Abstract
ABSTRACT Understanding the factors that affect tree hazard condition is important to mitigate it. To quantify these factors for Fraxinus americana L., Morus nigra L., and Populus deltoides Marshall trees we have conducted a survey recording their hazard rating according to the TRAQ methodology. The following covariates were registered for each tree: the number of single tree defects in root collar, trunk and scaffold branches, trunk diameter at breast height (DBH), crown class, crown asymmetry, and the size of the tree pit. A multi-category logistic regression model was fitted to the data using the backward step-wise procedure. Among the factors considered, only the species, crown class, number of single defects in root collar and scaffold branches significantly affected the tree hazard condition. P. deltoids trees had higher values of hazard compared to F. Americana and M. nigra trees. Trees with co-dominant crowns presented a higher probability of being classified as medium and high hazardous than trees with suppressed and dominant crowns. Finally, every single defect observed on root collar and scaffold branches increased the probability of a tree being of medium or high hazardous, by about 3.00 and 1.44 times, respectively. Other factors like crown asymmetry, DBH, and the tree pit size did not have a significant effect on the score of tree hazard. Our results demonstrate that the species selection is an important tool to manage urban tree hazard under the study conditions. At the species level, efforts should be directed towards minimizing defects in root collar, trunk and scaffold branches.
Highlights
Urban forests provide various ecosystem services to urban areas
The following covariates were registered for each tree: the number of single tree defects in root collar, trunk and scaffold branches, trunk diameter at breast height (DBH), crown class, crown asymmetry, and the size of the tree pit
Visual observations of tree hazard condition were made during the winter of 2011 according to tree hazard recognition methodology of the International Society of Arboriculture (TRAQ methodology) and guidelines reported by Smiley et al (2007)
Summary
Urban forests provide various ecosystem services to urban areas. Through proper design and management, urban forests can mitigate many of the environmental impacts of urban development by moderating climate, reducing building energyuse and atmospheric carbon dioxide, improving air quality and reducing noise levels. Inappropriate tree selection and maintenance can increase the negative impacts of urban forests on the environment, population, infrastructure and the ownership of citizens. Such negative impacts are related to pollen production and chemical emissions from trees, an increase in building energy use, infrastructure repair, sidewalks breakdown, luminaires obstruction, a hindrance to vehicular and human transit and visual discomfort (Randrup et al, 2001; Smiley et al, 2007). Other negative impacts are related to the increase of tree hazard condition due to the aging of the urban forest, which can be accelerated due to an inadequate species selection or unsuitable forest management. A hazardous tree may have a lower risk if there is not any target nearby or high if there are one or more targets below its crown or within its fall area, defined by its height
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