Species Differ in Responses to Tree Shelters

Materials And Methods

The earlier report on the 2 experiments (Witmer et al. 1997) described materials and methods completely, so only the main features are repeated here.

Trees in Experiment 1 were planted in May 1994 at Root’s Nursery in southeastern Pennsylvania, located in USDA Hardiness Zone 6. Seven species and cultivars were planted in 6-tree plots, replicated in 3 randomized complete blocks. Tubex^® tree shelters 8 to 11 cm (3.1 to 4.5 in.) in diameter and 1.5 m (5 ft) in height were placed over 3 trees in a subplot; the other 3 trees served as controls. After the second growing season, shelters were removed from 2 trees in each subplot and from the remaining tree after the third season.

Trees in Experiment 2 were planted in April and May 1995 at Nittany Trees nursery in central Pennsylvania, near the border of USDA Hardiness Zones 5 and 6. Nine species or cultivars were planted in 9-tree plots, replicated in 3 randomized complete blocks. Two sizes of Tree Pro Sr.^® Tree Protectors were used, 9 cm (3.5 in.) and 18 cm (7 in.) in diameter, both 1.5 m (5 ft) in height; each size was placed over 3 trees in a subplot, leaving 3 trees as controls. Shelters were removed after the second or third growing season, as they had been at Root’s Nursery.

Trees were measured every year after summer growth was complete: total height, caliper at 15 cm (6 in.) above ground, and diameter at 137 cm (4.5 ft). An indicator of trunk sturdiness was calculated by dividing caliper by height.

Analyses of variance were conducted in which blocks, tree varieties, and shelter treatments within varieties were the main sources of variation. Duncan’s New Multiple Range Test was used to separate means at the 0.05 significance level.

Results

Statistically significant differences were detected among varieties and among tree shelter treatments nested within varieties (Table 1). These occurred in all years in measurements of height, caliper, and diameter at breast height. Blocks and interactions of blocks with varieties and treatments had much smaller effects, which were nonsignificant in most cases.

Three species suffered severe dieback and mortality presumably due to low winter temperatures: Acer capillipes (24 of 30 trees died), Acer davidii (all 30 trees died), and Carpinus japonica (7 trees in shelters died and 9 trees in shelters had dieback, whereas 9 control trees were healthy). Therefore, these species were excluded from the analyses.

Trees that remained in shelters for 3 growing seasons had similar dimensions as those sheltered only 2 years. Accordingly, these 2 groups were treated as one in the analyses.

Heights of most varieties within 9-cm (3.5-in.) shelters increased greatly by the end of the second year but only moderately in the case of Malus ‘Adirondack’ (Table 2). The heights were even greater in the 18-cm (7-in.) shelters, except for the 2 Prunus varieties. At the end of the second growing season, most trees had emerged from the tops of the shelters. In subsequent years, the height advantages became smaller. Beneficial effects on heights of the 2 magnolias at Root’s Nursery and Carpinus caroliniana in the larger shelters at Nittany Trees were especially dramatic, and the height advantage persisted after removal of the shelters. Acer griseum and Cercis canadensis also retained a height advantage after the fourth growing season.

Harsh winters presumably caused some dieback in 1996 or 1997 at Nittany Trees. Practically all of the affected trees were within shelters: 4 of 9 Carpinus trees in each of the shelter sizes, 8 of 9 Magnolia trees in each of the shelter sizes, 3 of 9 Cornus ‘Celestial’ trees in small shelters, and 2 of 9 Cornus ‘Constellation’ trees in small shelters.

The caliper growth of trees in shelters was greatly depressed by the end of the second growing season in most cases (Table 3). The 2 magnolias were an exception at both nurseries, even though their height growth as a percentage of the control exceeded all other species. In the fourth year at Root’s Nursery, the magnolias were the only varieties for which the calipers of sheltered trees were greater than for the controls. At Nittany Trees, the dieback of sheltered trees, to the ground in many cases, reduced calipers substantially.

The sturdiness indicator of trees within shelters, expressed as caliper of the trunk divided by height, was reduced the most at the end of the second growing season (Table 4). After the tops of trees emerged from shelters and they were removed, sturdiness improved but was still inferior to the controls. The one exception, Malus ‘Adirondack’, had a higher sturdiness value because its height was depressed by shelters proportionally more than its caliper. The reduction in sturdiness was not a severe problem, but a few trees tended to bend over and were fastened to the stakes that were left in place after shelters were removed. The 18-cm (7-in.) shelters caused less of a reduction in sturdiness than the 9-cm (3.5-in.) shelters.

The shelters required little maintenance after they were installed shortly after planting. A few of them were bent over by a wet, heavy snowfall because their wooden stakes were defective. Branches inside the shelters remained relatively small. They could be pruned quickly when shelters were taken off. The plastic shelters appeared to deteriorate very little, and could be reused.

Discussion

Tree shelters were found to be useful in the nursery production of some species but not others. The heights of Magnolia ‘Merrill’ and Magnolia stełlata in shelters were about half again as tall as the controls, and their calipers were also somewhat greater. This size advantage occurred in USDA Hardiness Zone 6, but near the edge of Zone 5 it was nullified by dieback after severe winters. The modest height increases of Acer griseum, Cercis canadensis, Cornus ‘Celestial’, and Cornus ‘Constellation’ in shelters were offset by decreases in caliper and sturdiness; growing them in shelters appears to be advantageous mainly if they are to be sold before they attain 2.5 cm (1-in.) in caliper. The same may apply to Carpinus caroliniana in 18-cm (7-in.) shelters, but not the 9-cm (3.5-in.) size. Prunus serrulata ‘Kwanzan’ and Prunus subhirtella autumnalis can attain 2.5 cm (1 in.) in caliper and 2.7 m (9 ft) in height in 2 seasons inside 18-cm shelters, but after the third season there may be little advantage. Malus ‘Adirondack’ and Syringa ‘Summer Snow’ grew somewhat faster inside shelters for 2 seasons, but the height advantage disappeared afterwards, and caliper growth was inferior at all times.

The 18-cm shelters resulted in superior height and caliper growth compared to the 9-cm shelters and exhibited only minor differences in sturdiness values. In evaluating shelter size, the greater costs for materials and installation of the larger ones should be considered in relation to their biological benefits.

Additional benefits of shelters may apply in some situations. Where wildlife problems occur, shelters protect against deer browsing and rubbing their antlers, and also against bark gnawing by mice and voles. Furthermore, pruning can be deferred for 2 years and thus reduce costs and perhaps alleviate the scheduling of labor.

The results of these 2 experiments are consistent with other studies (Burger et al. 1992; Burger et al. 1996; Burger et al. 1997), although the other 2 studies were conducted with different species in a Mediterranean-type climate. Species differed in their responses to shelters. Generally they were taller than controls initially, and caliper growth was reduced. Root growth also was decreased in 1 study (Burger et al. 1997) and can be anticipated in other cases in which calipers are reduced.

Conclusions

The utility of tree shelters in the nursery production of smaller types of landscape trees depends on ascertaining biological effects, which then must be evaluated for economic and other practical benefits. The magnitude of height increases and caliper decreases that generally are caused before trees emerge from shelters can differ greatly among species. Tree shelter sizes and variable climatic conditions such as harsh winters also play a role.

Nurseries that may want to use tree shelters to accelerate height growth or protect against wildlife damage should investigate their effectiveness on particular varieties grown under local conditions over a period of at least 4 years. Generally the greatest increases in height and decreases in caliper occur in the first 2 years, after which shelters should be removed. After that time, height advantages decline and caliper growth recovers.

The most encouraging results were obtained with magnolias in a warmer portion of Hardiness Zone 5. Acer griseum, Carpinus caroliniana, Cercis canadensis, and Cornus kousa × florida hybrids also showed some promise. The faster growing Prunus serrulata and P. subhirtella varieties appeared to benefit only marginally. Malus and Syringa reticulata, each represented by only 1 cultivar, grew better without shelters.