Potential to introduce invasive trees and insects
James Steenberg, Halifax Tree Project
2021-08-20
Urban forests of course bring us city dwellers a myriad of benefits, but they can also have an occasional dark side, as this HTP blog series is investigating. One of these ecosystem disservices is that some urban trees and urban tree species can be vectors for invasive species.
The term invasive can be unclear or even elusive, but typically it refers to introduced, non-native species that can proliferate with exuberance in their new habitat. They are frequently hardy, pioneer species, generalists that can survive in a wide array of environmental conditions, including degraded ones, in the city. The other main attribute of invasive species is that they have some measure of adverse impact on local ecosystems and biodiversity, local people, local infrastructure, or any combination thereof.
Sometimes invasive species are insects that hurt trees. There is a prominent local example of urban trees leading to the introduction of an invasive insect right from our very own Victorian-styled Public Gardens in central Halifax1. Some time in the late nineteenth century, a handful of European beech (Fagus sylvatica) trees were brought ‘across the pond’ from England to be planted in the Public Gardens. European beech is similar in appearance and in the same genus as our native American beech (F. grandifolia), having simple, oval leaves with toothed margins and the signature smooth, silver-grey bark. A cultivated variety of European beech called ‘copper beech’ (Fig. 1) with deep purple glossy leaves is often favoured for urban plantings. Many Haligonians will recognize these as some of the signature trees in Point Pleasant Park.
Fig. 1. Left: A magnificent copper beech in a Dartmouth front lawn. Right: Effects of beech bark disease deep in Nova Scotia’s rural woodlands. (Source: both by Peter Duinker)
Unfortunately, these new arrivals at the Public Gardens were lousy with beech scale (Cryptococcus fagisuga), a scale beetle that punctures the bark of beech trees to feed. Beech scale carries on its person a species of fungus called Nectria faginata (and sometimes another native fungus called Neonectria galligena). The insect-fungus complex is collectively called the beech bark disease (BBD), and when the scale beetle feeds, the fungus infects the bark and leads to knobby ‘cankers’ on the tree’s trunk and limbs that eventually break open and rot, girdling and killing the tree2. BBD affects European and American beech in both urban and rural settings, though is far more damaging to our native beech. From the Public Gardens epicentre, BBD has spread across eastern North America, devastating the species – killing newly infected beech with lethal efficiency and subsequently reducing the next generations of this long-lived, late-successional, old-growth species to a stunted, gnarly understory tree.
Sometimes invasive species are trees that hurt other trees (and ecosystems). The perfect example is the Norway maple (Acer platanoides) – the most abundant street tree in urban Halifax, as is the case in many other North American cities. You can read about it in the HTP street tree blog series. It’s native to Europe and western Asia and was planted in abundance (ad nauseum?) after the decline of North America’s first favourite street tree, the elm (Ulmus spp.), due to yet another invasive pest in the mid-twentieth century, the Dutch elm disease. Norway maple was planted so extensively in the wake of the elms because it grows fast, propagates easily, and can take a serious licking and still keep ticking. This latter quality is useful in the urban landscape, but it’s also a core tenet of invasiveness.
Norway maple has since fallen into ill repute among urban foresters and ecologists alike because, among some other failures as a street tree, it is invasive and colonizes natural areas within and outside of cities, outcompeting native tree species and providing degraded habitat/forage for native critters. Moreover, Norway maple casts such a heavy shade that it can prevent the regeneration of other trees and even understory plants. This lack of vegetation underneath its canopy leads to erosion and soil loss3. Fortunately, while Norway maple is still easily found growing wild in Halifax’s urban forest, it hasn’t been planted in parks or streets by the municipality in well over two decades.
Lastly, sometimes invasive insects and invasive trees join forces as a villainous duo. Enter the tree of heaven (Ailanthus altissima). The tree of heaven is so named because it grows with such speed towards the heavens – sometimes more than a metre in height in a year. It produces a massive seed-crop each year and can reach sexual maturity and begin producing seed in its first few years of life. It is also allelopathic, meaning the tree generates toxins that inhibit other plants from establishing near it. The tree of heaven can sprout in the smallest of asphalt cracks in the most dense, heavily built-up neighbourhoods and grow to a full-stature tree. This trait actually makes the tree of heaven an important species in some of the highest density – and often marginalized – neighbourhoods where it becomes one of few species able to survive the conditions and provide shade and other needed ecosystems services.
And yet, if the tree of heaven is within a stone’s throw (or more accurately, within the effective seed dispersal range) of natural areas, it becomes an aggressive invader and muscles out nearly all other tree species. But the worst is yet to come: the invasive tree of heaven is a ‘twofer’. It is the primary host of the spotted lanternfly (Lycorma delicatula), which is an invasive insect that was introduced to the United States and made the jump from tree of heaven to many native tree species and some crop species like cherries, apples, peaches, and (heaven help us!) grapes and hops4. In my years exploring Halifax’s urban forest, I have only seen a couple of trees of heaven and they do not seem to establish here as well as they do in other cities, but I have concern.
Fig. 2. Right: a new annual crop of tree of heaven growing through the cracks in an alleyway in Toronto (Source: James Steenberg). Left: The spotted lanternfly (Source: Scientific American).
I do believe that there is room in the urban forest for introduced, non-native species. There is such wide breadth of values associated with urban trees, including social and cultural ones. Tree species from around the world are important for these values in a diverse Canadian city. However, when it comes to bringing in new species and avoiding introducing invasive ones, I definitely call up the second guiding principle of Halifax’s urban forest master plan: precaution. When it comes to handling our existing roster of invasive trees and insects, on-going monitoring and collaborative research are critical.
References
1 Houston, D. R. (2005). Beech bark disease: 1934 to 2004. What’s new since Ehrlich? Proceedings of the Beech Bark Disease Symposium. Newtown Square, PA: USDA Forest Service.
2 Loo, J. (2009). Ecological impacts of non-indigenous invasive fungi as forest pathogens. Biological Invasions, 11, 81-96.
3 Schollen & Company Inc. (2008). Glen Stewart Ravine management plan technical report. Toronto, ON: City of Toronto.
4 USDA. (2021). Spotted lanternfly. USDA, Animal and Plant Health Inspection Service. Retrieved from: https://www.aphis.usda.gov/aphis/resources/pests-diseases/hungry-pests/the-threat/spotted-lanternfly/spotted-lanternfly