A brief review of the impact of stand density on variables affecting radiata pine stand value

September 2000, updated February 2005

Dr Euan G. Mason

University of Canterbury

Christchurch, New Zealand

Introduction

This document contains a brief review of research that relates to the effects of stand density on product value in radiata pine plantations. It covers research relating to radiata pine conducted in New Zealand primarily, but where research conducted elsewhere fills gaps references to those studies are included. Factors that interact with spacing such as genotype and site are also covered.

In New Zealand both initial stocking and final crop stocking have gradually diminished over the last few decades. As more tending investment has been placed in each tree managers have planted and retained fewer trees thereby increasing growth per tree at the expense of growth per hectare (James 1989). Selection ratios have also dropped with improvements in establishment practices and in genetics (Trewin & Cullen 1985, Mason 1992, Wilcox & Carson 1990). Final crop stocking has been a particularly controversial topic. Not all the research leading to these developments can be included here, but main contributions that specifically address the question of stocking will be cited.
 

Defining stand density

Stand density reflects the extent to which trees use a site. At any given age, density in an unthinned stand might be expressed as stems per hectare, but measures which relate numbers of stems to average tree size are generally more independent of age and site quality.

Reineke (1933) defined stand density indices as linear relationships between the logarithm of stems per unit area and the logarithm of mean dbhob with functional parameters that varied slightly for different species. This implied a limiting relationship between average size and stocking. A similar assumption was implied by the use of tree area ratio (Chisman & Schumacher 1940), where the area occupied by a given tree in a fully stocked stand was expressed as a quadratic function of dbhob.

Relative spacing, the ratio between average distance between trees and average height of the dominant stems, has also been used successfully to represent density (Beekhuis 1966).

Measures of stand density commonly used in modern growth models are basal area per unit area (the sum of stem crossectional areas at breast height, usually derived from measures of dbhob), and numbers of stems per unit area.

Garcia (1984, 1990) and West et al. (1982) used representations of tree canopy as measures of density. In the former case, canopy closure was derived from levels of thinning and pruning of stands assumed to have 100% closure, whilst in the latter, the total length of crown per hectare was directly estimated from tree height, pruned height, and numbers of stems per hectare. These models were built to fill a need for more sensitive characterisation of growth and yield in heavily thinned and pruned stands of radiata pine in New Zealand.

Stand density measurement allowed the production of yield tables sensitive to stand density (MacKinney et al. 1937), known as "variable density" yield tables. MacKinney et al. (1937) also improved on graphical techniques by using least-squares regression to estimate parameters of functionalised yield curves. Variable density yield tables for radiata pine in New Zealand were prepared using graphical techniques by Lewis (1954). These tables and the growth and yield modelling techniques subsequently pioneered by Clutter (1966) allowed broad approximations of the consequences of adopting different stand densities. Stand density has many impacts that are not properly represented by these models, however, as will be demonstrated in this paper.

"Stems per hectare" is the measure most often used in radiata pine plantations to control density, and it is important to distinguish between effects of initial stocking, final stocking, selection ratio (the ratio between initial and final crop stocking), and the timing of thinning.

Effects of density expressed as stems per hectare vary with other factors such as tree size, site quality, genetics, tending regime, rotation length, and exposure. These and other factors will be covered here.

Density affects growth rates, mortality, tree form, branching, wood properties, wind stability and weed growth. These outcomes of density management form the outline of this paper. I have briefly mentioned a few papers on the topic at the end. These are mentioned because density management practices adopted by a plantation manager will ultimately depend as much on objectives of management and on economic assumptions as on the biological capability of a tree species.
 

Effects of spacing on growth rates

Foliage

Plant dry matter production is generally a linear function of intercepted radiation (eg: Monteith 1977, Biscoe & Gallagher 1975). In New Zealand (Grace et al. 1987), intercepted radiation increased with increasing amounts of foliage, at least up to a leaf area index (LAI) of 3.5. Hunter et al. (1987) found annual radiata pine bole volume increment/ha was linearly related to foliage mass and % foliar nitrogen. At some maximum LAI, the canopy can be considered "closed". For agricultural crops, LAI values greater than 4 were considered to represent a closed canopy (Biscoe & Gallagher 1975). However, for radiata pine aboveground production was found to increase approximately linearly with LAI values as high as 10, and showed a declining rate of increase up to LAI values of 20 or more in unthinned stands (Beets & Pollock 1987). It is, therefore, important to understand the effects of stand density on foliage mass.

Foliage mass of radiata pine was reported to increase with age until an equilibrium level was reached (Madgwick et al. 1977), and the same was reported for Douglas-fir, with the rate of approach to the maximum greater at higher stockings (Long & Smith 1984). Not all crops exhibit this pattern of foliage mass with time, however; Switzer et al. (1968) found that in a Pinus taeda stand foliage increased to a maximum, then declined to a reasonably high equilibrium level, while that of Betula spp. increased to a maximum which was also the equilibrium. Kuuluvainen (1991) reported that foliage in a naturally regenerated stand of Pinus sylvestris increased with time to a maximum, and then declined sharply. The current annual increment of bole volume peaked before the time of maximum foliage. Kuuluvainen suggested that the decline in foliage and growth was due to competition stress, immobilisation of nutrients, and more mutual shading of trees as stocking diminished. Beets & Pollock (1987) found that radiata pine LAI increased with age to a maximum at age 6, and then declined.

Madgwick et al. (1977) documented the impact of stocking on foliage mass for radiata pine between the ages of 6 and 22 in Kaingaroa Forest. Data were highly variable, but it appears that foliage mass was independent of density at stockings greater than 400 stems/ha., but diminished with diminishing stocking below this level. This is consistent with the behaviour of model PPM88 (Garcia 1990), where the difference in bole growth rate between stands of 100 stems/ha. and 200 stems/ha. is as great as that between 200 stems/ha. and 500 stems/ha.. It should be noted that most of the argument relating to final crop stocking focuses on stockings below five hundred stems per hectare, a range in which stocking should profoundly affect foliage mass/ha, light interception, and therefore productivity.

Canopy depth also varies with spacing. Radiata pine canopy depth was found to increase by 1.6 m for every 1 m increase in between-tree spacing (Beekhuis 1965). Beekhuis pointed out that the overriding influence on canopy depth in young stands (less than 25 m in MTH) was height growth, with a 1 m increase in canopy depth for every 3 m increase in height.
 

Diameter at breast height and basal area

Growth

Using data from 15 Nelder spacing experiments, Mason (1992) determined that basal area growth per hectare increased linearly with stocking up to age four in young radiata pine plantations across a wide range of stockings up to 12000 stems per hectare. Individual tree diameter growth was unrelated to stocking for the first 4 years and then diminished with stocking during the fifth growing season. The Tikitere agroforestry experiment yielded consistent results, where a stocking range of 300-2400 stems per hectare did not affect individual tree diameter growth until the fifth growing season (Knowles et al. 1999b). Menzies et al. (1989) found the same results except that competition began during the 4th growing season on a very productive farm site at low altitude.

Figure 1 – Mean top height (m) and dbhob (cm) at Tikitere, age 26, vs stocking (after Knowles et al. 1999)

Stocking affects tree dbhob growth non-linearly after competition begins, a fact that is built into many yield tables (eg: Lewis 1954) and growth and yield models (eg: Garcia 1984, 1988, 1990). At Tikitere (Figure 1), final crop stockings from 50 to 400 stems/ha decreased mean dbhob at age 26 from 80 cm to 50 cm, while volume productivity per hectare increased from 200 to 1000 m³/ha (Knowles et al. 1999b). The culmination of current annual increment in basal area was earlier for higher stockings. Mason (1992) also found an earlier culmination for basal area growth at higher stockings in Nelder experiments.

Despite arguments for lower final crop stockings (eg: Maclaren 1995), Whyte & Woollons (1990) reported that, in a thinning experiment in Kaingaroa Forest, yield from a stand thinned to 300 stems/ha was so much greater than that of a stand at 200 stems/ha, that volumes of the largest 200 trees/ha in the 300 stems/ha plots were almost as great as the entire yield from plots thinned to 200 stems/ha.. Some of the earlier arguments for very low final crop stockings (see James (1990) for a summary) may have been motivated by the use of models built using data from only more highly stocked stands, and were therefore based on extrapolations.

Selection ratios

Varying selection ratios between 1 and 6 did not affect diameter growth of radiata pine at age 19 (Maclaren & Kimberley 1991). The authors concluded that improved selection of large trees may have counteracted effects of spacing. Subsequent work showed, however that correlations between dbhob at young ages (age 7) and dbhob at rotation age (32) may be very weak. The correlation was poorer where stands had been thinned. These facts suggested that the cost of higher initial stockings may not be justified by the selection of large trees (Maclaren 1995).
 

Height

Growth

Although height growth is much less affected by stocking than diameter growth, several researchers have found that height growth diminishes as stocking diminishes. Menzies et al. (1989) noted an increase in height growth during and after the 3rd growing season with increasing spacings from 200-800 stems/ha. Mason (1992) found a reduction in height growth with decreasing stocking below two thousand stems per hectare in Nelder experiments. These effects were apparent by age 3, before stocking had affected dbh. Maclaren et al. (1995) studied the effect in older stands and found a 2 metre reduction in mean top height for every halving of stocking. They noted that detection of the effect was dependent on a constant selection ratio among thinned stands and postulated that exposure of trees at lower stockings to wind might explain the phenomenon. An alternative hypothesis is that trees may detect infrared radiation from neighbours at high stockings and respond by accelerating height growth. This is an area for future research.

At Tikitere (Figure 1) the 50 stems per hectare treatment was almost 10 meters smaller in mean top height than the four hundred stems per hectare treatment at age 26 (Knowles et al. 1999b).

Selection ratios

Dominance is apparently a poor basis for selection in young crops. An even chance of dominants at age 5 being suppressed by age 12 was reported by Sutton (1973).

Maclaren & Kimberley (1991) found an increase in height growth with selection ratio that ultimately caused an increase in volume at age 19 (Figure 2).

Figure 2 – Effect of selection ratio on total volume/ha at age 19 (after Maclaren & Kimberley 1991)
 

Effects of spacing on mortality

The "normal" survival of unthinned radiata pine as a function of tree height has been reported by Penistan (1960). In general, stocking diminishes naturally with height, and this fact is built into growth and yield models (eg: Beekhuis 1966, Garcia 1988). It should be noted, however, that there has been a decline in rates of mortality of radiata pine in the Central North Island region over 30 years (Klitscher 1987). Control of Sirex and Dothistroma outbreaks, improved genotypes, and improved management practices were proposed as reasons for the changes in trends, but causes have never been definitively identified.

A geometrical appraisal of growth processes in agricultural crops has led to a "law" of stand density which is actually a hypothesis of a power relationship between numbers of plants per unit area, and average plant mass (Yoda et al. 1963). As explained by Drew & Flewelling (1977), given certain assumptions, mean plant weight should be directly proportional to plants per unit area to the -3/2 power. Assuming a proportionality between mean tree weight and dbhob to the 5/2 power, this weight and stocking relationship is equivalent to Reineke's (1933) stand density index.

There is some evidence that the "law" might be applied to New Zealand's radiata pine crops (Drew & Flewelling 1977), but Zeide (1987) points out that two necessary assumptions required by the "law":

(i) complete canopy closure is maintained by the combined action of crown growth and self-thinning;
(ii) plants of the same species are always allometrically identical

Effects of spacing on tree form

Taper

It is well established that trees grown at lower stockings will have greater taper than those grown at higher stockings. At Tikitere, for instance, where initial stockings ranged from 300-2400 stems per hectare and final stockings from 50 to 400 stems per hectare, taper decreased markedly as stocking increased (Knowles et al. 1999b). Trees on the edges of shelter belts have been shown to have greater taper than those within shelter belts (Tombleson & Inglis 1986).

These effects may be caused by an increase in tree sway at lower stockings (Jacobs 1954), the effect of stocking on height growth mentioned above, and also by a more rapid rise of the canopy level (Beekhuis 1965) at higher stockings. There is a need to fit taper equations to data from these experiments in order to assess fully the effects of stocking on taper.

If tree sway and reduced height growth are the dominant causes of increased taper with stocking, then reducing initial stockings should increase log taper even for the same final crop stockings. Although increased taper was not explicitly reported, it was implied by the results of a study of selection ratios conducted by Maclaren & Kimberley (1991). The implication is that raising selection ratios and keeping stocking high for a longer period in the rotation may reduce taper even at low final crop stockings, but this topic requires more research.
 

Stem quality

Higher selection ratios may allow managers to grow more stems of high-quality for any given final crop stocking, but this effect may not be as marked as managers commonly believe. Sutton (1973) found that leader malformation present at age 5 often disappeared by age 9, but that stem malformation was more persistent. Only 38% of leaders malformed at age 3.5 resulted in multileadering at age 22 (Maclaren 1995). Sutton recommended that the order of selection criteria for crop trees be amended to:

1) stem form;

2) leader malformation; and

3) dominance.

Nonetheless, specific tests of selection ratios from 1 to 6 showed that stem form improved markedly with increasing selection ratio (Maclaren & Kimberley 1991). As selection ratio moved from 1 to 6 percentage of pruned logs that were straight, round, and had no scars moved from 74% to 94% (Figure 3), and from 70% to 90% for unpruned logs. James (1979) had studied the impact of selection ratio and tree breed on the proportion of defective stems in the same experiment. Increasing selection ratio improved the quality of the crop trees, but the rate of improvement slowed with increasing selection ratio. There was a marked improvement changing from a selection ratio of 1 to 2, but much less improvement going from 2 to 3.

Figure 3 – Effect of selection ratio on proportion of pruned logs that are straight and round (after Maclaren & Kimberley 1991)

It should be noted that in the experiment quoted above, researchers selected the crop trees. In practice, thinning contractors usually select crop stems, and they may pay little heed to stem form. In the area of toppled trees reported by Mason (1985), a return to the site after thinning showed that the contractor had selected large trees but his choice was unrelated to stem form. In such circumstances selection ratio could have minimal impact on final crop quality.

The importance of selection ratio may vary with genotype. Studies by James (1979), Maclaren & Kimberley (1991) and by Wilcox & Carson (1990) showed that improved breeds had more acceptable trees. When evaluating the effects of improved breeds and the consequences of breeding on selection ratio, more emphasis should be placed on stem quality than on leader quality.

Use of radiata pine cuttings may also influence choice of selection ratio, with cuttings from aged parents requiring lower selection ratios than seedlings in order to achieve an equivalent quality among crop trees (Menzies et al. 1989).
 

Effects of spacing on branching

Branch index

Branch index (BIX) is the mean of the largest branch in each of the four quadrants of a log. Strength & stiffness of wood obtained from logs decreases with BIX (Bier 1986).

A model to predict branch index included dbhob at age 20, the inverse of predominant mean height at final thinning, site index, and log height class. Initial spacing (1370-5100 stems/ha) was not related to BIX (Inglis & Cleland 1982). The formula was:

Where BIX=branch index, D₂₀=mean dbhob at age 20, HC=height to top of log, and HTT=predominant mean height at last thinning

However, at Tikitere branch index was strongly correlated with spacing; BIX in the second log diminished from 12.3 to 5.5 centimetres as spacing increased from one hundred to four hundred stems per hectare (Figure 4) (Knowles et al. 1999b). Shelter belts also show strong correlations between spacing and BIX (Tombleson & Inglis 1986). In Inglis & Cleland’s (1982) model, dbhob at age 20 reflected stocking, as did the inverse of predominant mean height.

Figure 4 – Branch index and number of whorls in the second log vs stocking at Tikitere (after Knowles et al. 1999)
 

An unpublished revision of the branch index model included initial stocking, however in a validation of the revised model it was suggested that the model also needed final crop stocking as an independent variable (Tombleson et al. 1990). Departures from the model showed that BIX of the second log decreased with site index and spacing. The revised model apparently included regions and had different coefficients for different 6 m logs up the stem. I could find no record of formal publication of the revised model.
 

Branch angle

Unpublished data at the University of Canterbury shows that branch angle decreases with lowering stocking for unimproved radiata pine, and that highly improved breeds have lower angles irrespective of stocking.

There are no published references to stocking effects on branch angle.
 

Internode index

Studies of the effect of spacing on internode index, the ratio of all branch segments over 60 centimetres for the total 5.5 metre log, have yielded conflicting results. Knowles et al. (1999b) found that as final crop stocking increased from 100 to 400 stems/ha whorl numbers decreased (Figure 4) and internode index in the second log increased from 0.04 to 0.22. However, Maclaren (1989) reported that internode index was unrelated to stocking between 117 and 383 stems/ha. Grace & Carson (1993) assumed that a measure of spacing was not required in their model that predicted internode length from mean annual rainfall, altitude, and "level of genetic improvement" after examining a few plots that included different stocking levels. Woollons et al. (in prep) found that internode index was independent of stand density, across a large dataset that would have included a range of selection ratios. It is likely that internode index increases with stocking as a consequence of increased height growth with stocking, and this effect would only be detectable when selection ratios were held constant as at Tikitere.

Carson et al. (1988) found that internode lengths varied with both genotype and site, and so both factors should be taken into account when manipulating stockings to change internode length.
 

Implications for pruning

Having larger branches at the beginning of the rotation means that pruning will cost more, and also that occlusion of pruned branch stubs may take longer. While these observations are based on anecdotes, it should be noted that diameter over occlusion is related to diameter over stubs (DOS) (Park 1980), and to the extent that DOS increased with spacing, diameter over occlusion would also increase. With more tapered stems in a lowly stocked stand, more frequent and smaller pruning lifts would be required to maintain the same DOS as in a more highly stocked stand. This would affect the profitability of a clearwood regime.
 

Branch models

At least two branch models have been built for radiata pine in New Zealand. The model of Grace et al. (1999) includes a spacing index that allows branch size to increase as relative spacing increases. Accurate estimates of height growth are critical for this model to predict branch formation accurately.

A different, probabilistic approach to modelling branches has been taken by Woollons et al. (in prep.). This reduces the dependence of the model on estimates of height growth that are notoriously difficult to predict from year to year. Variation in stocking influences branch size through its effect on dbhob in this model, and so stocking is not used directly as an independent variable.
 

Effect of genotype

Implications of effects of stocking on branch size may vary with genotype. Shelbourne (1970) reported that, compared to unimproved radiata pine, the "850" breed of radiata pine had 19% smaller branches in the first log, and 11% smaller branches in the second log.

Watt (1999) reported that the "850" breed had 1 cm smaller BIX than the "870" breed, and that this was correlated with smaller internodes for the "850" breed.
 

Effects of spacing on wood quality

Wood density

Wood density has been found to increase with increasing "distance" from the pith of a tree in a nationwide survey of wood qualities (Cown et al. 1991). "Distance was measured in terms of growth rings and was found to be "largely independent" of dbhob growth rate.

At Tikitere, mean-tree wood density decreased from 378 kg/m³ to 362 kg/m³ as stocking went from 100 stems/ha to 400 stems/ha. This was explained by a higher proportion of wood within the first few growth rings at higher stockings.

Wood density decreased markedly with latitude and altitude (Cown et al. 1991). This may be a function of temperature, which is very highly correlated with altitude and latitude in New Zealand (Norton 1985). Wood density is likely to be an important issue on lower quality sites, therefore.
 

Wood stability

Shrinkage of wood follows the density pattern, with higher shrinkage at higher densities. One might expect higher shrinkage on average with lower stockings, but whether or not differences in shrinkage within boards would increase with spacing may depend on the sawing patterns adopted.
 

Stiffness

Along with stability, stiffness is an extremely valuable wood quality. Little is known about the reasons for variation in stiffness of radiata pine within a tree, but stiffness tends to increase with "distance" from pith and it is hypothesised that decreases in microfibril angle with ring number from the pith may explain the phenomenon (Walker & Butterfield 1996).

Recent research at the School of Forestry (Lasserre et al. 2004) has shown that increasing initial stocking from 833 stems/ha to 2500 stems/ha increased stiffness in the first 10 growth rings by 40%.  This effect was shown to be independent of genotype and site.  The same study (Lasserre et al. in prep) showed that stiffness was much more highly correlated with microfibril angle than with density.
 

Corewood

It is well known that the inner portion of a radiata pine stem contains wood of lower density, smaller tracheids, higher longitudinal shrinkage, and lower stiffness (Cown et al. 1991).

Using an assumption that there were a constant number of corewood rings in all trees, West (1997) examined the impacts of site and silviculture on amount of corewood. Different stockings with constant selection ratios were simulated in Standpak (Whiteside et al. 1997), and the exercise suggested that there would be little difference in corewood proportion with changes in stocking. However, using different selection ratios (and initial stockings) to achieve the same final crop stocking would probably show an increase in corewood proportion with decreasing selection ratio, as early individual tree growth would be more rapid with lower selection ratios.

The assumption of a constant number of corewood rings in West’s (1997) analysis may be debatable. If corewood results from a mechanical process related to stem size, then the results of adopting a lower selection ratio might be quite different. Given that corewood occurs all the way up a tree, the name "juvenile wood" is not logical, and causes of the phenomena that make up corewood remain topics for future research.
 

Compression wood

Compression wood has lower cellulose, more lignin, reduced tracheid length, and is undesirable. It generally grows on leaning stems (Sinnott 1952).

Toppling of juvenile radiata pine is a common cause of reaction wood in tree stems (Mason 1985, 1992, Harris 1977). For any given toppling rate, lower selection ratios would limit the ability of a manager to thin out previously toppled trees containing large amounts of compression wood.

As lower stockings may promote wind damage (Fraser 1964, Somerville 1979), proportions of trees with compression wood may increase as stocking decreases.
 

Resin pockets

Resin pockets caused a loss in value of veneer bolts of up to 45% in one careful study (Park & Parker 1982). Causes of resin pockets are unknown, but if, as is sometimes postulated, they result from tree sway, then lowering stocking may increase resin pocket frequency by increasing tree sway (Fraser 1964). This topic requires more research.
 

Impacts of spacing on log grade

Two studies have been made of the impact of spacing or selection ratio on volumes of different log grades.

Maclaren & Knowles (1999a) did a "MARVL" (Method for Assessment of Recoverable Volume by Log Type) assessment (Deadman & Goulding 1978) of the Tikitere experiment at age 21 and simulated results at older rotation ages. This study examined different final crop stockings with identical selection ratios. Results showed that, as expected, far greater volumes were produced at higher stockings (400 stems/ha). There were greater amounts of pulpwood at lower stockings (100 stems/ha), but volumes of all other log types increased with stocking. In particular, small branched saw logs were much more abundant at higher stockings. Piece sizes were much greater at lower stockings, and it was assumed that lower stockings produced higher clearwood indices (small end diameter - defect core diameter) (Maclaren & Knowles 1999b).

Maclaren & Kimberley (1991) conducted a MARVL assessment of the experiment reported by James (1979) that examined alternative selection ratios (1-6) to achieve the same final crop stocking. There was an increase in height growth with increasing stocking, but no increase in diameter growth. This effectively reduced the predicted taper, resulting in greater volumes of pruned logs as well as greater volumes overall with higher selection ratios. Increasing selection ratio from 1 to 6 increased pruned volume/ha from 116 m³ to 146 m³ and total volume/ha from 340 m³ to 397 m³. Higher selection ratios also produced a greater proportion of straight logs. As in James (1979) study, the rate of increase in volume and log straightness slowed with increasing selection ratio.
 

Effects of spacing on wind damage

Effects of spacing on wind damage have been rarely studied. Anecdotal evidence suggests that while trees can develop resistance to persistent exposure, lower stocked stands are more susceptible to wind damage. Research supports these hypotheses.

Wind tunnel tests with model trees showed that Increasing distance between trees from 25% of height to 40% of height doubled the bending moment experienced by individual trees (Fraser 1964).

Radiata pine trees subjected to sway developed more taper and larger anchoring roots than adjacent trees prevented from swaying (Jacobs 1954). This implies that trees would be more susceptible to wind throw immediately after thinning, an implication that is also supported by anecdotal evidence.

Somerville (1989), in a review of wind impacts on forests reported that there was generally more damage during storms with lower stockings.

At Tikitere wind damage increased with lowered stocking rates. 41% of trees were damaged at 50 stems/ha while only 24% were damaged at 400 stems/ha (Knowles et al. 1999b).
 

Impact of spacing on weed growth

Higher spacing may reduce impacts of weeds, but the topic has been rarely studied. Knowles et al. (1999a) reported an inverse non-linear relationship between canopy closure and pasture production. Canopy closure was linearly related to basal area for young crops, but rose to an asymptote at 60 m²/ha. Increasing initial stocking would reduce pasture production at a rapid rate according to their models.

The initial growth model for radiata pine (Mason 1992, Mason & Whyte 1997) when combined with Knowles et al.’s (1999a) model suggests effects on pasture productivity could be important by age 3 on a typical Central North Island site at 200 m altitude.
 

Rectangularity of initial spacing

Plantation managers typically plant trees at rectangular spacings, with smaller distances between trees within rows than between rows. This reduces costs of site preparation, planting, and post-planting care. The effects of rectangularity in spacing have not been fully investigated for radiata pine.

Sutton (1970) noted that rectangularity of initial spacing had no detectable effect on branch size, with rectangularities up to 1.8 x 7.3 m.

Tikitere’s twin row treatment, at 100 stems/ha but with distances within pairs as if they were planted at 400 stems/ha can be compared with the more evenly spaced 100 stems/ha treatment. Results at age 25 showed a reduction in dbhob from 75 cm to 69 cm due to rectangularity, and increase in mean top height from 33.1 m to 34.6 m. This equated to a drop in volume from 400 m³ to 368 m³ (Knowles et al. 1999).

Grace (1990) used a process-level model of light interception to evaluate a variety of initial spacing configurations through simulation. Regular (square) spacing produced the highest rates of net photosynthesis in a ten year old stand. As rectangularity increased, growth dropped by a small amount (3%). She also found seasonal differences in orientation of rows relative to a north-south direction.

Group planting is an alternative planting configuration that may facilitate selection of crop trees. The idea is to plant the selection ratio number of trees in each group, and then aim to finally select only one tree from each group. Although such experiments have been initiated in New Zealand, results have not been reported.
 

Financial analyses of alternative spacings

The choice of planting spacing, selection ratio, and final crop stocking is heavily dependent on objectives of forest owners, product prices, costs, and interest rate assumptions. I have noted a few instances where relevant financial analyses have been done.

Maclaren (1989) found that the financially optimum final crop stocking for clearwood regimes of radiata pine varied inversely with interest rate. Optimum spacing was lower with a lower site index, because lower site indices resulted in larger branches in the second log. The optimum increased with rotation length and decreased with an increased price for clear wood.

Whiteside et al. (1997) reported that simulations followed by financial analyses showed 250 stems/ha was the most profitable stocking for clearwood regimes, and that 350 stems per hectare should be adopted for unpruned regimes.

Choice of selection ratio was found to be highly dependent on interest rate, with high interest rates resulting in low selection ratios (MacLaren & Kimberley 1991).
 

Concluding remarks

More than just final crop stocking must be considered when designing a regime. Initial stocking, selection ratio, and timing of thinnings can affect productivity, tree geometry and wood value. It is also clear that financial factors and assumptions profoundly affect choice of stocking. Management of spacing, however, is a key to good silviculture, and more research is needed to improve our understanding of its effects.
 

References

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Beekhuis, J., 1966, Prediction of yield and increment in Pinus radiata stands in New Zealand, New Zealand Forest Service, Forest Research Institute Technical Paper No. 49, 39 pp

Beets, P.N., & D.S. Pollock, 1987, Accumulation and partitioning of dry matter in Pinus radiata as related to stand age and thinning, New Zealand Journal of Forestry Science 17 (2/3): 246-271

Bier, H., 1986, Log quality and the strength and stiffness of structural timber, New Zealand Journal of Forestry Science 16 (2): 176-186

Biscoe, P.V., & J.N. Gallagher, 1975, Weather, dry matter production and yield, IN: Landsberg, J.J., & C.V. Cutting (Eds.), Environmental effects on crop physiology, Academic Press, London; 75-100

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Chisman, H.H., & F.X. Schumacher, 1940, On the tree area ratio and certain of its applications, Journal of Forestry 38: 311-317

Clutter, J.L., 1963, Compatible growth and yield models for Loblolly pine, Forest Science 9 (3): 354-371

Cown, D.J., D.L. McConchie & G.D. Young, 1991, Radiata pine wood properties survey, New Zealand Forest Research Institute Bulletin No. 50, 50 pp

Deadman, M.W., & C.J. Goulding, 1978, Method for assessment of recoverable volume by log types, New Zealand Journal of Forestry Science 9: 225-239

Drew, J.T., & J.W. Flewelling, 1977, Some recent Japanese theories of yield-density relationships and their application to Monterey pine plantations, Forest Science 23 (4): 517-534

Fraser, A.I., 1964, Wind tunnel and other related studies on coniferous trees and tree crops, Scottish Forestry 18: 84-92

Garcia, O, 1984, New class of growth models for even-aged stands: Pinus radiata in Golden Downs Forest, New Zealand Journal of Forestry Science, 14 (1); 65-88

Garcia, O, 1988, Growth modelling - a (re)view, New Zealand Forestry 33 (3); 14-17

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Acknowledgement

This review was made possible by a grant from Forestal Mininco, and I am very grateful for the company's interest.