INTRODUCTION
It is now common practice to enclose newly planted tree seedlings in open-ended plastic tubes. The latter are manufactured under a variety of brand names and can be collectively described as 'tree-shelters'. They are available in various sizes (height and width) and colours. The plastic may be rigid or flexible and is manufactured at several thicknesses. In some instances holes are provided in the side-walls to improve air-circulation within the shelter.
Tree-shelters are intended to provide physical protection and favourable microclimatic conditions for seedling establishment and growth. Rapid growth rates have been reported in a variety of contexts (eg Applegate and Bragg, 1989; Beetson, Taylor and Nestor, 1991). Minimisation of evapotranspiration and increased carbon-dioxide within the shelter are considered to be important in explaining the acceleration in growth. Other explanatory factors are likely to be related to the particular wavelengths of light admitted and their influence on growth hormones (Sure Gro, 1992). The stimulation of phytohormones, in particular auxins and gibberellins is important in this regard, and has implications for tree-shelter design. For example, the thickness and colour of the plastic will influence the intensity and composition of light within the shelter. This in turn will influence phytohormone levels, and hence tree growth and development.
Some observations of the effects of two types of tree-shelters (Tubex® and Sure Gro®; Figures 1 and 2) on the growth of Grevillea robusta are presented here.
Grevillea robusta established in tree-shelters
PROPAGATION AND PLANTING
Seed of G. robusta was obtained from a garden specimen in Wagga Wagga, NSW. Seedlings were raised in 240mm open-ended tubes (diameter 45mm). Site preparation consisted of mechanical removal of grasses over 2 x 2m squares at each planting location. The soil was cultivated and mounded using hand tools, and each plant placed within a water-retaining basin (approx. diameter 50cm). Planting occurred in summer (early January 1993), approximately 24 weeks after germination. Twenty-four seedlings were placed 5m apart in 3 north-south rows (Figure 3). Distance between rows was 5m. Each seedling received approximately 20gms of Osmocote® and an initial 4L of water. Synthetic weed-mat (43x43cm) was placed around each plant.
Sure Gro® heavy duty shelters (height 450mm; diameter approx. 255mm) were erected over 9 seedlings (Figure 2). Tubex®, a rigid cylindrical shelter (height 1600mm; diameter 120mm; see Figure 1) were placed over 9 seedlings also. These were pushed 3-4cm into the soil and secured to stakes. Both shelters are fabricated from translucent plastic, the latter is cream-coloured and the former green. The remaining 6 seedlings were left without a shelter i.e to act as a ‘control’. Stone and grass-clippings were applied as mulches in all instances.
Shortly after planting all the control seedlings and 4 of the those in the Sure Gro® shelters were severely damaged by rabbits. Access to the 4 sheltered seedlings is thought to have been facilitated by the thickness of mulch and the consequent reduction in the effective height of the shelter. The remaining 5 undamaged seedlings were compared to 5 randomly selected seedlings in the Tubex® shelters. Height and stem diameter was recorded at establishment and at 4, 6, 12 and 30 months (Figures 4 and 5). Stem diameter was measured at 4cm above soil surface. The t-test was utilised to ascertain the statistical significance of differences in height and stem diameter.
RESULTS
At planting (0 months) the height of G. robusta seedlings ranged from 30 to 45cms, with the mean height of those enclosed in Tubex® 38cm, and those in Sure Gro® 38.8cm. These differences are not statistically significant (p=0.05). However, significant differences in height emerged in the months following planting; at 4, 6, and 12 months the mean height of G. robusta in Tubex® was greater than the mean height of plants enclosed in Sure Gro® (Figure 4). These differences are statistically significant (p=0.5), with the difference in mean height ranging from 64cm (6 months after planting) to 53.8cm (12 months after planting). The differences in mean height at 30 months were not significant.
For stem diameter, Sure Gro® showed the larger mean values (Figure 5) with statistically significant (p=0.05) differences occurring at 4, 6 and 12 months after planting. These differences increased with time, ranging from 2.6mm at 4 months to 8.2mm at 12 months. Differences at planting (0 months) and 30 months were not statistically significant.
DISCUSSION
CONCLUSION
REFERENCES:
Applegate, G.B., and Bragg, A.L., 1989. Improved growth rates of red cedar (Toona Australis (F. Muell.) Harms) seedlings in growtubes in North Queensland, Australian Forestry 52(4): 293-297.
Beetson, T.B., Taylor, D.W., and Nestor, M.R., 1991. Effects of treeshelters on the early growth of four Australian tree species, Australian Forestry 54(1&2): 60-65.
Sure Gro, 1992. Tree growth and survival rates, Technical Bulletin 4.
Figure 1: Grevillea robusta emerging from a Tubex® tree-shelter some 4 months after planting.
Figure 2: G. robusta standing well clear of a Sure Gro® tree-shelter (also 4 months after planting).
Figure 3: Planting site in 1993, showing N-S rows with seedlings established in Sure Gro® and Tubex® tree-shelters.
Figure 4: Comparison of the mean height of G. robusta in Tubex® and Sure Gro® tree-shelters over 30 months.
Figure 5: Comparison of the mean stem diameter of G. robusta in Tubex® and Sure Gro® tree-shelters over 30 months.
