Abstract
Cooperators in the Municipal Tree Restoration Program planted nine Callery pear (Pyrus calleryana Decne.) cultivars in 11 Pennsylvania, U.S. communities for evaluation as street trees, comparing two cultivars (three in one case) in each community. Cooperators measured them annually with standardized methods for 3 years and then at 3-year intervals until the 12th year. The most noteworthy differences occurred in tree height and crown width. The tallest were Aristocrat™, ‘Cleveland Select’, and ‘Redspire’, attaining more than 8 m (26 ft) on average by the twelfth year and even 10.3 m (34 ft) in one community. ‘Autumn Blaze’, evaluated only at one location, was ≈1.5 to 2 m (5 to 6.6 ft) shorter in the 12th year. Heights of the other cultivars, tested at just one or two locations, were similar to the tallest ones. Crown widths differed more in the first 9 years than at the twelfth when on average most were ≈6.5 m (21.5 ft) wide. Cleveland Pride®, ‘Cleveland Select’, Valiant®, and ‘Whitehouse’ were narrower than the others until the ninth year, but only ‘Cleveland Select’ at ≈5.6 m (18.5 ft) remained narrower in the twelfth year and not everywhere. All cultivars were in good health during the whole period, although the foliage of ‘Whitehouse’ exhibited minor injuries in many years. As street trees, the Callery pears were not invasive and did not yet experience branch breakage, which can become a serious problem. All of the cultivars are too tall to be planted under utility wires.
This final report brings to a conclusion a series of evaluations of nine Callery pear (Pyrus calleryana) cultivars planted in Pennsylvania, U.S. as street trees. Two previous reports (Gerhold and McElroy 1994; Gerhold 2000) provided preliminary data up to 9 years after planting, and results have now been extended to the twelfth year. Performance test plantings of the Municipal Tree Restoration Program (MTRP), which include many more species and localities, have encouraged municipalities to improve their tree programs. The test plantings serve as demonstrations of utility-compatible trees, and results provide information helpful for selecting appropriate cultivars that can be planted under utility wires.
Callery pears have been quite popular after introduction of the ‘Bradford’ cultivar in 1963 as a result of their attractive flowers and foliage, and tolerance of urban conditions. Their features as landscape trees have been evaluated in various locations, including Alabama (Fare et al. 1991), New Jersey (Kuser et al. 2001), and Ohio (Haserodt and Sydnor 1983).
MATERIALS AND METHODS
Plants and Experimental Design
Cooperators planted the nine Callery pear cultivars in 11 Pennsylvania communities during 1988 to 1993. Community representatives chose the planting sites with the advice of utility foresters, service foresters in the Bureau of Forestry, or Penn State extension urban foresters.
Each test planting in a community consisted of two paired cultivars, except in Tioga where there were three. A typical test consisted of two cultivars planted alternately within 4 to 10 plots that contained 4 to 16 trees each for a total of 50 trees. All test trees were planted along streets and under electric conductors. The utility company arranged for the removal of large trees in the plots that interfered with utility lines, thus making space for planting in some of the plots.
Plant Materials
The test trees came from several nurseries as balled-and-burlapped stock, and the same nursery always supplied both cultivars for a community. Calipers ranged from 4.4 to 5.1 cm (1.76 to 2 in) and heights from 2.8 to 5.0 m (9.2 to 16.5 ft). The wide range in heights presumably could be explained by genetic differences among cultivars and diverse cultural differences among nurseries that supplied the trees. ‘Redspire’ typically was 0.2 to 0.6 m (0.7 to 2 ft) taller than its companion cultivar; ‘Cleveland Select’ planting stock tended to be shorter but not in all cases.
No unusual climatic events occurred during the study. Wet snow or ice before leaf-fall can cause devastating branch breakage in this species, but none was reported at any of the study sites.
Measurements and Analysis
A trained cooperator inspected and measured the trees soon after planting, then annually during the first 3 years, and at 3-year intervals afterward. During September or October, a service forester or extension urban forester, using standardized procedures, measured tree height, trunk diameter, and crown width; classified the health of foliage, branches, and trunk separately; and rated maintenance needs and overall quality. The health of foliage and branches was estimated according to the percentage injured using a 5-point scale: 1 = 65 to 100, 2 = 45 to 60, 3 = 25 to 40, 4 = 5 to 20, 5 = <5. The scale for trunk injuries was 1 = tree dead, 2 = severe injuries, 3 = moderate, 4 = slight, 5 = no injuries. Causes of injuries such as by diseases, insects, drought, or mechanical damage were identified and recorded when possible.
An analysis of variance (using MINITAB’s General Linear Model) of quantitative data collected in a given year calculated means and determined statistical differences between cultivars at the 5% level of probability. Each test location in every year constituted a separate experiment with plots providing replication within a community. These results, along with comments provided by cooperators, were used to characterize performance of the cultivars.
RESULTS
The most extensive information was obtained for Aristocrat (in four communities), ‘Cleveland Select’ (in seven), and ‘Redspire’ (in four) (Table 1). These three cultivars turned out to be the tallest after the third year (Figure 1); their average heights in the 12th year were ≈8.5 m (28 ft). ‘Autumn Blaze’ at 6.7 m (22 ft) was the shortest, although evaluated in only one community, where it was 1.2 m (4 ft) shorter than ‘Redspire’. The other cultivars, evaluated in one or two communities, were in most cases similar in height or not quite as tall as the largest cultivars.
The average crown widths of some cultivars differed by ≈2 m (6.6 ft) through the sixth year (Figure 2), but most converged at ≈6 to 7 m (19.8 to 23 ft) in the 12th year (Table 1). ‘Cleveland Select’ was narrower, most commonly between 5 and 6 m (16.5 to 19.8 ft). Cleveland Pride®, Valiant®, and ‘Whitehouse’ were as narrow as ‘Cleveland Select’ until the 9th year but afterward spread out much more.
Trunk diameters at a height of 1.35 m (4.5 ft) varied considerably among communities. Aristocrat™ had the largest trunk diameters, but trunks of all cultivars were sturdy enough to provide good support.
The foliage and branches of all but one of the cultivars remained healthy throughout the 12 years as indicated by ratings above 4.0. The one minor exception was the foliage of ‘Whitehouse’ at both locations where it was planted. Anthracnose or fireblight were suspected causes of injuries to leaves but were not verified. Fireblight can be serious in southern states (Fare et al. 1991) but was not a problem in this study.
Two undesirable characteristics of Callery pears that have been reported elsewhere did not affect the trees in the performance tests. No breakage or tearing out of branches was reported, although it is known that severe damage to branches and trunks can occur when heavy snow or ice accumulates on leaves before they are shed late in the fall, often aggravated by wind. Nor was any indication of invasiveness reported, although there were grassy areas adjacent to some of the plots where seedlings could grow.
CONCLUSIONS
All of the Callery pear cultivars that were evaluated for 12 years performed well as street trees, although the likelihood of breakage by storms in the future could cause some to be less desirable than others. Some were distinctive in height or crown spread. ‘Autumn Blaze’ was somewhat shorter than others, and ‘Cleveland Select’ was narrower. The latter, also known as ‘Chanticleer’ pear, was selected as the 2005 Urban Tree of the Year by the Society of Municipal Arborists (2004). All of the Callery pears were still growing in height at the 12th year, and most of them were already interfering with utility wires, so none of the tested cultivars should be planted below overhead wires.
Acknowledgments.
Financial support for the Municipal Tree Restoration Program was provided by Allegheny Energy, Asplundh Tree Expert Co., Baltimore Gas & Electric, Bartlett Tree Expert Co., Duquesne Light, Environmental Consultants Inc., FirstEnergy Corp., Hazlett Tree Service, PECO Energy Company, PPL Electric Utilities, and UGI Utilities. Service foresters of the Pennsylvania Bureau of Forestry and extension urban foresters of Penn State University assisted with the measurements.
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