“Decarbonization has design value. It’s part of the design process, not a separate thing,” said Marieke Lacasse, FASLA, principal at GGLO, during the second in a series of webinars organized by the ASLA Biodiversity and Climate Action Committee.
Lacasse outlined practical strategies landscape architects can apply to reduce the carbon footprint of parks, plazas, residential communities, and other landscapes.
A landscape’s footprint depends on how much concrete, steel, aluminum, and other carbon-intensive materials it incorporates. These materials create high amounts of embodied carbon in comparison with other materials, like wood, which store carbon.
“A highly active urban plaza will have lots of hardscape,” Lacasse said. But designers can still find ways to minimize hard surfaces and structures to reduce climate impacts.
One tool for figuring out how to cut these materials is Climate Positive Design’s Pathfinder. It shows landscape architects the carbon impacts of the materials they choose for a project.
Climate Positive Design and the ASLA Field Guide to Climate Action call for 70 percent of all landscapes to be “softscape,” covered in trees, plants, and soils, and 30 percent to be hardscape.
To get there, Lacasse said landscape architects should cut unnecessary concrete surfaces and swap in decomposed granite, local stone or wood pavers, or recycled materials.
Conventional asphalt, which is commonly used in streets and trails, can be swapped for pervious asphalt, which is lower in carbon. Existing asphalt can also be cut and reused.
Reducing high-carbon materials overall provides benefits. “We can rethink our designs. Do you need a 10-foot-wide concrete or asphalt path, or can you make it 5-feet-wide and add decomposed granite to the edges?”
“Instead of a concrete seat wall, design a wood one. Instead of concrete retaining walls and terraces, slope the grade,” she said.
Reducing materials with high embodied carbon is one step for reducing emissions from landscape architecture projects. The other side is significantly increasing the amount of carbon stored in a landscape. If the amount of carbon sequestered is more than the amount emitted by its construction or operations, it’s climate positive — a net carbon sink.
Biodiverse landscapes, with layers of trees and understory plants, store more carbon. But maximizing carbon storage in a landscape is complex. “We look at the growth speed and life span of trees and plants, the planting strategy, how to weave in biodiversity,” Lacasse said.
Trees, plants, and soils all store carbon but at different rates. “Evergreen trees have a faster carbon absorption rate. Deciduous trees have larger canopies and denser wood so they store more carbon, but their absorption rate is slower.”
Large trees store more carbon than smaller trees, so it’s important to keep existing trees whenever possible.
For a new two-acre community in Seattle, with community gathering places, playground, and a creek, GGLO first created a design using conventional concrete, asphalt, and metal play equipment (see images at top).
Using the Pathfinder tool, they found the site would take 92 years to become climate positive. That is how long it would take for the carbon stored in the trees, plants, and soils to be greater than the carbon emitted through the site’s materials and construction.
But with some changes to the design — including recycled asphalt and concrete; bamboo decking; and wood seatwalls, play structures, and surfaces — it could take 32 years to reach climate positive, a decrease of 60 years.
Andrew Wickham, ASLA, a project leader at LPA Design Studios, explained how landscape architects can better measure carbon in their projects and for their clients, using a mix of tools like Carbon Conscience, Climate Positive Design, iTree, Tally, and the Embodied Carbon in Construction (EC3) calculator.
“We can’t manage carbon without measuring it. These tools can make designers feel empowered,” he said.
From his work with the tools, he found that “the decarbonization potential is higher with hardscapes. Reducing the use of these materials is better than pumping up planting. You can’t plant the heck out of a project to offset embodied carbon. Reduce first and sequester second.”
And Mariana Ricker, ASLA, an associate with SWA, explained how landscape architecture firms can weave carbon considerations into their internal design workflow. “It is just another layer in the process.”
To make that workflow clearer, SWA developed its own decarbonization guide. “We use it to advocate to clients, empower our designers, and prioritize. Not all projects have the same carbon potential.”
She urged landscape architects to carefully consider the materials they select for projects. “It’s important to be as low carbon as possible upfront.”
“When a material has been produced, those embodied carbon emissions are in the atmosphere. We have a limited time span with the climate crisis. Reducing emissions must be our priority. Sequestration happens later, over time.”