Managing biological carbon storage in urban areas: Assessing the influence of above-ground ecosystem structure on soil carbon

Background/Question/Methods

Despite urbanisation being a major driver of land-use change globally, there have been few attempts to quantify and map ecosystem service provision at city-wide scales. One service that is an increasingly important feature of climate change mitigation policies, and is associated with a wide variety of other additional benefits, is biological carbon storage. Using a typical UK city (Leicester) as a case study, we examine the quantities and spatial patterns of (i) above-ground carbon stored in vegetation and (ii) below-ground carbon within soils. We also assess the potential for managing the urban ecosystem structure, across multiple spatial scales, in order to augment the below-ground carbon pool. Carbon storage estimates were generated from field and GIS data. Greenspace across the entire urban area was classified into three landcover categories effectively stratified by vegetation height: herbaceous vegetation (comprising grasses and non-woody plants), shrubs & tall shrubs (consisting primarily of woody bushes and trees with a mean height typically less than 5 m), and trees (mature trees over 5 m tall). Soil samples (at depths of 0 - 7 cm and 7.1 - 14 cm) and above-ground surveys were conducted at sites randomly distributed throughout each of the different landcovers.

Results/Conclusions

We find that urban areas are not depauperate in terms of above- or below-ground carbon storage as is often assumed, especially in national estimates of this ecosystem service. The structure of the urban ecosystem (e.g., proximity of roads, tree cover, tree density, non-tree woody vegetation cover and sealed surface extent) explained less than 10% of the variation in soil carbon density and concentration. Our results therefore demonstrate that relationships between the two biological stores are decoupled in urban areas and managing above-ground ecosystem structure will not increase the below-ground carbon pool. Nevertheless, at a time when governments are setting greenhouse gas emissions reduction targets, there is a need for reliable data to help establish and underpin realistic trajectories for such decreases, along with acceptable and robust policies for meeting these goals. Here we illustrate the potential benefits of accounting for, mapping and appropriately managing above- and below-ground urban carbon stores, even within a typical densely urbanised European city.