Tackling Tradeoffs: Tree Functional Groups and Ecosystem Services in Tree Planting
Article: Wood, S.L.R & Dupras, J. (2021). Increasing functional diversity of the urban canopy for climate resilience: Potential tradeoffs with ecosystem services? Urban Forestry & Urban Greening 58: 126972. https://doi.org/10.1016/j.ufug.2020.126972
Planting Plans to Save Ecosystem Services
As populations in cities rise, many people increasingly depend on the ecosystem services, or natural benefits to us, that urban trees provide such as clean air, temperature regulation, and storm water control. At the same time, climate change, pests, and diseases threaten the ability of these urban trees to continue providing these services. Québec City in Canada, for example, is expected to experience hotter and drier conditions, which may further stress the city’s trees. Adding to this stress is the non-native emerald ash borer (Agrilus planipennis), a bright green beetle that threatens the ash trees that make up 13% of tree cover in the city.
The city has several options to approach replanting in areas where ash trees have died. While the city could plant the same typical tree species, alternatively, it could use the “functional group” of species to guide these decisions. This is done by considering the physical characteristics of different trees and assessing their tolerance to future climate stressors. Then species are grouped by how similar traits (“functional group”). There are different ways to use functional group to guide planting. For example, planners may choose to maximize representation of one or a few functional groups of trees they know are poised to handle specific anticipated challenges, such as drought, or they may choose to evenly include trees from a range of functional groups to maximize the functional diversity of trees. How well do these functional strategies of replanting city canopies protect ecosystem services?
Simulating Replanting Strategies
Drs. Sylvia L. R. Wood and Jérôme Dupras of the Université du Québec sought to explore the influence of these different strategies on ecosystem services by simulating these strategies over time using a computer model. They used the publicly-available street and parkland data provided by the Department of Horticulture and Urban Forestry in Québec city, which included 103,269 trees from 187 species, and they classified these species into six functional groups such as “shade-intolerant spine species tolerant to drought” and “shade-tolerant species with moderate growth, medium-high wood density.” They then used the U.S. Forest Service’s UFORE urban tree model, or i-Tree Eco, to quantify five ecosystem services based on these trees present in Québec City: carbon storage, carbon sequestration, air pollution removal, avoided stormwater run-off, and energy savings provided by city trees.
After establishing this baseline, Drs. Wood and Dupras tested three replanting strategies in a computer model that ran for both 10 and 20 years. The first strategy, “business as usual”, added planted trees to the city to replace dying ash trees with a selection of species from the current list of species present, keeping the functional group distribution the same. The second strategy, “conifer-focused replanting”, replaced dying ash trees with trees from two drought-tolerant conifer functional groups that were under-represented in the city, thereby increasing the overall drought tolerance of the urban canopy. The third strategy, “stratified replanting”, selected an even number of stems from five of the functional groups (excluding the group that ash belongs to) to replace dying ash trees. In this way, the conifer-focused replanting strategy was designed primarily to increase ecosystem services, while the stratified replanting strategy was designed to maximize the representation of functional groups.
Stratifying for Success
While the conifer-focused strategy did greatly increase ecosystem services, particularly by reducing energy costs of nearby buildings, the stratified replanting strategy had the greatest level of carbon storage, greatest biodiversity, and was the least vulnerable to pests. In particular, when considering the vulnerability of planted trees to regional pests, the model predicted that more trees in the conifer-focused replanting strategy would be susceptible to regional pests than the stratified and business-as-usual strategies. While all replanting strategies lost species due to the death of ash trees, the stratified strategy lost the least and had the most even representation of tree functional groups.
Drs. Wood and Dupras’s study suggests that a replanting method aiming to increase representation of functional groups may not result in the greatest possible increase in ecosystem services in Québec City, but could still be more resilient and provide more ecosystem services than a business-as-usual approach. This strategy also seems more feasible for urban foresters to implement than a conifer-focused one, as this gives foresters more flexibility to match different species to specific site conditions that might increase their chances of success even more. Using functional diversity to guide replanting is a new concept that has not yet been widely applied. As city planners and urban foresters are now tasked with anticipating new environmental conditions and threats, Drs. Wood and Dupras’s research demonstrates how considering the “function” of trees can help tackle these challenges and ensure the maintenance, or even increase, of ecosystem services in our cities in the future.