Abstract
The study of approximately 5,000 trees growing in the Quebec City area permitted the identification of the major problems related to the urban environment. These trees include ornamentals, those they grow along streets, and those in urban parks and woods. The results show that abiotic diseases are far more important and cause much greater damage than biotic diseases.
In comparison with forest trees, city trees growing in back yards, parks and recreational areas are confronted with disturbances which affect very rapidly their health and vigor. Many studies have been conducted and carried out on the perturbations causing wood decay and losses in our natural forests, but little is known about the insects and diseases which kill our city trees, nor about their importance.
Since the summer of 1980, consultants Dery, Rocray et associes have studied many trees in the Quebec City area. The firm has gathered and compiled information that was eventually presented as a report to the Ministere de I’Environnement in March ’82. The report discusses: 1) the species frequently encountered, 2) the principal causes of deterioration, 3) The qualitative and quantitative scale of severity for the problems encountered, 4) the interrelations that exist among the causes, and, finally, 5) proposes recommendations aimed at more appropriate protection for urban trees.
Methods
All the information needed to carry out the study was gathered with inventories that were especially conceived to evaluate with precision the health of trees growing in an urban environment. Four types of inventories were chosen.
Expert appraisement
The tree evaluation done on privately owned trees (more than 400) were noted on a special form. Most of these trees were ornamentals.
Systematic inventories
This type of inventory was used on private and public land having hundreds of trees. These were numbered to make compilation easier. The systematic inventory is more elaborate and has a greater precision than sample-plots and randomly selected transects.
Sample-plots
Within public parks and extended urban woods, sample-plots were used because of the great number of trees. This type of inventory gives a picture of the site, without studying each tree in particular. The sample-plots represented approximately 5% to 10% of the total area for each site.
Randomly selected transect
To complete the information on each site, we made randomly selected transects in combination with sampleplots.
For the purposes of the study, we have divided the trees into three classes in relation to their geographical location: ornamental trees, right-of-way (ROW) trees, and trees in wooded sites (See details in Table 1).
Classes of trees in relation to their location.
Results
The urban environment contains a vast variety of tree species that forms a heterogeneous population. Thus, this population is susceptible to a large number of diseases and insects which are specific to each genus. On the other hand, the great diversity of species keeps most pathogenic agents to an endemic level that does not affect the entire population. This chapter considers the trees most frequently encountered in the Quebec City area, the disturbances that affect them, and their importance to the tree’s health. Table 2 lists the ten most important species in this region; this table also includes the geographical class to which they belong.
The ten most important species.
The following diseases and insects were the most commonly observed during the study.
Abiotic diseases
Injuries
Injuries were caused to roots, trunk and branches by mechanized machinery such as lawn mowers, snow blowers, automobiles. They were also caused by objects attached to branches and boles.
Construction and grade change
Contractors do not care about or are not aware of the cultural requirements of trees. Trees surrounding newly built houses are seldom chosen for conservation on the criterion of health; in fact, they are most often chosen for their height and their appearance.
Transplantation shock
Many trees were in bad condition and even dying because they had been selected without taking the site into consideration. Others were not planted in the proper manner.
Chemicals
Many tree owners care too much about their plants and want protection against pathogens and insects. They do not follow the directions on labels, or use appropriate apparatus for the application. The result: burned foliage. Chemical problems also include over-fertilization.
Temperature
During the winter of 1981 (especially during February), the Quebec region experienced successive periods of temperatures well below freezing. Many trees such as Malus sp., Betula sp., Populus sp., Aesculus hippocastatum and Quercus rubra died as a result of this perturbation.
Salt
Ornamentals and ROW trees growing along city streets were adversely affected by the use of deicing salt.
Biotic diseases
Cytospora canker
This disease frequently affected the Colorado spruce and the white spruce.
Black knot
Both ornamental and wild black cherry and choke cherry were hit by this disease.
Fire blight
Fire blight is one disease that has an endemic presence on mountain-ash and on hawthorn.
Nectria dieback
The dieback is affecting mostly the Chinese elm, the Norway maple and both European and American linden.
Dutch elm disease
Elms growing in the Quebec region were originally affected by the native beetle. Recent inventories proved that the European beetle is now present in the area.
Observed insects
During our inventories, we observed many species of insects, but those that suggested the need for a certain kind of control measure were the following:
Birch leafminer, Fenusa pusilla
Birch case bearer, Coleophora fuscedinella
Mountain-ash sawfly, Pristiphora geniculata
Spruce budworm, Choristonevra fumiferana
Eastern tent caterpillar, Malacosoma americanum
Forest tent caterpillar, Malacosoma disstria
Sugar maple borer, Glycobius speciosus
Quantitative scale
The quantitative scale of severity for the problems observed within our inventories permits us to establish a frequency for each of them, and this, in relation to the principal species of trees affected. Table 3 presents the results obtained for trees growing as ornamentals, in private woodlots or along streets.
Quantitative scales for ornamental, private woods and ROW trees.
Since public woods were inventoried using sample-plots and randomly selected transects, it is impossible to give a quantitative scale for these tree populations.
Qualitative scale
The qualitative scale gives the number of trees affected by a disease or by insects. This scale was established by taking into consideration the following criteria: 1) the presence of severe disturbances that can result in the death of a tree; and 2) the disturbances that most frequently appeared during the inventories. There are many other kinds of diseases that affect trees in all manners. These were not considered in our study because they are less widespread and do less damage to the trees.
Discussion
Among the diseases and the insects problems we observed during the inventories, many are severe enough to kill trees. Abiotic diseases are the most destructive agents for trees in the urban environment; as a matter of fact, 52% of ornamental trees and 73% of private woodlot trees were affected by these disturbances, compared to respectively 30% and 10% for biotic diseases. Mechanical injuries and construction disturbances (including grade changes) are the abiotic problems most frequently observed. As for biotic diseases, wood decay is the number one destructive agent. Finally, the insect problems that required the destruction of trees are very limited. The sugar maple borer is the most important cause of death in both ornamental and private woodlot trees.
Acknowledgments
The forestry engineer’s firm Dery, Rocray et associes would like to thank La Direction des Reserves Ecologiques et des Sites Naturels (ministere de I’Environnement) for providing the funds needed for the study. Special thanks also go to the municipalities which contributed to the realization of the field work. Copies of the complete study (E.N.V.-16) are available upon request at the following address: Direction des Reserves Ecologiques et Sites Naturels, Ministere de I’Environnement, Centre Innovation, 2360, chemin Ste-Foy, ler Etage, Ste-Foy (Quebec), Canada G1V 4H2.
Footnotes
↵1 Presented by Pierre-Emile Rocray at the Northeast Forest Pathology Workshop in May 1982.
- © 1983, International Society of Arboriculture. All rights reserved.
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