Green infrastructure has a lot of benefits: nature can improve people’s mental and physical health; vegetation helps reduce building energy use by providing insulation and cooling; and plants and soils store carbon.
The problem is there’s no way of evaluating whether green infrastructure projects really live up to their promises. How much carbon does a street tree actually sequester? And what’s the greenhouse gas impact of growing seedlings in nurseries, assembling the raw matterials for potting soil, and transporting mulch to where it needs to go?
The solution, according to a group of researchers in Finland: develop carbon footprint standards (known as Environmental Product Declarations or EPDs) for plants, soils, and mulches similar to those that already exist for building materials. This would provide an objective check of cities’ and developers’ claims about the environmental benefits of green infrastructure projects, and help landscape designers plan, construct, and maintain green spaces in the most climate-friendly way.
“Landscape designers will need to know how much carbon typical plants and growing media can sequester. This should be seen as basic product information that enables us to mitigate climate change,” says study team member Matti Kuittinen, adjunct professor of architecture at the University of Aalto in Finland.
But that basic information isn’t so simple to assemble. As a first step to developing such standards, Kuittinen and his colleagues analyzed how carbon flows through plants, soils, and mulches in different phases of their life cycle. Based on this analysis, it’s clear that standard methods for assessing the carbon footprint of building materials will need to be tweaked when applied to components of green infrastructure, the researchers report in the International Journal of Life Cycle Assessment.
One challenge: plants are alive. So the amount of carbon stored when, for example, young shrubs are planted in a park may not correspond to the amount stored years later after they’ve grown to maturity. The upshot is that applying conventional life cycle analysis approaches to green infrastructure could actually be misleading and suggest that plants are a climate burden.
On the plus side, there are already widely used methods for modeling the carbon stock of trees and carbon storage in urban forests. “Bushes, herbaceous plants and their carbon dynamics with soil are clearly more challenging,” says Kuittinen.
One source of complexity is that neither soils nor mulches have a clearly defined “end of life” stage: soil might be removed from a site and deposited elsewhere, but it’s still soil; mulch decomposes over time, yet there’s no clear demarcation of when it ceases to be mulch and becomes something else. The researchers couldn’t find any established methods for modeling the environmental impact of mulches.
But one step at a time, says Kuittinen: “Next steps in translating our findings into EPDs will be compilation of data on the carbon uptake potential of bushes. We will start to gather samples and measure their carbon contents,” he reports.
Source: Kuittinen M. et al. “Environmental Product Declarations for plants and soils: how to quantify carbon uptake in landscape design and construction?” The International Journal of Life Cycle Assessment 2021.