sub-humid with a distinct dry summer season. The long-term rainfall average is 660 mm, evenly distributed through the year, but with considerable annual variability. Summer evapo-transpiration usually exceeds summer precipitation by twofold to threefold. In the 1993±1994 season the rainfall was 596 mm with potential evapo-transpiration of 934 mm. The trial was laid out in a split-plot design, with the pasture treatments at the main plot level and genotype treatments at the subplot level, replicated in three blocks. Prior to planting at 1:4 m  7 m spacing (1000 stems/ha) the tree rows were ripped to 60 cm depth in an east±west direction. At the time of this study, in the winter at ages 3 and 4 years, the trees were unpruned and had been thinned down to 800 and 600 stems/ha, respectively, by removing the poorest trees. For this study, we selected the lucerne and no understory as the main plot factors, and clone 3 and seedlings radiata pine as the sub-plot factors. Two trees from each subplot were sampled each year and they covered the range of tree size within the treatment. Diameter at breast height (DBH) and total tree height was recorded for each sample tree. The aboveground tree was then harvested and their dry weight estimated (Bandara, 1997).

M.K. Gautam et al. / Forest Ecology and Management 177 (2003) 387±397

2.1. Root excavation Coarse roots were de®ned as those with a diameter greater than 2 mm. Before removing the root system a nail was driven into the stump on its north side and a line was drawn along the direction of the ripline on the surface of the stump. A tractor with a hydraulic-ram of 2.5 t lifting power and a four-footed steel frame was used to pull the root system from the saturated soil in winter. However, as this often broke roots, particularly the lateral roots, it was also necessary to establish relationships between root diameter at 10 cm from the root core (dia10) and biomass of complete roots, i.e., from the stump to the point along their length where they were 2 mm in diameter. To establish the relationship between root dia10 and biomass, 4±6 complete lateral roots per tree, over the range of dia10, were excavated and their biomass determined. The position of these sample roots was marked on the root core, and their direction of spread recorded. 2.2. Root measurements In the laboratory the extracted root system was placed upside down with the stump placed in a hole made in the centre of a 1 m  1 m  1 m platform (Balneaves and de la Mare, 1989). The platform was marked so that, by using the east±west line on the stump, it was possible to accurately de®ne the coordinates of each lateral root. The dia10 of all ®rstorder lateral and vertical roots were measured to the nearest millimetre. Three to four unbroken vertical roots were sampled from each root system. The sample lateral and vertical roots and root core were dried at 70 8C to a constant oven dry weight and weighed to the nearest gram. The relationship between dia10 and lateral and vertical root biomass was established using linear regression methods. For each tree, the biomass of coarse root components, i.e., root core, vertical roots, lateral roots and lateral roots for the north and south quadrants and on the ripline (east and west quadrants), were calculated using the relationships between dia10 and root weight, as appropriate. Root:shoot ratio was calculated by dividing the total belowground biomass of coarse root components by the total aboveground biomass (Gautam, 1998; Bandara, 1997 unpublished data). To obtain fractional

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biomass allocation by components within the coarse root system, including lateral root biomass by direction (north, south or ripline), the biomass of a coarse root component was divided by the total biomass of the coarse root system. 2.3. Statistical analysis Linear regression models between (dia10)2 and root biomass (oven dry) were developed for each treatment from the sampled complete lateral and vertical roots for 1993 and 1994. The R2 of the models range from 0.86 to 0.96. As the slope of the regression models for 1994 were signi®cantly different from the 1993 ones it was not possible to pool root data for the 2 years. The lateral root models were derived for each of the understory and genotype combinations because the slopes of the models were different (p  0:007, data not shown). As the slopes were also signi®cantly different (p  0:001, data not shown) between genotypes for the vertical roots, models were derived for each genotype. Homogeneity of variance and normality of distribution were checked before any analysis of variance. Variance analysis was performed on all experimental variables using the SYSTAT (1992) software. To remove the tree-size effects, total tree biomass (aboveground biomass plus coarse root system biomass) were used as covariates in the analysis of root biomass data, where the covariates signi®cantly …p  0:05† contributed to the model. Lateral root dry matter for particular directions (i.e. north, ripline and south) were analysed separately. Probabilities 0.05 are highlighted in the tables. 3. Results 3.1. Tree height and diameter of sample trees The mean DBH of 3-year-old radiata pine was signi®cantly reduced by 38% due to lucerne competition (Table 1). On average, the mean DBH of clonal trees was 41% greater than seedling trees …p < 0:04† (Table 1). For the 1994 DBH data, there was a signi®cant understory  genotype interaction. This interaction was due to there being no differences between genotypes in the lucerne treatment, whereas with no

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Table 1 Treatment effects on DBH and height of sampled trees for 1993 and 1994 in a silvopastoral experiment in Canterbury, New Zealand Treatment

1993

1994

DBH (cm)

Height (m)

DBH (cm)

Height (m)

No understory Clone Seedling

5.6 3.6

3.4 2.3

10.9 8.6

5.5 4.2

Lucerne Clone Seedling

3.9 2.9

2.9 2.2

8.4 8.2

4.7 4.1

Standard error of mean

0.22

0.17

0.25

0.13

Source

Significance level (p)

Understory Genotype Understory  genotype

DBH (cm)

Height (m)

0.050 0.020 0.141

0.397 0.036 0.312

DBH (cm)

Height (m)

0.321 0.036 0.050

0.037 0.021 0.115

Table 2 Effect of treatment on root:shoot ratio and biomass for coarse root components in a silvopastoral experiment in Canterbury, New Zealand Treatment

Root:shoot ratio

Root biomass (kg/tree) Total root

Core

Lateral

Vertical

0.162 0.096

2.715 1.219

0.874 0.446

1.360 0.725

0.482 0.048

0.207 0.072

1.129 0.427

0.396 0.258

0.514 0.137

0.218 0.032

0.207 0.116

8.903 3.733

2.559 1.476

4.644 1.945

1.701 0.312

0.231 0.168

4.350 2.095

1.401 0.860

2.338 1.130

0.611 0.105

Standard error of mean

0.0206

0.1491

0.0800

0.0775

0.1844

Source

Significance level (p) Root:shoot ratio

Total root

Core

Lateral

Vertical

0.277 0.001 0.461 0.011 0.299 0.532 0.160

0.177 0.004 0.078 0.002 0.333 0.006 0.202

0.726 0.020 0.134 0.040 0.839 0.037 0.177

0.843 0.007 0.127 0.008 0.656 0.001 0.049

0.143 0.005 0.020 0.132 0.088 0.040 0.258

1993 No understory Clone Seedling Lucerne Clone Seedling 1994 No understory Clone Seedling Lucerne Clone Seedling

Understory Genotype Understory  genotype Year Year  understory Year  genotype Year  genotype  understory

M.K. Gautam et al. / Forest Ecology and Management 177 (2003) 387±397

understory treatment clonal trees had 25% greater DBH than the seedling trees (Table 1). Clonal trees were 28 and 24% taller …p < 0:04† than the seedling trees in 1993 and 1994, respectively, and in 1994 trees were signi®cantly taller in the no understory than in the lucerne treatment. These results are similar to the analysis based on all trees in the plots (Gautam, 1998). 3.2. Root:shoot ratio and biomass in coarse root components Tree root:shoot ratio was not altered by the presence of lucerne understory (Table 2). However, in both

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years, the ratio was signi®cantly different between the genotypes, being more than 1.5 times greater in clonal trees than the seedling trees. Root:shoot ratio increased by 34% between 1993 and 1994. The tree size covariates were only signi®cant …p  0:03† for the total coarse root and root core biomass and were not important for the analysis of lateral or vertical roots. However, for the root core biomass and total root biomass the slopes of the relationship were signi®cantly different between genotypes (Fig. 1a and c) and for the root core between the understory treatments (Fig. 1b). In general, clonal trees had substantially greater biomass allocation than seedlings to root core and coarse root system, and this

Fig. 1. The relationships for 1993 and 1994 data between total tree biomass, and (a) root core biomass by genotypes, (b) root core biomass by understory treatments and (c) total coarse root biomass by understory treatments. Slopes of the lines are signi®cantly different.

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was consistent over the 2 years (Fig. 1a and c). Fig. 1b also illustrates that in the lucerne treatment the trees had relatively greater root core biomass allocation than those trees in the no understory treatment. These ®gures, along with data presented in Table 2, also illustrate the increasing biomass between 1993 and 1994. There were signi®cant (p  0:04) year  genotype and understory  genotype interactions for the vertical root biomass (Table 2). The interaction between year and genotypes was due to the very much larger increase in vertical root biomass in clonal than the seedling trees from 1993 to 1994. Similarly, the understory  genotype interaction for the vertical roots resulted from the clonal trees having non-signi®cantly different biomass from the seedlings in the lucerne treatment, but having signi®cantly greater root weight than the seedling trees in the no understory treatment. However, in both understory treatments the clonal trees had about six times the biomass of vertical roots than the seedlings. There was also a signi®cant year  genotype  understory treatment interaction for the lateral root biomass (Table 2). The interaction was primarily due to the clonal trees being more responsive to reduced competition than seedling trees between 1993 and 1994. The lateral root biomass in clonal trees was greater than in seedling trees in both understory treatments for both years.

Table 3 Effect of treatments on fractional allocation of coarse root biomass in a silvopastoral experiment in Canterbury, New Zealand

3.3. Fractional allocation of coarse root biomass

(seedlings and clones in the no understory and clones in the lucerne treatment). There was a signi®cant year  genotype  understory interaction for fractional allocation of biomass to the root core (Table 3). In particular, the seedling trees in the lucerne treatment allocated a third less to the root core in 1994 than they did in 1993. In addition, the allocation to the root core was greater in the seedlings than in the clonal trees in both understory treatments in both years, and for the clonal trees the fractional allocation was not altered by understory treatment (Table 3).

About half of the coarse roots biomass were as lateral roots and the vertical roots made up the smallest fraction (under 20%). There were, however, some signi®cant treatment effects on the fractional allocation pattern (Table 3). In general, clonal trees allocated more biomass to lateral and vertical roots and less to the root core than the seedling trees (Table 3). The low allocation to vertical roots (7.5%) in the seedling trees compared to 17% for the clonal trees, was particularly noticeable. There was a signi®cant genotype  understory interaction for the fractional allocation of biomass to lateral roots (Table 3). This interaction was due to lateral roots accounting for 43% of total root biomass for seedlings in the lucerne treatment while they accounted for 53% for the other treatments

Treatment

Core

Lateral

Vertical

0.325 0.399

0.517 0.546

0.158 0.056

0.335 0.597

0.493 0.327

0.172 0.075

0.284 0.399

0.529 0.532

0.187 0.069

0.310 0.417

0.540 0.532

0.150 0.052

Standard error of mean

0.0270

0.0478

0.0306

Source

Significance level (p)

1993 No understory Clone Seedling Lucerne Clone Seedling 1994 No understory Clone Seedling Lucerne Clone Seedling

Understory Genotype Understory  genotype Year Year  understory Year  genotype Year  genotype  understory

Core

Lateral

Vertical

0.030 0.001 0.017 0.010 0.061 0.151 0.031

0.207 0.020 0.006 0.096 0.095 0.341 0.207

0.856 0.001 0.546 0.988 0.334 0.864 0.869

3.4. Spatial distribution of lateral root biomass The distribution of lateral roots between the north and south sides and along the ripline changed between 1993 and 1994. In 1993 the distribution was 28, 36 and

M.K. Gautam et al. / Forest Ecology and Management 177 (2003) 387±397 Table 4 Effect of treatment on lateral root biomass in three positions Treatment

Root biomass (kg/tree) North

1993 No understory Clone Seedling Lucerne Clone Seedling 1994 No understory Clone Seedling Lucerne Clone Seedling

Ripline

South

0.394 0.176

0.540 0.188

0.425 0.361

0.153 0.057

0.214 0.035

0.147 0.045

393

Table 5 Effect of treatment on fractional allocation of lateral root biomass (to total biomass) of coarse root components at three positions in a silvopastoral experiment in Canterbury, New Zealand Treatment 1993 No understory Clone Seedling Lucerne Clone Seedling

North

Ripline

South

0.137 0.135

0.191 0.141

0.189 0.270

0.138 0.146

0.225 0.066

0.131 0.116

0.073 0.108

0.311 0.329

0.145 0.095

0.089 0.103

0.316 0.289

0.135 0.139

0.0452

0.0498

0.648 0.369

2.786 1.217

1.211 0.360

0.341 0.237

1.420 0.604

0.577 0.288

1994 No understory Clone Seedling Lucerne Clone Seedling

Standard error of mean

0.0708

0.2373

0.1079

Standard error of mean

0.0315

Source

Significance level (p)

Source

Significance level (p)

Understory Genotype Understory  genotype Year Year  understory Year  genotype Year  genotype  understory

North

Ripline

South

0.116 0.185 0.561 0.003 0.590 0.618 0.670

0.016 0.004 0.126 0.001 0.049 0.026 0.440

0.049 0.016 0.184 0.001 0.727 0.012 0.085

36% for these sides, respectively, while in 1994 the distribution was 16, 24 and 60%, respectively. In part, these changes are re¯ected below. While lucerne competition resulted in about half the lateral root biomass along the ripline and clonal trees had more than twice the ripline root biomass compared to seedling trees, there were signi®cant year  understory and year  genotype interactions (Table 4). These interactions were due, respectively, to there being no signi®cant differences between treatments in 1993, but signi®cant biomass differences and greater increases with time in the no understory than in the lucerne treatment, and in the clonal than the seedling trees. Similarly for the biomass of lateral roots on the south side, the lucerne treatment was only 45% of no understory treatment (Table 4). The signi®cant year  genotype interaction for the south side lateral

Understory Genotype Understory  genotype Year Year  understory Year  genotype Year  genotype  understory

North

Ripline

South

0.820 0.503 0.884 0.078 0.988 0.625 0.748

0.750 0.014 0.042 0.001 0.954 0.153 0.626

0.279 0.843 0.668 0.219 0.117 0.457 0.317

root biomass resulted from the genotypes being not signi®cantly different in 1993, but the clonal trees increasing signi®cantly in biomass by over three times by 1994, while the seedlings biomass remained static. The biomass on the north side doubled between 1993 and 1994 but there were no signi®cant differences due to genotype or understory (Table 4). There was a signi®cant interaction between understory  genotype in the fractional allocation at the ripline (Table 5). The fractional allocation on the ripline was signi®cantly lower (at 0.18) in seedling trees with lucerne competition, whereas the clonal trees in both understory treatments and the seedlings in the no understory treatment were the same at 0.25. The fractional allocation to the ripline doubled between 1993 and 1994 (Table 5). For lateral roots on the north and south sides there were no signi®cant differences in fractional allocation.

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4. Discussion 4.1. Tree size, root biomass and root:shoot ratio Overall the clonal trees were both taller and had greater DBH than the seedling trees but their growth was reduced more by lucerne competition (Table 1). Furthermore, the growth differences between the no understory and the lucerne treatments for the clonal trees increased between ages 3 and 4 years but decreased or remained static for seedling trees. The result was that at age 4 years only the clonal trees in the no understory treatment have signi®cantly larger diameters than the other treatment combinations. Tree growth in the lucerne treatment was reduced due to competition for water and perhaps nutrients (Mead and Mansur, 1993; Yunusa et al., 1995b; Gautam et al., in press, 2002). Several studies have shown that root production is often determined by availability of soil moisture (Squire et al., 1987; Hallgren et al., 1991; Gautam et al., in press), and nutrients (Nambiar, 1980; Warren et al., 1987). Moisture can often be limited by the presence of understory competition (Sands and Nambiar, 1984; Mead and Mansur, 1993; Yunusa et al., 1995a,b; Gautam, 1998). Likewise, this study found that trees had higher lateral root biomass growth in the no understory treatment where soil moisture was higher, than in the lucerne treatment in both years 3 and 4 (Yunusa et al., 1995a; Gautam et al., 2002). The greater root biomass in the south compared to the north was associated with the moist, cool soil environment on the south side (Yunusa et al., 1995a; Gautam et al., 2002). The unpruned trees intercepted the rain coming from the south, and this led to increased soil moisture content on the south compared to the north side of the trees. Furthermore, shading on the southern side reduced lucerne growth close to the trees, particularly in the autumn (Pollock et al., 1994; Yunusa et al., 1995b). These moisture competition effects were more marked in the clone, whose growth was affected more by competition, than in the seedling trees. The root:shoot ratio was not altered by lucerne competition, despite the changing growth patterns above- and belowground, suggesting a functional equilibrium between root and shoot (Drew and Ledig, 1980; Clinton, 1990; Chang et al., 1996). Differences

with genetic makeup of radiata pine have also been noted by others (Theodorou and Bowen, 1993). Changes in root:shoot ratio with tree age in planted radiata pine has been poorly studied (Madgwick, 1994) but the increase found here between ages 3 and 4 years accords with data presented by Gadgil (1979) and Beets and Pollock (1987). 4.2. Allocation patterns In general, clonal trees allocated a higher proportional biomass to the lateral roots than the seedling trees, which suggests that the superior growth of the clonal trees is associated with greater allocation to lateral roots. This vigour of clonal trees was most marked, there being a twofold to fourfold increase for root core, lateral and vertical root biomass production between ages 3 and 4 years. The fractional allocation to lateral roots in seedling trees was reduced, particularly along the ripline, by understory competition while that of the clonal trees were not altered (Tables 3 and 5). Further the seedling trees had a higher fractional allocation to the root core as a result of understory competition, particularly at age three (Table 3). It seems that the seedlings adapted to competition by reducing allocation to lateral coarse roots and this resulted in the root core and stem growth being less affected. In contrast, the clonal trees in this study had higher root biomass than the seedlings (Table 2) and were able to respond to reduced competition in the no understory treatment without altering overall allocation patterns (Table 3). The increased fractional allocation of biomass to the ripline at age of 4 years, indicated that this region was a particularly favourable microsite for tree root growth. The allocation along the ripline doubled between ages 3 and 4 years so that by age of 4 years, 60% of lateral roots was in this zone. The allocation tended to decrease …p ˆ 0:08† on the north side between ages 3 and 4 years. The ripline and south side were moister and cooler than the north side of the trees (Yunusa et al., 1995a; Gautam et al., 2002). It is also possible that increased allocation to lateral roots along the ripline was also associated with the removal of some competing trees along the rows by thinning. Thus at age 4 years half the sampled trees had had a tree next to it removed in an earlier thinning,

M.K. Gautam et al. / Forest Ecology and Management 177 (2003) 387±397

compared to only a sixth of the sample trees at age three. Plant root morphological plasticity (i.e. ability to alter root growth) enables a plant to change its root growth pattern as it encounters different soil conditions (Hutchings, 1988), and this is clearly illustrated by the cloned trees. Similarly, root growth towards and into localised patches of moist soil or to nutrients has been demonstrated (Jackson and Caldwell, 1989; Pregitzer et al., 1993). The increased plasticity of root growth in the clonal trees compared to the seedlings was highlighted by its reaction to ripping and growth on the south side. 4.3. Seedlings versus clones and tree toppling The differences found in root growth between clonal and seedling trees could have been caused by several factors that are dif®cult to separate in this study. Firstly, they could have resulted from genetic differences between genotypes (Roberts and Wareing, 1975; Nambiar et al., 1982; Theodorou et al., 1991). The growth and form (GF) tree improvement rating was 17 for the tissue-cultured clone and 14 for the open pollinated seedlings (Mead et al., 1993). The greater GF tree improvement rating in clonal trees could partially explain its growth vigour. Secondly, the clonal trees were physiologically older and this can be a more important factor controlling tree root growth pattern than the genetic differences (Brown, 1974). At time of planting the clone was 6 years from seed and seedling trees were 1-year-old (Mead et al., 1993). Finally, propagation method also affects tree root growth (Ritchie et al., 1992). In other studies height growth of radiata pine cuttings was superior to seedlings (Burdon and Bannister, 1985; Whiteman et al., 1991). Fielding (1970) suggested that, at least during the ®rst 3 years of plantation, trees from cuttings tended to have larger root biomass, and a larger root:shoot ratio than trees raised from seed. In the Lincoln University Agroforestry trial, there was considerably lower toppling incidences in clonal trees compared to seedling trees (Mead et al., 1993; Gautam et al., 1999). At the age of 2 years 10% of the clonal trees in the no understory plots had toppled compared to 86% of the seedling trees. This may be attributed to clonal trees having a greater root:shoot ratio, higher biomass of and allocation to both lateral

395

and vertical roots, as well as a lighter and more porous crown architecture (Bandara et al., 1999). The more porous crown probably reduced the wind force on the clonal trees (Burdett, 1979) while at the same time these trees had better anchorage. 5. Conclusions There were marked differences between the tissuecultured clonal trees and the seedlings. The clonal trees had higher aboveground growth, root:shoot ratio, and coarse root biomass as well as a greater allocation to coarse roots, particularly to lateral and vertical roots. The two groups of trees differed in genetic improvement, physiological age and method of propagation, all of which could have affected these growth patterns. There was a functional equilibrium between roots and shoots that was governed by both tree type and tree age and this remained largely unchanged with different understory environments. However, the two tree types responded differently to competition. The clonal trees, which were more affected by competition, did not alter their fractional allocation to coarse lateral roots, while the seedling trees reduced allocation to lateral roots. As lucerne competition became less from ages 3 to 4 years the seedlings responded by increasing allocation to lateral roots at the expense of the root core. These results suggest that careful selection of appropriate genotypes or planting material could be worthwhile to improve both tree production and competitive ability under silvopastoral systems. Also the greater stability of the clonal trees to toppling by wind, due to a combination of crown architecture differences and rooting patterns, suggests that carefully selected clones could help overcome this silvicultural problem. The ripped zone greatly enhanced tree root production by creating favourable microsites. Also herbicide spraying along riplines and later thinning reduced competition in this zone. Similarly, the more shaded and moister south side of the trees had greater lateral root development. All these changes illustrate the morphological plasticity of roots that enable them to exploit favourable soil conditions. Furthermore, managers could exploit this root plasticity to their advantage.

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Acknowledgements We are grateful to Drs. J. Sasse, C. Frampton, E. Mason and N.B. Comerford for their helpful comments. We also thank Dr. G. Bandara for providing aboveground biomass data. The support of the technical staff of the Field Service Centre at Lincoln University is appreciated.

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