Tree Condition Associated with Topping in Southern Illinois Communities

Topping, or “the drastic removal of large branches with little regard for location of the pruning cut” (Iles 1989), continues to be widely practiced in communities for clearing utility lines and reducing tree size despite education efforts and condemnation by trained arborists (e.g., Iles 1989; Fazio 1998). A recent survey of home owners in the U.S. Inland Northwest who allowed their trees to be topped addressed the widely held belief that topping is an innocuous and often necessary practice to reduce both tree crown size and danger to property (Fazio and Krumpe 1999). Many individuals surveyed believed that topping ultimately benefited tree health—underscoring the divergence in opinion between tree care professionals and many owners or stewards of urban trees.

Tree health problems associated with topping include disrupted root:crown balance, increased susceptibility to pathogens, decline in tree health, and formation of hazardous defects (Kaiser et al. 1986; Iles 1989; Watson 1991). Although anecdotal evidence regarding the damaging nature of topping is undisputed, we are unaware of any published data quantifying a relationship between topping status and risk of tree failure. Understanding the strength of this relationship may contribute to a more accurate assessment of the true costs associated with this practice.

The objective of this study is to compare the current condition of trees subjected to topping relative to nontopped individuals. Species variation and specific health problems associated with topping are also evaluated.

METHODS

This study was conducted in 4 southern Illinois communities (Anna, Herrin, Mt. Vernon, and Willisville) participating in a street tree inventory program managed through Southern Illinois University Carbondale (Karlovich 1999). Community size ranged from approximately 600 to 17,200 residents, with the responsibility for street tree maintenance under the jurisdiction of the Department of Public Works in all cases. Additionally, Mt. Vernon had an active tree board and Tree City USA status. Anna was a former Tree City USA at the time of this study. Patterns of tree care appeared to be typical of similar-sized communities throughout the U.S. Midwest.

An inventory was conducted from February through August 1998 on all trees within the effective city right of way, typically between street and sidewalk or a distance not exceeding 7.6 m (25 ft) of the street centerline where sidewalks are absent. Data collected included tree species, evidence of prior topping, and a description of symptoms of tree damage or stress, including presence of broken branches, evidence of insect infestation or disease, and presence of bole cavities. An assessment of tree condition, determined to be either good or poor, was made on the basis of present hazardous condition or likelihood of becoming hazardous in the near future. Trees classified as good met all of the following conditions:

• no more than minimal cracks or splits

• cavities not exceeding 30% of stem diameter

• no broken branches exceeding 20 cm (8 in.) diameter

• no more than minimal evidence of insects, disease, or parasitic plants

• less than one-third of root system exposed

• lean less than 30%

Failure to meet any one of the above criteria resulted in classification as poor condition. Data from all four communities were pooled as part of an effort to create a regionwide street tree database. Statistical analyses used a Chi-square test (SAS Institute 1989) with α < 0.05.

RESULTS AND DISCUSSION

A total of 5,252 trees were surveyed. Acer saccharinum was the most commonly occurring species, accounting for 16.6% of all street trees (Table 1*). The 4 most frequently occurring Acer spp. together comprised 40.2% of all trees sampled. The 15 most frequently occurring species and genera accounted for roughly 75% of all trees measured and are considered for further analyses. Among these species, 27.5% of trees had been subjected to topping prior to the survey. Gleditsia triacanthos had a significantly higher likelihood of being topped than other species, reflecting a large-scale, onetime planting of this species in a commercial area where crown maintenance is particularly intensive.

A greater than average likelihood of being in poor condition was observed among the 3 most commonly occurring trees species, with A. saccharinum more than twice as likely to be in poor condition relative to the entire street tree population (Table 1). Juniperus virginiana, Liquidambar styraciflua, Malus spp., and Pinus strobus were more frequently in good condition than the average species, with 3% or fewer surveyed being in poor condition. Better condition among these species may be attributed in part to small branch size and overall small stature precluding some hazard-forming conditions.

Topped trees were 192% more likely (P < 0.001) to be classified as poor condition than those not subjected to topping. (Table 2). All species for which more than 3% of the population were in poor condition showed a numerically greater likelihood of being in poor condition when topped. No significant difference was observed from the mean response to topping, suggesting that adaptability to this practice does not differ among species.

The occurrence of dead branches of any size was 40% more likely in topped trees (Table 3). More frequent occurrence of dead branches in topped trees may be attributed to poor attachment and therefore greater vulnerability to breakage among the adventitious and epicormic sprouts resulting from topping. The only species differing significantly from the mean response was J. virginiana, for which topped trees were more than 5 times more likely than nontopped trees to contain dead branches. The high visibility of dead branches in this evergreen species may have contributed to this value.

Insects, disease, and parasites were 77% more likely to occur in topped than in nontopped trees, with no significant differences observed between species (Table 4). Decreased tree vigor resulting from crown:root imbalance and slowed wound closure believed to be associated with improperly pruned trees may contribute to this phenomenon (Iles 1989). Two species, A. negundo and Ulmus americana, did show large numeric but statistically nonsignificant differences compared to the sample population as a whole. Eight species showed a numerically neutral or positive response to topping, suggesting that conclusions may be changed with a larger sample size.

Topped trees had a 143% greater likelihood to have large cavities than did nontopped trees (Table 5). Acer saccharum and Quercus imbricaria both showed significantly less likelihood than the average species in the occurrence of cavities, although both had a greater than 50% increased likelihood of cavity occurrence in topped trees. These differences may reflect interspecific variation in the ability to compartmentalize wounds. Research to determine the relationship between topping, species, the extent of cavity formation, and structural failure would be helpful in developing more objective measures of hazardous condition.

CONCLUSIONS

Although the nature of the study does not permit the establishment of a cause-and-effect relationship, a correlation between conditions associated with hazard tree formation and topping is quantified. These data suggest that topped trees are more likely to be, or become, unsafe. This suggests that topped trees should be subjected to more frequent and careful observation in tree risk management programs.

Species comparisons in this study are subject to the inherent bias generated by episodes of popularity and disfavor among planted street trees. For example, popularity of Acer spp. during a time prior to the widespread planting of G. triacanthos may result in the more recent choices looking better, due in part to the greater vigor of younger, compared to older, trees. The relative condition or incidence of hazards in the species and genera discussed here apply only to this region or others with similar community forest demographics.

The information presented in this paper should be viewed as a step in the objective evaluation of topping and its role in community forest management. Community forestry professionals may use this information or make similar analyses using their own data to demonstrate to individuals or municipalities the value of seeking tree management alternatives to topping. A combination of several data sets at the regional level using similar sampling criteria would both decrease sampling error associated with a small population and broaden the inference space of this study.