Comparing square and rectangular spacings in
Eucalyptus nitens
using a Scotch plaid design
Adam M. Gerrand
a,*, William A. Neilsen
baVanuatu Department of Forests, PMB 064, Port Vila, Vanuatu bForestry Tasmania, G.P.O. Box 207B, Hobart, Tasmania 7001, Australia
Received 8 November 1998; accepted 8 November 1998
Abstract
A spacing experiment using a Scotch plaid systematic design was established to investigate a wide range of square and rectangular spacings on the growth and branching habit ofEucalyptus nitens(Deane et Maiden). The Scotch plaid covered stockings ranging from 278 to 2500 stems per hectare (SPH). Spacings used ranged from a rectangularity of 1, for square spacing, to 3, with a spacing of 6 m2 m. The design used here effectively resulted in single tree plots. The ®rst 6 m of stem
was assessed for branch size and direction. The experiment was measured at age of 5 years. There were no signi®cant differences for tree growth between square and rectangular spacing forE. nitens. The number of branches greater than 35 mm increased rapidly at lower stockings. There were no differences in largest branch size with rectangular spacing compared with equivalent square spacing. For the experiment more rectangular spacing had signi®cantly fewer large branches (>35 mm diameter) than square spacing, though the difference was not great. There was no between row bias of large branches with rectangular spacing and there was no signi®cant bias in an E±W direction and only a slight bias to a northerly (Sun) direction. The number of large branches increased signi®cantly with height over the assessed 6 m of stem. Because there were no adverse effects of rectangular spacing, on growth or branching habit ofE. nitens, rectangular spacing could be used in the establishment of plantations, providing ¯exibility of layout of the plantations and thus improving machine access between rows for weed management and other maintenance operations.#2000 Elsevier Science B.V. All rights reserved.
Keywords:Spacing;Eucalyptus nitens; Rectangular spacing; Eucalyptus plantations; Branching; Early growth
1. Introduction
Nelder spacing trials are a method of looking at a wide range of spacings most often by using a series of circles, or arcs, with trees planted along radii from the centre like spokes in a wheel (Nelder, 1962). They are more ef®cient than spacing plots in that they can look
at a wider range of stockings in a given area and so suffer less site variation. However, uniform sites are critical. A variation of this design has been developed by Lundgren using a Scotch plaid layout (Zavitkovski et al., 1983). This design has the advantage of parallel planting lines and easier determination of tree spacing and space occupied (Zavitkovski et al., 1983; Adlard et al., 1992). With this layout a range of square and rectangular spacings can be easily evaluated. *Corresponding author.
A speci®ed stocking rate can be achieved by a range of between row and between tree distances. These can be grouped into square spacings where the inter-row distance equals the inter-tree distance, or rectangular where the row distance is wider than the tree distance. Increasing the row distance is desirable as it can reduce establishment costs and improve access to the stand.
A Scotch plaid experiment ofE. nitens was estab-lished at Upper Castra in Tasmania, Australia, to investigate the effect of rectangular spacing on growth and branching habit.
2. Methods
2.1. Experimental design
The experimental area was a highly productive site in northern Tasmania which has been described pre-viously (Neilsen and Gerrand, 1999). The area was in a temperate environment at an altitude of 310 m with a mean annual rainfall of 1250 mm. The soils were
deep, well-drained krasnozems (USDA classi®cation Hapludox).
The design of the Scotch plaid was a variable within-plot spacing experiment established to inves-tigate a wide range of stockings and rectangularity. Four `Scotch Plaid' plots were established where the spacing was uniformly increased along each side in order to cover a wide range of stockings within a relatively small area (Fig. 1) (Zavitkovski et al., 1983). The plots were 40 m40 m. The stockings in these
plots ranged from 278 to 2500 stems per hectare (SPH) and spacings varied from square to a rectangular factor of three at a spacing of 6 m2 m (Table 1). Plots
were of single tree and only trees of regular spacing, i.e. in the centre of a rectangle or square, were measured. Of 121 planted trees only the 25 at actual operational spacings were measured in each quadrant (Fig. 1).
2.2. Establishment
The area was prepared in the same way as an adjacent plot experiment (Neilsen and Gerrand,
1999). Toorongo provenance E. nitens (Pederick, 1979) container grown stock were planted in late winter, 1992. Complete stocking was ensured by planting two seedlings 20 cm apart at each planting space. Seedlings which were not required were removed progressively over the ®rst 12 months as it became apparent that the remaining tree of a pair was going to survive and be vigorous. Seedlings for removal were chosen randomly except where one had failed to establish satisfactorily.
2.3. Measurement
The experiment was measured at age 5 of years, following canopy closure at the 1000 SPH stocking. Diameter breast height over bark (DBHOB) and height were measured on all trees. Height to the junction with the stem of the ®rst branch carrying green foliage (green crown height) was measured. Forks and ramicorns (large high angle branches) were recorded. Two measures of branching were used, largest branch on the lower 6 m of stem and the number of branches greater than 35 mm diameter on that section of stem. Branch angle was also esti-mated in four classes. Tree form was scored, for defects, sweep, kinks and lean. A system of scoring stem straightness on a scale of 1 to 4 was used for assessment of severity of form defects (Gerrand et al., 1997).
To obtain further detail on branch development, all branches in the ®rst 6 m of stem over 35 mm were scored for size, height of occurrence, in 2 m zones, orientation for compass direction and in relation to row and inter-row direction. Straightness of the branches was also noted.
2.4. Analysis
For branch direction, data was analysed for N±S, E± W and row direction. For analysis of N±S bias branches within 22.58N was assigned a value of 2, NW and NE a value of 1, E and W zero, SE and SWÿ1 and Sÿ2. Similar values were assigned to determine E±W bias. For row direction, branches within 22.58of row direction were assigned a value of 1 and those between row a value ofÿ1. Branches within the 22.5±
67.58zone to the row direction were assigned a value of zero. For rectangular spacing row direction was considered as the shortest spacing distance and between row direction the widest spacing. For the square spacings row direction in the four quadrants was used, with two north±south and two east±west. For analysis of branching with height the number of branches for each 2 m zone upto 6 m on each tree were summed. For initial analysis of the branch direction data the non parametric sign test was used.
Regression analysis and ANOVA were used for analysing tree growth, branch size and direction by
Table 1
Treatments represented in the Scotch plaid spacing experimenta
Stocking (SPH) Tree space (m2) Spacing (row
tree) Rectangularity Tree numbers in plot
2500 4 22 1 1
rectangularity data. Analysis of the Scotch plaid designs have been dealt with by various authors (Zavitkovski and Strong, 1983; Zavitkovski et al., 1983; Adlard et al., 1992).
3. Results
3.1. Growth
Mean DBHOB was linearly related to stocking with higher stocking having lower mean DBHOB. Mean DBHOB varied from 18 cm at 500 SPH to 11 cm at 2500 SPH. Mean height was greatest at median stock-ings and was signi®cantly less at wide or very close spacings.
3.2. Branch size
For all trees there was a relationship between largest branch and stem DBHOB. Largest branch was strongly correlated with stocking with higher stocking having smaller branches. Mean largest branch was 18 mm at 2500 SPH, 40 mm at 1000 SPH and at 750 SPH, it increased to about 46 mm (Fig. 2).
3.3. Rectangularity
There were no signi®cant differences for tree growth between square and rectangular spacing. There
were no differences in largest branch size with rec-tangular spacing. For the Scotch plaid more rectan-gular spacing had signi®cantly fewer large branches (>35 mm) than square spacing though the differences were small (Fig. 3).
3.4. Branch orientation
For the analysis of 716 branches greater than 35 mm diameter there was no signi®cant bias of large branches in the E±W direction but there were signi®-cantly more larger branches developed on the northern (Sun) side of the trees. There was no bias between row direction for large branches, even for strongly rectan-gular spacings. There was a signi®cant increase in number of large branches with height, there being 189 branches in the ®rst 2 m, 253 in the 2±4 m zone and 274 in the 4±6 m zone. E. nitens branches were observed to curve away from other established branches and tended to grow towards clear space.
4. Discussion
4.1. Effect of rectangularity on growth and branching
The effect of rectangular spacing on branch growth has depended on the branching habit of the species being studied. For Douglas ®r the branching habit was
strongly radial so that branches growing in light gaps were larger and related to the space available, while for Pinus radiata D. Don branches curved towards light and there was no radial pattern of branch size with rectangular spacing (James and Revell, 1978). With eucalyptus being crown shy, it was not known which pattern trees in the experiment described here would fall into.
Square spacings might be expected to achieve higher early volume productivity for a given site quality because stands more quickly reach canopy closure. However, this has not generally been the case. In P. radiata extensive trials in New Zealand showed no reduction in growth and no imbalance of branch development due to rectangularity upto a factor of 4 (Sutton, 1968; Sutton, 1970). In other species, Scots pine, birch and a number of eucalyptus species, rectangular spacing did not reduce growth (Lohani, 1980; Salminen and Varmola, 1993; Nie-misto, 1995). The rectangular shape of growing space did not in¯uence the size and number of branches of birch or Scots pine but heavier branches were signi®cantly greater in the between row spacings and in the rectangular spacings (Niemisto, 1995; Salminen and Varmola, 1993).
With Eucalyptus saligna Sm. at rectangular spa-cings of 5.5 m2.4 m compared to square spacings
of 3.7 m3.7 m, both nominally around 740 SPH,
rectangular spacing was observed to give larger branches (Glass, 1985). In that trial there was also some indication that there was a reduction in tree form in rectangular spacings due to larger branch sizes. In a plot experiment surrounding the Scotch plaid dis-cussed here there were no signi®cant differences for branches between the plots at 3.3 m3 m and 4 m2.5 m spacings, rectangularity's of 1.1 and 1.6, respectively (Neilsen and Gerrand, 1999).
In this Scotch plaid experiment for 5 year old E. nitens there were no signi®cant differences for tree growth between square and rectangular spacing. There were no differences in largest branch size with rec-tangular spacing. For the experiment more recrec-tangular spacing had signi®cantly fewer larger branches (>35 mm) than square spacing, indicating that within row spacing had some measure of control of general branch growth.
Unlike other studies, there was no between-row bias of large branches with rectangular spacing. There was a signi®cant but slight bias (15%) of number of large branches facing north, towards the Sun, but otherwise there were no differences at all.E. nitensindicated an ability to access clear space, with branches subjected to competition curving away from competing branches from adjacent trees.
In this Scotch plaid experiment, the number of large branches increased signi®cantly with height in the 6 m log. Branch size was limited by effective stocking and as the stand grows a reducing number of trees, only dominant and co-dominants, were likely to in¯uence the branch size (James and Bunn, 1978). Therefore, an increasing number of large branches with increasing height was expected.
4.2. Management considerations
The ability to use rectangular spacing provides opportunities to adapt spacing to management con-siderations providing ¯exibility of layout of planta-tions (Adlard et al., 1992). Machine access between rows in established plantations is important allowing mechanical operations such as fertilising, slashing or spraying for weed management, and other mainte-nance. Row widths greater than 3.5 m generally allow suf®cient access for machines for the ®rst few years when weed control operations are likely to be needed. Extreme rectangularity may not be acceptable to conservative managers but lower levels of rectangular spacing could be used con®dently. Approximately 1100 SPH are required, for management purposes of volume production and branch control, on the site studied here (Neilsen and Gerrand, 1999), so that appropriate spacings would be 3.5 m2.6 m or 4 m2.25 m.
Acknowledgements
The authors would like to thank Forestry Tasmania and the Forests and Forest Industry Council of Tas-mania for funding the research. Les Johnston and Martin Piesse worked directly on the establishment of the project, and Colin Crawford and Devonport District staff assisted with the establishment and management of the experiment. We would also like to thank Carolyn Ringrose, Lindsay Wilson, Tom Lynch and Paul Dredge for assistance in measuring and data analysis.
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