THE AMERICAN ELM
Peter Duinker, Halifax Tree Project
2020-06-01
Second up in this series on Halifax street trees is the American elm (Ulmus americana L.). According to one of our studies (a master’s thesis by Bimal Aryal) of street trees in the HRM Centre Plan area, American elm constitutes just over 20% of the number of street trees. However, because the elms are so large, they account for almost 35% of the basal area of street trees.
Permit me a small diversion into tree measurements. The basal area of a tree is the area of the cross-section of the trunk measured at 1.3 m from the ground. When we measure trees, whether in the streets or the woods, the second thing we do – after noting the species – is measure the diameter at breast height (DBH). There are many other measurements we can make on a tree, but DBH is doubtless the single most important dimension we can measure, and easy too. From a tree’s DBH, we can estimate many other dimensions of the tree, such as basal area, amount of biomass in the various parts of the tree, and others.
Urban foresters and researchers are frequently interested in knowing the importance and extent of the ecosystem services provided by trees in the city. The importance part can be gauged by asking urban people how deeply they value trees in the city and why. We have done a fair amount of this kind of research in Halifax and other Canadian cities. To get at the extent of some ecosystem services, particularly the ecological ones, we calculate them based on relationships built by researchers who have painstakingly measured complicated variables such as the total mass or area of foliage on a tree. The mass of foliage carried by a tree is a very important indicator of the extent of many ecosystem services, like carbon uptake from the atmosphere, pollution abatement, water retention, air cooling, and others (see a 2017 report by David Foster and me entitled “The HRM Urban Forest in 2016”, available here). But it is impossible to measure the amount of foliage on a tree without taking all the leaves off and weighing them, drying a sample so you can estimate dry weight, and coming up with a quantity of mass. That is called destructive sampling. So if we can find an easy-to-measure dimension of a tree – like DBH – that provides a satisfactory estimate of the foliage mass of a tree, we can use it non-destructively to estimate the mass.
Urban foresters and researchers usually want to know about an entire population of trees in a certain area, not just the dimensions of a single tree. It is not particularly intuitive to add up the diameters of trees in a population. That’s why we calculate basal area, a measure for which foresters have a strong intuitive grasp. In the woods, we calculate basal area as an area per unit area, such as square metres per hectare (the latter is ten thousand square metres). For urban forests, we often just use the basal area to get a sense of the relative contributions of different tree species to the overall canopy.
OK, back to the elm. According to John Farrar (Trees in Canada, 2017), there are 18 species of elm worldwide, six native to North America, and three native to Canada (the other two are Ulmus thomasii Sarg., or rock elm, and Ulmus rubra Muhl., or slippery elm). American elm, also called white elm, has by far the largest natural range in Canada – the other two appear naturally in southern Ontario and southern Quebec, but American elm grows in the wild from Nova Scotia to southeastern Saskatchewan, with considerable range all around the Great Lakes. As a child in the 1950s and 60s, I remember iconic elms on many farms in southern Ontario (see photo on the right). Elms do indeed grow in extensive stands, but many of us know of elms in the fence rows of farm fields and along the streets of our towns and cities.
American elm has many redeeming features as a tree species in the streets. It is long-lived (easily over a hundred years), and grows tall and wide, which means that a single tree can deliver ecosystem services well beyond the immediate location of the trunk. Most elms grow with multiple leaders (not a single dominant stem all the way to the top), and that makes them among the easiest trees to train around power lines without a tangle of twigs and foliage to prune every few years (see photo below, left). Unlike the Norway maple, featured in last week’s column, American elm has strong branch attachments, and the wood is strong too – it’s among the toughest woods to split, so people generally avoid it for firewood.
Our love of the elm in the streets is not a consequence of lovely flowers, attractive fragrances, or colourful autumn leaves – rather, it is because of its shape (see photos). American elm has a shape people call umbrella-like. If you have a street lined on both sides with nothing but mature elms, the stunning streetscape has what people call a cathedral-like image – tall pillars on each side with a high arched roof. There are several such streets in Halifax – Kline St. (see photo above, right) and Swaine St. in the centre of the Halifax Peninsula are two of my favourites.
No discussion about elm in North America can avoid mentioning Dutch elm disease. The disease is caused by two species of wilt fungus of the Ophiostoma genus. The fungi were first identified in Europe in the early 1900s, and experts believe that some form of human movement brought the fungi to North America. The fungus is spread from tree to tree by elm bark beetles, some native and some from Europe. All three of Canada’s native elm species are highly susceptible to the fungi. In southern and central Ontario, where I lived from the 1950s through the 1970s, I recall seeing rural landscapes rife with dead elm trees. I also recall the Government of Ontario offering subsidies to farmers to cut down the dead elms in their woodlots, and I even earned some income after high school helping a friend log those elms at his property.
I could go on and on about Dutch elm disease – its story is at once most fascinating but also depressing. If you want to learn more, you can find abundant information online. For this article, I want to relate how lucky we are that our substantial elm population in Halifax has largely escaped the ravages of Dutch elm disease. No one really knows why. Towns such as Truro and Shubenacadie were hit hard by the disease, but we have essentially escaped. We have a couple of cases a year in Halifax (that we know about), and these trees are usually cut down as soon as the disease is identified.
You might wonder about why we are planting more elms in the streets when we know what Dutch elm disease can do. Many professionals would say that American elm, aside from susceptibility to Dutch elm disease, is the best possible street tree you can have (because of factors I related above). Plant breeders in the USA have worked very hard over the past several decades trying to develop cultivars of elm that are more resistant to the disease than elms from wild populations. Several of these that have been available for many years: “Princeton”, “Valley Forge”, and “New Harmony”. Only disease-resistant cultivars of elm are now planted in the streets of Halifax.
Like so many Haligonians, I do adore elms in the streets of the city. However, I am not a fan of planting so many of one species, or even one cultivar of a species, in a streetscape. For both aesthetic diversity and risk reduction, I would far sooner see a diverse mix of many species on a length of street. In a neighbourhood of several residential streets, one could imagine a strong interplanting of twenty or thirty species. That way, if any particular species is in trouble due to blowdown issues or diseases or insect pests, no street would be without at least some trees remaining healthy. There are sad photos online that show streetscapes in cities where, after the elm populations were decimated by Dutch elm disease, the subsequent plantings of nothing but ash trees all had to be cut down because they were simultaneously killed by emerald ash borer. So, high species diversity is a strategy to reduce the risk of losing all the trees in a neighbourhood to a calamity.
There are some magnificent elm trees in Halifax. One particularly large specimen is at 273 Bedford Highway (see photo below, left), potentially the largest elm around the Halifax Harbour and Bedford Basin. On the peninsula you can find many elms of awesome proportion. Another huge beauty is on Pepperell St. just west of Preston (photo below, right). As I walk around the streets of the peninsula and look up at the tree canopies, I notice that our elm population is showing signs of old age – many trees have dead branches high up in the crown. I sincerely hope that we find ways to keep our elms in good shape while the younger trees planted recently under the HRM Urban Forest Master Plan make their way into the upper canopy. By the way, a fascinating row of young trees to watch over the years is the recently planted series of elms (and oaks) along the active transportation path on the harbour side of Barrington St. north of the Cornwallis St. intersection.
