In the wake of the ice storm of December 2013, it was reported that Toronto lost about 20% of its tree canopy. That was the figure that was bandied about, yet it might as well have been pulled from the same thin, wintery air we’ve been breathing up here for what seems like eternity. It will be many more months, if not years, before we get a handle on how devastating the ice storm was to our green infrastructure.
Trees in the city are at an inherent disadvantage. In the city, condos, apartments and tall office buildings have supplanted the supremacy of the tree and are part of the canopy, if not the only canopy in many areas. Buildings have become the overstorey and canopy trees have been demoted to understorey status.
But canopy trees do not act as if they are understorey minions. Typical canopy tree are large trees and are programmed to grow large. But since they grow in isolation on city streets and don’t experience the same competition from their peers to keep their canopies in check as they would in a forest, they may exercise their freedom to branch out in an open canopy form as “specimen” trees. Picture the last tree standing in a farmer’s field. They may look beautiful with branches out-stretching, however, without the pressure to keep branches upright and the canopy tight, i.e., buffering of other trees, they risk damage from catastrophic weather events such as high winds or ice storms.
The ravines are ribbons of green that wind through the city. Here, the trees retain their status as the overstorey. A tighter-knit community holds better against the elements.
However, the edges of ravines where the forest meets the road result in a hybrid situation: closed canopy on the forest side and open canopy as branches reach towards the light offered by the freedom of the open road. And you can imagine the outcome after the ice storm – broken branches brought down hydro lines running along the roadside, littering sidewalks and roads.
Yet, other roadside trees stood the test…
The most vulnerable trees were open canopy and the more resilient ones resided in forests. Of course there were exceptions. Trees which dropped branches damaged healthy trees in a domino effect. In the winter wallop that hit my garden, my neighbour’s red oak dropped large branches which stripped some of the foliage on my cedar trees. Trees weakened by disease or previous damage were more susceptible to failure. But what about species of trees? Were native trees better able to withstand the onslaught?
The ice storm came across Lake Ontario in a mess of mixed precipitation which touched down in a patchy mosaic across the Greater Toronto Area. Some neighbourhoods were hit hard while adjacent ones were unscathed. From assessing neighbourhoods around me which were hit worse than others, it was apparent which trees failed and which fared better. And it appeared that both native and non-natives failed and were hardy.
Some trees are susceptible to branch failure according to Urban Forestry in Toronto. These include black locust (Robinia pseudoacacia)*, silver maple (Acer saccharinum), Manitoba maple (Acer negundo)*, American liberty elm and pin oak (Quercus palustris). I would include Norway maple (Acer platanoides)*, Siberian elm (Ulmus pumila)*, birches (Betula spp.; native and non-native) and larger specimens of honey locust (Gleditsia tricanthos). American liberty elm (Ulmus americana ‘American Liberty’) is a dutch elm disease cultivar which has the classic American elm shape which is susceptible to wind and ice damage.
Trees with a dominant stem rather than co-dominant stems fared well. Conifers in the classic Christmas tree shape shed snow and ice relatively easily because they have very flexible branches near the tips compared with hardwood trees. The one hardwood tree which has a similar trunk is the linden tree (Tilia spp.). Since the native basswood (Tilia americana) is not usually planted as a street tree due to salt intolerance, usually I only encounter exotic lindens (e.g., Tilia chordata).
Conifers with high “hedge tolerance” had uneven outcomes. Cedars, junipers and yews which had room to grow as trees and had full foliage fared better than top-heavy trees.
In cities, tall buildings may dominate. But if we add green roofs, can buildings function like a tree canopy? Sure, it’s better to have green roofs on those buildings than no green roofs. But green roofs are not an ideal substitute for green space on terra firma. Green roofs are much more restrictive in biodiversity both in planting material and ecosystem inhabitants and the restrictions increase with height. You aren’t going to see squirrels, even flying squirrels at 30 storeys, let alone the array of pollinators you would experience in a canopy firmly rooted in the ground. So, to keep those ecosystem connections, it is better to retain tree canopies be they open or closed on the ground in cities and elsewhere.
In Tree Debris (Pt. II) we’ll tackle the massive clean up job.
Originally published February 26, 2014 on Native Plants and Wildlife Gardens