Carbon Sequestration across Urban Vegetation Types in Changing Climate in Finnish Cities

Urbanisation and climate change are global megatrends. Currently, more than half of the world's population lives in urban areas and this number is expected to increase in the future. Urban areas are vulnerable to climate change-induced extreme weather events due to urban characteristics such as the urban heat island (UHI), but are also large sources of anthropogenic greenhouse gas emissions. Urban green spaces are increasingly being explored as a solution to offset these emissions and adapt to climate change in cities. Our knowledge of urban carbon sequestration is mainly derived from research in natural ecosystems and is lacking in the urban context. Urban environments are characterized by a high degree of complexity due to their fragmented and heterogeneous nature and intensive anthropogenic management and modification. More knowledge on carbon sequestration in different urban vegetation types and the influence of management is needed to guide the planning of resilient and climate-smart urban green spaces.

Here, the JSBACH, a process-based land surface model, was used to understand how carbon sequestration in different Nordic urban vegetation types responds to possible future climates in Finnish cities. JSBACH, previously tested for urban conditions in Helsinki, was used to simulate seven urban vegetation types in 20 Finnish cities between the years 2006 and 2100. The urban vegetation types used were urban lawn, park site with Tilia cordata, urban birch-dominated forest, mesic meadow and dry meadow. In addition, irrigated versions of urban lawn and park with Tilia cordata were also simulated. JSBACH was driven by daily EURO-CORDEX data from global models CanESM2, MIROC5 and CNMR-CM5 using  RCP4.5 and RCP8.5 emission pathways downscaled to the EUR-44 domain.

Based on these simulations, urban ecosystems with trees were more consistent carbon sinks and less sensitive to future weather conditions than vegetation types dominated by grasses. Drought decreased primary production in some vegetation types during summertime, but on an annual scale, productivity was mainly driven by the length of the growing season.  In these simulations irrigation caused a decrease in Net Ecosystem Production (NEP) compared to their non-irrigated counterparts, highlighting the role of moisture as a driver of respiration.