Halifax Tree Project

The Silver Maple

Peter Duinker, Halifax Tree Project

2020-06-22

Welcome to summer! Most of the street trees are now in leaf, the tardy catalpas finally catching up to the early bloomers like Norway maple. This week we feature Acer saccharinum L., a tree species that is not native to Nova Scotia but which grows as far east as New Brunswick. Like willow trees, it is common along the low-lying shorelines of the St John River valley. Silver maple grows widely across the eastern USA, almost reaching the Gulf of Mexico.

I think of silver maple as a big tree. Of course, I’ve seen lots of small ones, but in the streets of Halifax, for example, we have only huge ones. Data collected by Bimal Aryal, my former graduate student, in the Centre-Plan area of Halifax bear this out. Silver maple does not make it into the top ten most-abundant tree species when it comes to stem count, but by basal area it ranks fourth. Silver maple used to be planted fairly frequently in city streets in eastern Canada, but urban foresters seem to be shying away from it on account of its weak wood. Large silver maples lose a lot of twigs and branches during windstorms, and that makes for much cleanup.

Silver maple is the fifth tree species in this series (to date, we have covered Norway maple, American elm, Littleleaf linden, and red oak), and readers may see the pattern here of featuring only large-stature tree species so far. This is a consequence of queuing up the species by their prominence in our streets, either by number of individuals or their size (as reflected by basal area). Much debate rages in the realm of urban forestry about whether, in the street environment, we should favour large trees or small trees at maturity.

Consider the difference between a mature silver maple at 25 metres tall (a height typical of our large-stature street trees), and a mature serviceberry (Amelanchier spp., to be featured next week) at 10 metres tall. One thing we can be sure of is that the serviceberry will have far less foliage than the silver maple, and because many of the ecosystem services provided by trees is directly proportional to the amount of foliage they have, the silver maple, tree for tree, provides a much greater quantity of ecosystem services. The services I am thinking of in this analysis include things like stormwater attenuation, carbon capture and storage, atmospheric cooling, shading of built infrastructure, and pollution abatement.

Let’s examine for a moment the merits of urban cooling. Transpiration is the process in which trees take up water from the soil through their roots and send that water up the trunk to the leaves and ultimately into the air. The water is important to keep the leaves healthy and to move nutrients from the soil up to the leaves so they can make sugars (during photosynthesis) to grow the tree. But here is another important fact. When the water molecules exit the tree’s leaves, they reduce the temperature of the air. Think about it this way - when you are at the ocean beach and come out of the water after swimming, you shiver and want a towel to dry off and keep warm. The same cooling effect is happening with tree leaves. From us after a swim, it’s evaporation. From the trees, it’s transpiration.

So, if a large silver maple is transpiring a bathtub’s worth of water every hour, that is cooling the air substantially. Especially on hot days in summer, we want trees in the city to combat what is called the urban-heat-island effect. Roads, cars, and buildings all produce heat and release it to the environment, so cities are warmer than the surrounding rural areas, be they farms, forests, or waters. On a hot summer day, trees can be important agents of cooling the city – and thereby making it more comfortable for people – by transpiring a lot of soil water. The larger the tree, the greater the cooling effect.

Finally, even if the serviceberry has a fulsome crown, the position of that crown low to the ground means that some of the ecosystem services, leaf for leaf, would be less because the leaves do not reach past the heights of most residential housing along our streets. The higher up the leaves are – within reason – the more of some ecosystem services we can enjoy.

However, there are downsides to having tree crowns way up there. The crowns catch more wind, and are therefore, all other things being equal, more prone to windthrow. Plus, tall trees in full leaf are carrying their foliage high and are therefore top-heavy, also contributing to windthrow potential. A tall tree coming down in a hurricane has a much greater chance of damaging buildings than does a short tree. And then there are the electricity and communications lines strung between poles stuck into the same spaces as the trees are planted. Considerable resources need to be spent, more or less continuously, to prune trees away from power lines. Trees are the major culprit when there are power interruptions in the city during high-wind events. In many cities around the world, power lines are underground – Halifax is not one of those cities!

In sum, urban foresters in Canada today seem to be planting a wider and wider diversity of tree species in the streetscape, a welcome development for many reasons. Since we have available a considerable range of large-stature tree species with relatively strong wood and branch connections, silver maple, at least in my experience, seems to be less favoured as a street tree.

Some tree species hybridize with closely related species. This means that they can interbreed and produce offspring trees that have characteristics of both parent species. It’s rather like a horse and a donkey interbreeding and producing a mule. We have at least two examples of hybridization in the Acadian forests of the Maritime provinces. One is black spruce and red spruce. We don’t have a name for their joint offspring. The other is silver maple and red maple, and we do have a name for that – it’s Freeman maple (Acer x freemanii). While the cross occurs frequently in nature (including in the St. John River valley), it is also produced in the nursery, and Oliver Freeman did this in 1933 in the US National Arboretum in Washington, DC. Freeman maple has overtaken silver maple as a street tree because it has better survival and growth characteristics than its parent species in the harsh street environment.

By the leaves, how can you tell a Freeman maple from a red maple and a silver maple? This cultivar of Freeman has the toothiness along the leaf edge of red maple, but the deeper lobe indentations of the silver. I have provided photos of all three species’ leaves. As for notable specimens of silver maple in the streets of Halifax, my go-to street is Williams St. There are at least three large silver maples, probably in the twilight of their lives (note the thinness of the crown in the photo above).