Height, caliber growth and biomass response of ten shade tree species to treeshelters

D.W. Burger, G.W. Forister and P.A. Kiehl

Department of Environmental Horticulture

University of California

Davis, CA 95616

Abstract

Ten tree species, common to the California landscape, [Sequoia sempervirens (D. Don) Endl., Quercus lobata Née, Quercus agrifolia Née, Alnus rhombifolia Nutt., Lagerstroemia indica 'Watermelon Red', Ginkgo biloba L., Platanus racemosa Nutt., Fraxinus latifolia Benth., Maytenus boaria 'Green Showers' and Pinus canariensis Sweet ex K. Spreng.] were grown from liners with or without treeshelters in the landscape for two years. Periodic (~ every two months) height and caliber measurements were taken and, at the end of the two years, all trees were harvested for fresh and dry weight biomass determinations. Response to the treeshelter microenvironment was species-dependent. Height was greater for sheltered versus unsheltered trees during the first 30-250 days for all species After two years, only G. biloba and P. canariensis trees grown in shelters were taller than their unsheltered counterparts. Stem caliber was often reduced for sheltered trees. Treeshelters may be useful for tree establishment in the landscape, but should be removed once the tree has grown out of it. It is likely staking will be required after removal of the treeshelter.

Treeshelters are cylindrical or square, translucent, polypropylene tubes of varying heights (60-150 cm) originally developed in Great Britain to protect newly transplanted trees from browsing animals. However, it was observed that trees growing in shelters responded with significant increases in height sometimes exceeding 600% that of unsheltered trees (4, 8). These growth enhancements have been attributed to the microenvironment changes around sheltered trees (1, 8, 10). Comparisons between the interior and exterior of treeshelters show the interior environment to have a lower light intensity and a higher temperature, relative humidity and CO2 concentration especially if the shelter is free of any ventilation holes (1, 6). The low-light conditions are known to stimulate stem elongation responses by increasing internodal distances. Other responses such as above-ground biomass accumulation are not always apparent or observed and the effects of treeshelters on below-ground growth are just beginning to be extensively studied (unpublished data).

Treeshelters have been successfully used for tree establishment (12), nursery tree production (1, 13), rural releaf efforts (5), oak regeneration and rehabilitation (6), and landscape revegetation (3). They have been recommended for use in myriad landscape planting and revegetation situations and environments (6, 12) and found unsuitable in others (3). Most of the work with treeshelters to date has been conducted in temperate climates where treeshelters can effectively increase the growing season (9). In California's Mediterranean climate, the use of treeshelters in the landscape has been based on data and information gathered from studies conducted in other climates. Only recently has information been gathered on their use in Mediterranean climates (2,3). The objective of this two-year study was to assess survival and determine the usefulness of treeshelters on ten shade tree species important to the California urban landscape. In doing so, measurements were made of the trees' height and stem diameter (caliber) and assessments of the trees' responses (e.g., ability to stand upright after treeshelter removal) to the treeshelter microenvironment.

Materials and Methods

In April 1993, ten replicate liners each of ten tree species were planted in a randomized complete block design with paired-tree treatments and two blocks. The following seed-propagated tree species were used: Sequoia sempervirens (D. Don) Endl, Quercus lobata Née, Quercus agrifolia Née, Alnus rhombifolia Nutt., Lagerstroemia indica L., Ginkgo biloba L., Platanus racemosa Nutt., Fraxinus latifolia Benth., Maytenus boaria Molina and Pinus canariensis Sweet ex K. Spreng. The liners were planted on 1.8 m centers in rows 1.8 m apart. Each block consisted of two rows; all trees in one row were sheltered (122-cm tall, tan Tubex® shelter) while trees in the adjacent row were unsheltered (control). The paired-tree arrangement provided a clear comparison between sheltered and unsheltered tree treatments for all ten species and reduced the potential effects due to soil type variation and varying environmental conditions. The trees were irrigated with an automatic drip system that provided plentiful amounts of water for the duration of the experiment. Trees were fertilized once, six weeks after planting, with Agriform 20-10-5 plus minors fertilizer tablets (Grace/Sierra, Milpitas, CA). The fertilizer tablets were pressed 8-12 cm into the soil immediately after an irrigation.

At intervals of between 60-90 days the trees' height and stem diameter (caliber) were measured and observations of the trees overall appearance and quality were made. As trees grew and eventually emerged from the treeshelter, the shelter was removed; stakes (two per tree) were used for trees unable to support themselves. Unsheltered trees were staked as needed and none of the trees in either treatment were pruned. After one year, half the trees were measured (height, caliber at ground level and caliber at one m above the ground). The two caliber measurements were used to calculate taper (caliber at 1 m minus caliber at ground level divided by 1 m to obtain a taper estimation in cm/m). Taper was not calculated for L. indica trees since both sheltered and unsheltered trees developed a multi-stem growth habit making accurate caliber measurements impractical and the data questionable. At the end of the second year, trees were harvested by cutting the trees at ground level. Leaves and stems were cut into pieces, weighed (fresh weight), placed in paper bags and dried for at least 1 week at 70 C to obtain dry weights. Collected biomass data included top (leaves, stems) fresh (TFW) and dry weight (TDW). All data were analyzed using the General Linear Model (GLM) Procedure of the SAS statistical system (11).

Results

Survival. Unsheltered tree liners (in particular, P. canariensis, L. indica and M. boaria) were subject to damage or destruction from birds immediately after planting. In these cases, treeshelters were necessary for survival.

Height increase. The influence of treeshelters on height and caliber development is species dependent (Figs. 1-5). All species had height enhancements when grown in shelters. Four species [Q. lobata (Fig. 1B), G. biloba (Fig. 3B), F. latifolia (Fig. 4B) and M. boaria (Fig. 5A)] had nearly immediate (within the first 30 days) enhancements in height. However, the enhanced height of M. boaria trees growing in shelters ceased after 100 days. From then until the end of the experiment, sheltered and unsheltered M. boaria trees grew at essentially the same rate. In all other species the height enhancements due to the treeshelter occurred between 100-250 days. When trees grew taller than the shelter and the shelter was removed, the height increase rate declined [see S. sempervirens (Fig.1A) , Q. lobata (Fig.1B) and Q. agrifolia (Fig.2A), L. indica (Fig.3A) and F. latifolia (Fig.4B)]. At the end of the experiment (700 days), tree heights were the same for sheltered and unsheltered trees except for G. biloba (Fig.3B) and P. canariensis (Fig.5B) where sheltered trees were taller than unsheltered ones.

Trunk malformations occurred in several A. rhombifolia trees once they emerged from the top of the shelter (Figure 6). This problem was present, but less significant, for other tree species. This may be a problem especially when treeshelters are used in areas having winds that come most often from the same direction.

Caliber development. Trees growing in shelters tended to have reduced caliber development, but they were rarely significantly lower (Figs. 1-5). S. sempervirens (Fig.1A) and Q. lobata (Fig.1B) showed some caliber reduction in sheltered trees during the early stages of growth (days 200-400) and F. latifolia (Fig.4B) was the only tree species to show significantly reduced stem caliber at the end of the experiment. None of the tree species growing in shelters could stand alone once the treeshelter was removed and had to be staked.

Top fresh and dry weight and taper (Year 1). After 1 year, top fresh and dry weights of S. sempervirens and P. racemosa were significantly reduced in sheltered trees (Table 1). Statistically significant reductions in top fresh and dry weight could not be shown for either of the Quercus species. Taper was significantly reduced (63-87%) in all sheltered trees that had grown out of the treeshelter during the first year (Table 1). Several A. rhombifolia trees in shelters were damaged or died during the first summer (undetermined reasons) preventing accurate measurements the first year.

Top fresh and dry weight (Year 2). After two years, the top fresh and dry weights of unsheltered trees were greater than those of sheltered trees for all species except G. biloba (Table 2). The increases for TFW were 25-72% while those for TDW were 23- 72%.

Discussion

The combination of increased height, decreased TFW, TDW and decreased caliber of sheltered trees resulted in trees incapable of standing alone after shelter removal. Trees growing in shelters had limited opportunity for movement that usually occurs in unsheltered or unstaked trees. It has been shown that trees kept from moving (e.g., rigidly staked) grow taller and have reduced stem caliber development and taper (7). Caliber and taper development were further diminished by the reduced light intensity found inside treeshelters.

When the shelter was not removed soon after the trees grew out of them, the trunk was subject to deformities due to predominant winds creating a bend at the location on the trunk corresponding to the height of the shelter. This tendency to bend at the top of the treeshelter could be accentuated if trunk taper and reaction/compression wood formation are diminished.

The reduced top biomass of sheltered trees after one year is not surprising since the light intensities inside shelters are roughly half of that of full sun (1). Though the carbon dioxide concentration can be up to 30% higher inside the shelter (1), the overall photosynthetic rates of sheltered trees are most assuredly lower than those of unsheltered ones since light was a limiting factor. Reduced photosynthetic rates may then lead to a reduction in root biomass if a change in the photosynthetic partitioning ratio does not occur.

The effect on top growth for most of the species tested was sustained for the duration of the experiment (2 years). In contrast, height differences between the sheltered and unsheltered trees of most species were evident only during the first growing season. Similar to what Dunn et al. (3) found, unsheltered trees had slower rates of height increase during the early stages of the experiment, but caught up by the end. This suggests that sheltered trees benefit from the favorable microenvironment early in their development, but as the trees grow out of the shelters, its effect diminishes.

Treeshelters stimulate rapid height increases of trees and protect trees from browsing animals and from chemical or mechanical weed control (9). If treeshelters are to be used, they are probably most effective when they are kept to no more than 60 cm tall and removed when trees grow out of them. Alternatively, trees may (and should) be staked when they grow taller than the shelter.

Literature Cited

1. Burger, D.W., P. vihra and R. Harris. 1992. Treeshelter use in producing container-grown

trees. HortScience 27(1):30-32.

2. Costello, L.R., A. Peters and G.A. Giusti. 1996. An evaluation of treeshelter effects on plant

survival and growth in a Mediterranean climate. J. Arboriculture (in press)

3. Dunn, G.M., M.S. Cant, and M.R. Nester. 1994. Potential of two tree shelters to aid the early

establishment and growth of three Australian tree species on the Darling Downs, south-east Queensland. Aust. For. 57(3):95-97.

4. Frearson, K. and N.D. Weiss. 1987. Improved growth rates within treeshelters. Quart. J. For.

81(3):184.187.

5. Keyser, J.M. 1989. Treeshelters for rural releaf. Urban Forestry Forum. American Forestry

Assoc. 9(4):5.

6. Minter, W.F., R.K. Myers, and B.C. Fischer. 1992. Effects of tree shelters on northern red oak

seedlings planted in harvested forested openings. North. J. Appl. For. 9(2):58-63.

7. Neel, P.L. and R.W. Harris. 1971. Motion-induced inhibition of elongation and induction of

dormancy in Liquidambar. Science 173:58-59.

8. Potter, M.J. 1988. Treeshelters improve survival and increase early growth rates. J. Forestry

86(6):39-41.

9. Potter, M.J. 1991. Treeshelters. Forestry Commission Handbook 7. HMSO Publications

Centre. P.O. Box 276, London, SW8 5DT.

10. Rendle, E.L. 1985. The influence of treeshelters on microclimate and the growth of oak. Proc.

6th Natl. Hardwoods Prog., Oxford Forestry Institute.

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12. Sun, D., G. Dickinson, and A. Bragg. 1994. The establishment of Eucalyptus camaldulensis

on a tropical saline site in north Queensland, Australia. Agric., Ecosys., and Environ. 48:1-8.

13. vihra, P., D.W. Burger and R. Harris. 1994. Treeshelters for nursery plants may increase

growth, be cost effective. Calif. Ag. 47(4):13-16.

The authors gratefully acknowledge the support of the Elvenia J. Slosson Foundation and the staff of the UC Bay Area Research and Education Center, Santa Clara, CA.

Table 1. Top fresh and dry weight (TFW, TDW) and taper (cm/m) of five tree species that grew out of the 122-cm treeshelter after one year, n=10

TFW, g TDW, g Taper, cm/m
Control Shelter Control Shelter Control Shelter
Sequoia sempervirens 1213z a 316 b 386 a 102 b 2.1 a 0.4 b
Quercus lobata 350 a 210 a 162 a 96 a 1.6 a 0.2 b
Quercus agrifolia 289 a 221 a 143 a 113 a 1.1 a 0.3 b
Platanus racemosa 4735 a 2243 b 1518 a 720 b 3.0 a 0.7 b
Fraxinus latifolia 1658 a 1345 a 542 a 463 a 1.2 a 0.4 b

z Means followed by different letters are significantly different from one another based on Scheffe's Multiple Mean Comparison procedure.

Table 2. Top fresh and dry weight (TFW, TDW) of sheltered and unsheltered trees after two years growth, n=10.

TFW, g TDW, g
Control Shelter Control Shelter
Sequoia sempervirens 7290 z a 3745 b 2969 a 1554 b
Quercus lobata 2624 a 1770 b 1470 a 986 b
Quercus agrifolia 3307 a 2470 b 1511 a 1149 b
Alnus rhombifolia 8622 a 4833 b 3473 a 2062 b
Lagerstroemia indica 2262 a 1689 b 1215 a 936 b
Ginkgo biloba 61 b 111 a 24 b 43 a
Platanus racemosa 12897 a 9511 b 5148 a 3733 b
Fraxinus latifolia 4052 a 2551 b 2406 a 1362 b
Maytenus boaria 321 a 88 b 123 a 34 b
Pinus canariensis 968 a 536 b 348 a 201 b

z Means followed by different letters are significantly different from one another based on Scheffe's Multiple Mean Comparison procedure.

Figures 1-5. Height (cm) and caliber (mm) of trees grown with ( or ) and without ( - - - or - - -) shelters for 700 days. The horizontal line found in some figures represents the height of the treeshelter (122 cm). Each value is the mean of 8-10 replicate trees 1 Standard Deviation.


Figure 1A


Figure 1B


Figure 2A


Figure 2B


Figure 3A


Figure 3B


Figure 4A


Figure 4B


Figure 5A


Figure 5B


Figure 6. Trunk malformation (bending) of Alnus rhombifolia due to the predominant wind (from the right).