Decay in New York’s street trees – how often and how bad?

There are likely very few – if any – trees of any respectable size or age that don’t have at least small pockets of decay. In our urban trees, such areas of decay may be even more common, given how they are prone to a much greater amount of foot and vehicular traffic passing within close vicinity of their presence, as well as sometimes being pruned at (sometimes regular) intervals. Perhaps, decay in urban trees is even more important (in terms of its impacts beyond that of pure economics) than decay in rural or woodland trees, because of the more significant target zones. Despite this, little research has been done into the average amount of decay an urban tree may have, and how often decay will occur within its structure. The authors of this study seek to remedy that, by providing a foundation on which further research can be done.

This study sees most attention drawn towards the genus Acer (maples), whose species grace the streets of New York cities in great numbers. Acer platanoides, Acer rubrum, and Acer saccharinum are but three species of maple commonly found, and amongst other maple species they account for as much as 50% of all street trees. Perhaps their abundance is, in part, due to their selection following the removal of Ulmus americana after major outbreaks of Dutch elm disease in the 1930s. Many of the maples are also mature, and therefore the authors note that ascertaining extent and frequency of decay within individuals can be achieved with relative success, whilst being very important in terms of health and safety. To determine decay extent and gather data, the authors of this study used a resistograph, a sounding mallet, and undertook a visual inspection of the trees.

All trees within this study were over 30.5cm in DBH (thus, they could be considered mature), and were situated within the New York cities of Albany, Buffalo, Rochester, and Syracuse. Because all four cities had (mostly) complete records of their tree populations, identifying trees with a diameter of over 12in (30.5cm) was swiftly achieved, and from the pool of trees (67,000) that were within the criteria a total of 480 were randomly chosen from each of the four cities – of the 480 in each city, at least 90 were of the species Acer platanoides, Acer saccharinum, and Acer saccharum (other species included – but were not limited to – Acer rubrum, Fraxinus pennsylvanica, Platanus x acerifolia, Quercus rubra, and Tilia cordata). All trees were also split up into DBH classes of 30.5–45.7 cm (12-18 in), 45.7–61 cm (18–24 in), 61–76.2 cm (24–30 in), and greater than 76.2 cm (30 in).

For each individual tree, three resistograph measurements were taken (at the height of where decay was considered to be present, following sounding hammer application around the circumference of the tree and visual inspection – if no decay indicators were present, readings were taken at the DBH height; and never above 3.1m up the stem). Each measurement went to a depth of 38cm, so the authors did note that the much larger trees would not see an entire cross-section ‘sampled’, but instead perhaps only around half (which may have caused readings to not be as accurate when ascertaining decay extent). However, the resistograph is a good tool for assessing internal wood properties at a given point, and therefore it was determined that the resistograph would be used and, after a drop in wood resistance of 13mm or greater when in operation, it was assumed that decay was present within the tree being assessed. If decay was present on the outside of the tree, because the bark was dead or sapwood rot was present, but the inner core remained sound, the outer ‘shell’ was marked as zero (to factor into the calculations for t/R).

Fewer sugar maples (Acer saccharum) were sampled in Albany as the city has far fewer street trees of this species.

In relation to the decay frequency, the city of Syracuse had the highest rate at 61.2% of trees having decay (though across all four cities, the average was 58%), whilst sugar maples (Acer saccharum) were most frequently observed to have decay within (at 63% of all trees). Individuals with a DBH of 61-76.2cm (24–30in) were most likely to have decay, out of all the DBH classes assessed. As for decay severity, only 3.2% of the trees assessed had severe decay (where the sound wall thickness, based on Matthecks’ t/R formula, was from 0.1-0.3), though the range was from 1.5-4.5% across the four cities (and not significantly different). Silver maples (Acer saccharinum) were most often found to have severe decay, with 5.3% of those surveyed found to have a sound wall thickness of below 0.3, whereas sugar maples (Acer saccharum) were least likely at only 1.8% (therefore, there was a significant difference in terms of severe decay frequency between species). Additionally, severe decay was most frequency in trees with a DBH of 76.2cm and above, at nearly 7% – the next highest class range was 61-76.2cm, at around 3.5%.

Decay incidence by species.
The percentage of each group that was shown to have significant decay (t/R = less than 0.3).

Curiously, this means that whilst sugar maples (Acer saccharum) are most often going to harbour decay, they are the least likely of the species surveyed to suffer from significant decay. However, the authors note that silver maple (Acer saccharinum) is a species very prone to decay, and therefore it had been actively removed in the recent past by urban foresters prior to this study. Thus, it’s low ranking for decay frequency is perhaps skewed by past management practices, though ranking highest of the species in terms of decay severity, it is perhaps still evident at how poor of a compartmentaliser the species is. Despite this, all four cities had very few significantly decayed trees, though did have over half of the tree population suffering from some form of decay.

With regards to what this means for management practices, even though the frequency of significantly decayed trees was shown to be low from the sample, this may still equate to over 2,000 individuals across the four cities (of which most are of very significant size – over 76.2cm in diameter). This is certainly an important statistic from a health and safety perspective, as it means that there are many areas where there is significant risk to people and property. Therefore, it is imperative that management practices have the identification of decay extent as a top priority, and particularly for much larger trees. The research also shows that many trees do suffer from some degree of decay, and therefore establishing the causes of this, and what can be done to reduce the frequency of decay within urban trees, is required.

Source: Luley, C., Nowak, D., & Greenfield, E. (2009) Frequency and severity of trunk decay in street tree maples in four New York cities. Journal of Arboriculture. 35 (2). p94-99.

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Decay in New York’s street trees – how often and how bad?

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