Year after year, hundreds of Quebec doctors and public health experts urge various institutions to plant more trees to expand the urban canopy. They are right to do so, as they draw on numerous studies demonstrating the many benefits trees provide to community health. In this context, it is important to ensure that the urban canopy is resilient to current and future disturbances, in order to guarantee the long-term benefits of tree cover for the community. Lessons can be learned from this by looking at what happened to our elms and ash trees, which have disappeared.
[Cover photo: Historical photo: In the 1950s in Montreal, approximately 35,000 elm trees were decimated – a view of Jarry Park at the time (left) and today (right). Source: Montreal Archives and Google Street View]
Let’s start with the basics: what exactly is resilience? In ecology, it refers to “the ability of an ecosystem to withstand and survive alterations or disturbances affecting its structure or functioning, and to eventually find a new equilibrium” (free translation, Commission générale de terminologie et de néologie). One of the best examples of a forest with low resilience—which we’re seeing right now—is with the arrival of an exotic forest pest like the emerald ash borer (Agrilus planipennis) in an urban forest composed mostly of ash trees; the ideal feeding ground, in other words. With global warming and climate change fueling its spread, the emerald ash borer is spreading northward like wildfire. Threats to the forest canopy are present, and the urban forest was not prepared to face them; it therefore lacked resilience. The result: we’re facing an unprecedented scale of tree felling and planting, with nearly 45,000 ash trees to be cut down over the course of a few years. In fact, this isn’t entirely unprecedented: around the middle of the last century, an exotic insect appeared—the European elm bark beetle (Scolytus multistriatus)—and with it, a fungus (Ophiostoma ulmi) that killed 35,000 elm trees in the city of Montreal alone. You can see the parallel with what our ash trees are currently going through… In both cases, we were dealing with an urban forest that lacked resilience, even though ash and elm trees were among the species best adapted to the urban environment.
What is biodiversity?
Part of the solution was proposed by researcher Frank Santamour Jr. in 1990 [1], who issued recommendations regarding species diversity and representativeness based on their taxonomic classification. To reduce canopy loss during a future epidemic, Santamour suggested that no single species should account for more than 10% of the trees in a given area (e.g., the Green ash, Fraxinus pennsylvanica), that no genus should account for more than 20% (e.g., Fraxinus), and that no family should account for more than 30% of all trees present (the Oleaceae, in the case of ash trees).
In fact, beyond the simple “specific” biodiversity promoted by Santamour—a term that has almost fallen out of fashion today—we have realized that what we are really seeking are ecosystems that are resilient to various environmental threats. The arrival of the emerald ash borer brought to the forefront a concept that is now becoming the norm: functional biodiversity. To achieve this, we do not simply want a variety of species, but a variety of species with different response capabilities to their environment. This is referred to as functional diversity. It has now become the preferred approach to protect our urban forests from the various threats looming over them, because we now know that certain as-yet-unknown threats may arise in the near future…
Functional diversity was described by the eminent researcher David Tilman in 2001 [2] as the variety and abundance of different characteristics of organisms that influence their performance within a community (functional traits). These traits can be morphological (the physical characteristics of a plant), physiological (the organization and functioning of a plant), or phenological (the often annual changes in a plant’s functioning). These traits can vary in response to the plant’s environment and influence the tree’s resilience to environmental stresses. Since no single tree species can withstand all stresses, research tends to show that an ecosystem comprising a greater range of different functional traits offers greater stability in the face of disturbances [3].
The following figure clearly illustrates that species diversity alone is not synonymous with complexity and resilience in a plant community. In this example, for the same level of species diversity, it is easy to see which of the two ecosystems exhibits greater functional diversity (in terms of morphological traits in this case).
Différences de complexité entre deux communautés de 3 espèces (couleurs) chacune : la communauté 1 présente un seul groupe fonctionnel (forme d’arbre distincte) alors que la communauté 2, plus complexe, présente 3 groupes distincts. La biodiversité fonctionnelle y est donc plus grande (Image adaptée de Paquette, 2016).
Functional groups really work!
Plusieurs recherches récentes ont transposé le concept de diversité fonctionnelle à la forêt urbaine [4]. Une grande diversité fonctionnelle à l’échelle des quartiers, parcs et boisés urbains est importante afin d’assurer leur résilience face aux perturbations futures. Afin de mettre sur pieds une méthode aussi simple que les ratios de Santamour, mais plus représentative du fonctionnement des écosystèmes, l’approche par groupe fonctionnel a été développée [5]. Les espèces y sont regroupées selon leurs ressemblances en termes de traits fonctionnels, ce qui signifie que chaque espèce d’un même groupe fonctionnel génère un impact similaire sur l’écosystème. Au Québec, les chercheurs Alain Paquette et Christian Messier ont classifié les différentes espèces d’arbres en 10 groupes fonctionnels (tableau suivant). Cette classification peut donc être utilisée pour mesurer et comparer la diversité fonctionnelle de différents territoires ou à différentes échelles d’un même territoire (ex : rue, quartier, ville). Cette méthode permet également d’identifier les lacunes d’un territoire en termes de diversité fonctionnelle et de dresser une liste détaillée d’espèces à favoriser afin de maximiser la résilience de la forêt urbaine.
Several recent studies have applied the concept of functional diversity to urban forests [4]. High functional diversity at the scale of neighborhoods, parks, and urban woodlands is important for ensuring their resilience to future disturbances. To develop a method as simple as Santamour’s ratios but more representative of ecosystem functioning, the functional group approach was developed [5]. Species are grouped according to their similarities in functional traits, meaning that each species within the same functional group has a similar impact on the ecosystem. In the province of Quebec, researchers Alain Paquette and Christian Messier classified different tree species into 10 functional groups (see table below). This classification can therefore be used to measure and compare the functional diversity of different areas or at different scales within the same area (e.g., street, neighborhood, city). This method also makes it possible to identify gaps in an area’s functional diversity and to compile a detailed list of species to prioritize in order to maximize the resilience of the urban forest.
The 10 functional groups of tree species in Quebec, as established by Paquette and Messier in 2016
In an ideal world, since the full range of functional traits is represented by these 10 groups, each functional group should account for approximately 10% of the trees in a given area. In other words, planting or replacement efforts should be directed toward increasing the representation of groups that account for less than 10% of the total number of trees in an area. Conversely, species whose groups already present in the area account for more than 15% of the trees should be avoided.
Now that we have lost a significant proportion of our urban trees to the emerald ash borer, the best we can do is view this situation as an opportunity to restore a functional balance to our urban forest. From now on, we must plan our landscaping with diversity in mind—functional diversity, to be precise—rather than simplicity, as we did in the past. How often have we seen rows of a single tree species lining boulevards or, even today, in new residential developments? These are prime targets, even without the presence of exotic pests; high concentrations of a single species generally tend to attract more harmful insects, whether they are native or exotic. This is a relatively recent finding shared by the fields of forestry, agriculture, horticulture, and likely many others: everyone benefits from prioritizing species diversity.
A row of trees (planted too close together) consisting entirely of red oaks (Quercus rubra), which will soon attract pathogens and pests. It looks beautiful now, but it won’t be nearly as attractive if they all get sick at the same time…(Photo Trame-Verte)
In closing, let’s remember that the benefits of trees, as reported by researchers and doctors [6] [7], are primarily provided by large trees, and given the harshness of our urban environments, we must care for them throughout their entire lives. This means that in order to take care of us, trees need us to take care of them—it’s a win-win! When you think about it, caring for trees involves so much, yet most people might assume that if trees don’t require care in the forest (which isn’t entirely true, depending on the management objectives), they don’t require any more care in an urban setting… However, the opposite is true, because isolated trees are subjected to more stress than in the forest, compounded by the various pollutants found primarily in urban environments. This is precisely the role of an urban forestry expert: to raise awareness to the needs of our urban trees. On the other hand, if a majority of residents and stakeholders were aware of all the threats facing our trees, we would undoubtedly enjoy a larger urban canopy, and everyone would be better off! Let’s spread the word!
1] Santamour Jr, F. S.,1990. Trees For Urban Planting: Diversity Uniformity, and Common Sense. METRIA 7: Proceedings of the Seventh Conference of the Metropolitan Tree Improvement Alliance (pp. 57-66). Lisle, IL: Morton Arboretum
[2] Tilman, D., 2001. Functional diversity. Encyclopedia of biodiversity, 3(1), 109-120
[3] Manes, F., G. Incerti, E. Salvatori, M. Vitale, C. Ricotta, and R. Costanza. 2012. Urban ecosystem services: Tree diversity and stability of tropospheric ozone removal. Ecological Applications 22:349–360
[4] Nock, C. A., Paquette, A., Follett, M., Nowak, D. J., & Messier, C., 2013. Effects of urbanization on tree species functional diversity in eastern North America. Ecosystems, 16(8), (pp.1487-1497)
[5] Mason, N. W., Mouillot, D., Lee, W. G., & Wilson, J. B., 2005. Functional richness, functional evenness and functional divergence: the primary components of functional diversity. Oikos, 111(1), 112-118.
[6] https://www.journaldemontreal.com/2019/11/24/des-medecins-reclament-beaucoup-plus-de-place-pour-la-vegetation-en-ville-1
[7] https://www.tvanouvelles.ca/2020/02/20/verdir-les-villes-pour-vivre-en-meilleure-sante-plaident-600-medecins