Urban vegetation consists of trees, shrubs, and grasses. Data measured in forests are often used to estimate the carbon stock of urban vegetation. However, the urban environment differs from forests as a habitat, because it has a lot of small shrubs and new plantings are built on commercial growing media.
In recent years, attention has been paid increasingly to the research of carbon stocks of urban green areas, particularly of urban forests and trees. However, little research has been done on the carbon storage capacity of individual plant species. Shrubs are one of the most widely used vegetation type in the built environment and it is therefore important to have more detailed information on their carbon sequestration capacity.
In a recently published study, the actual dry weight of some commonly used shrubs in the built environment was measured to estimate the amount of carbon stored in the shrubs. The results can be used not only to estimate the actual carbon storage capacity of these species, but also to refine existing carbon models, point cloud-based estimates of carbon stocks in different regions, and to develop Environmental Product Declarations (EPDs) for vegetation in the Life Cycle Assessment (LCA).
Ten shrub species commonly used in the built environment were selected for the study. All the measured shrubs were full-grown and growing in a uniform planting area. In addition, their management history was known. The measured shrubs were randomly selected from the plantation area. The shrubs were pre-dried and cut after a few weeks of room drying. The below- and above-ground parts of the shrubs were separated and dried in an oven to determine the dry weight.
The total biomass of the shrubs varied widely, with the smallest shrub weighing only 0,09 kg and the largest weighing over 37 kg. In some species, such as Diervilla lonicera, the roots weighed more than the above-ground parts, underlining the importance of below-ground carbon storage (Figure 1).
The measured shrubs fell into the coarse size classes used in the planning work, i.e. small, medium, and large. The results of the measurements were used to refine the classification. The amount of carbon was considered both by size class and as a proportion of the area (Figure 2). Numerical values based on area were calculated according to commonly used planting densities. The results show that small shrubs store almost as much carbon per square meter as large shrubs. This finding suggests that small shrubs with strong roots are a valuable addition to a carbon-wise urban green in terms of carbon storage, which is good to remember in planning work.
The size classification used in the study provides a tool especially for landscape designers and architects, as it follows the classification already in use. Although Life Cycle Assessments (LCA) are commonly used to assess the environmental impact of construction projects and Environmental Product Declarations (EPDs) are available for construction products, such assessments have not yet been developed for ‘living products’ such as plants or growing media. EPDs for urban green would allow the role of vegetation to be more effectively integrated into the overall reduction of emissions from construction projects.
However, a more holistic approach is needed in the future for the development of EPDs that consider not only the carbon stock of the plant, but also the emissions generated throughout its life cycle. This would include emissions from the growing, planting, management, and degradation of the growing medium, as well as end-of-life emissions such as plant removal and further treatment. The results of this study provide a basis for the development of EPDs for shrubs.
Link to the original publication: https://www.sciencedirect.com/science/article/pii/S1618866724003583
Text by Taika Tommila.
Starting photo: Mari Ariluoma