Exterior characteristics and internal wood decay of hazard trees in Thunder Bay, Ontario

Abstract

One major component of urban forestry is the ability to recognize hazard trees and have them removed before they fail or cause damage to people or property. Thunder Bay's urban forest consists of approximately 20,000 street trees, many of which are at the end of their lifespan and becoming hazardous. Each year 200 or more street trees are removed by the city which provided an opportunity to undertake this research project to identify and possibly equate the relationship between exterior characteristics of trees deemed hazardous, with interior damages, i.e. wood decay. Secondary objectives of this project included measuring the occurrence and diversity of wood decay fungi on street trees in Thunder Bay, creating a resistograph measurement atlas, and determining if a relationship between resistograph measurements and wood strength values (density and MOE) is possible. In total, exterior characteristics were measured on 177 hazard trees destined for removal, 65 in the summer of 2011 and 112 in the summer of 2012. Of the trees measured, 19 species were represented, with white birch (26%), silver maple (16%), green ash (14%), and Manitoba maple (9%) being the most commonly occurring. White birch had the lowest health ratings, with crown die-back brought on by drought stress and bronze birch borer infestation. Damage by severe pruning on major limbs resulted in extensive decay observed among many of the white birch. Silver maple exhibited a decrease in trunk health and structure with increasing age. Large pruning wounds which facilitated invasion by decay fungi, and co-dominant forking were common problems encountered on silver maple. Green ash experienced damage to foliage and exhibited twig and branch die-back in the lower crowns of trees due to ash anthracnose. Although discs should have been removed from all 177 street trees, only 26 tree discs from this pool of trees were collected by Parks crews. An additional 44 tree discs were collected by Parks crews that came from trees not on the original tree removal lists. Due to the small sample size for each tree species, it was not possible to find a correlation between exterior characteristics and internal defects. However, all tree discs (70) were photographed and drilled with a resistograph (IML Resistograph F-series) and are illustrated along with the resistograph charts in Appendix III. Cracks and advanced decay were readily recorded with the resistograph, but incipient decay was not detected as it had readings similar to sound wood. A significant source of error noted was drill bit deflection caused by knots in the wood, contours, and meandering growth rings. One hundred and one trees were observed to be colonized by decay fungi, 22 of these trees were among the original 177, while the remaining trees were additional street trees observed in Thunder Bay. Twenty seven species of decay fungi were recorded, and among these, Cerreno unicolor, Hypsizygus tessulatus, Pholiota aurivella, Chondrostereum purpureum, Pholiota squarrosa, and Ganoderma applanatum were among the most commonly encountered. Particular attention needs to be paid to silver maple and white birch which were most heavily infected. Green ash exhibited the lowest infection by decay fungi. Sixteen resistograph readings from two bolts from a silver maple were taken to compare the values on the y axis with actual wood density and MOE values. Results of a linear regression model suggest that a relationship exists. Although more trees would need to be tested in order to strengthen the interpretation of the relationship. This could provide for a methodology to be created which would give urban foresters measurements that could more accurately predict when a tree is likely to fail.