A large proportion of CO2 emissions originate from urban areas. Thus, cities are increasingly searching for tools to reduce emissions in order to slow down the speed of climate change. A recent study, based partly on ICOS data, shows how planted street trees increase carbon absorption in cities.
Urban green areas absorb a significant amount of city greenhouse gas emissions. Trees can offer cities a variety of ecosystem services, ranging from carbon sequestration to cooling of local temperatures, reducing floods caused by storms and rains in addition to improving air quality. However, urban nature is highly diverse in terms of soil properties, plant species, and biomass, which creates a great deal of uncertainty regarding the potential of trees as carbon sinks.
“For cities to reliably calculate their own carbon sinks in urban vegetation and soil, they need more information on the carbon cycle in urban areas. Most of our knowledge of the carbon cycle is based on data and models from forested ecosystems, though urban nature and microclimates differ greatly from them,” says Leena Järvi, ICOS scientist and Professor in Urban Meteorology at the University of Helsinki.
Taking specific urban conditions into account, the Finland-based research team took on the task of bringing clarity to the carbon cycle dynamics in urban nature:
“The aim was to examine the carbon sequestration potential of two commonly used street tree species, considering the complexity of urban conditions. We wanted to evaluate different models for CO2 flux dynamics of planted urban street trees on day-to-day, season-to-season, and year-to-year basis. We also wanted to describe temporal variability of soil organic carbon beneath street trees,” says Minttu Havu, Doctoral Researcher in Atmospheric Sciences at the University of Helsinki.
After 14 years trees become sinks
The research team used an urban land surface model, SUEWS, for providing data on the urban microclimate, street tree photosynthesis and respiration, whereas soil carbon storage was detected using a software model called Yasso. The researchers examined the carbon sequestration of street trees and soil in specific areas in Helsinki, Finland, while studying the urban carbon cycle throughout the expected lifespan of the street trees.
“We estimated carbon sequestration potential for an average street tree and for the average of the diverse soils in the study area. Over the study period of 30 years, the soil respired more carbon to the atmosphere than the tree captured. This was due to the high organic carbon in the constructed soil which gradually respires to the atmosphere. However, the street tree plantings turned into a modest sink of carbon from the atmosphere on an annual scale, as tree and soil respiration approximately balanced the photosynthesis,” Havu explains.
The compensation point, when street tree plantings turned from an annual source into a sink was reached after 12 years by alder trees and after 14 years for linden trees. However, these moments naturally vary from site to site depending on e.g. planting density, tree species, and climate. Overall, the results indicate the importance of soil in urban carbon sequestration estimations.
“These results should however be upscaled with caution since they do not represent the full variety of soil growing media, planting densities, and plant types. The annual carbon sequestration depended on environmental factors, such as air temperature and humidity, indicating the need for modelling techniques that allow us to appropriately account for local climate conditions,” Havu concludes.
Greenhouse gas data from urban areas
The study used data from the ICOS urban ecosystem measurement station in Kumpula, Helsinki, Finland.
“The data we received from the station mainly covered turbulence and wind speed. Quantifying biogenic sinks is also important in the new ICOS Cities project, as they impact atmospheric carbon observations. The same models that were used in this project will also be used in the ICOS Cities project,” Järvi explains.
The ICOS Cities project is a Horizon 2020 project, that develops systematic observations to monitor the level of greenhouse gas emissions in urban areas. Urban areas contribute to a large share of global and European fossil fuel emissions - cities are therefore at the heart of emission reduction efforts.
Source and full article:
Biogeosciences, 19, 2121–2143, 2022
Piece of news about the article in the regional newspaper Helsingin Sanomat (in Finnish).
"Kaupunkipuiden istuttaminen luokin uuden päästölähteen hiilinielun sijaan" (HS 15.1.2022)