Atop most of the world’s land surface, there is a layer of soil varying in depth from a mere few centimetres through to over ten metres in well established peat bogs. The majority of the world’s cities were founded in the most sensible places for human inhabitation – fertile lands close to a source of water. As time advanced, so did the extent and population density of many of these cities and towns and gradually bare and vegetated soils came to represent the minority land use within city borders.
Today, in many cities, parkland and other greenfield sites are considered premium resources. Townhouses bordering parks fetch the highest house prices and nearby green space is prerequisite for the majority of buildings of state and homes of the rich. Urban vegetation takes the form of trees planted by the roadsides, parks large and small, turf playing fields, urban farms and forestry and, importantly, residential gardens.
Across the world, city planners are coming to value the importance of greenery and soil within the city bounds and seek to maximise it. Why have they come to this conclusion? Not only to drive up the house prices and provide a pleasant environment to live in for city citizens but also because of the key influence that soils and vegetation have on natural cycles of heat, water and pollutants in the surface to atmosphere interface. These ‘services’ that the soils provide are termed “ecosystem services” and they are as important in the city as they are in natural environments. Ecosystem services might be cultural (house prices, space for recreational activity, aesthetic value) or provide habitats for animals and plants. They might be productive (city farms and allotments) or they might regulate the environment (take up carbon, capture pollutants such as heavy metals, cool the city down).
In the bottom panel of the above graphic I have drawn three different land surfaces and how they impact three important things; heat, water and carbon dioxide. The size of the arrow is a rough indicator of the magnitude of the movement of that commodity. Outbound heat is much greater from the tarmac surface than from the vegetated or soil surfaces for several reasons. Firstly, tarmac is a dark surface and it absorbs a larger amount of incoming solar radiation. But soils can be dark too – so why no large heat loss from soil surfaces? Because soils are often wet as well as dark, in fact quite often a darker soil is a wetter one. And heat is required to evaporate this water, therefore less heat is absorbed and the warming effect is reduced. On vegetated soils the plants mediate this evaporation through their water-upake systems, a process known as transpiration. Back to the tarmac. The second reason it is a heating surface is that it does not absorb water, it pools on the surface. So instead of a consistent and steady evaporation flux you have either rapid evaporation from a pool on the surface, or if there is drainage, little cooling effect from water. In fact the tarmac is a barrier to downward water flow (infiltration) which is why urbanisation in areas that are naturally floodplains can cause serious flooding issues if nowhere is provided for displaced water to go. Finally, another important ecosystem service is the regulation of carbon dioxide by vegetation. This effect in some city neighbourhoods could be substantial enough in the summer to balance out the carbon dioxide from human sources (UC Santa Barbara, 2012) though it would take a lot of vegetation to do this at the city scale – to the extent that such an environment might no longer qualify as an urban space (University of Helsinki, 2012).
For further information on this fascinating and complex subject, I recommend this detailed review by Pataki et al (2011) which is available open-access here. This article provides analysis of key ecosystem services provided by urban ecosystems and their related costs (here termed ‘ecosystem disservices’). It also provides a useful summary in Table 1. of the magnitude of impact of each of the discussed ecosystem services and the level of uncertainty associated with current knowledge.
In the next technical article on Ground to Sky, I will describe in more detail the influence of land surface type on heat, including the startling impact that hot cities have on the lower atmosphere. Before this, I will write the first of what I hope will be many insight articles into my life in science, focusing on taking field measurements of soil properties and greenhouse gases and why measurements are critically important now, as environmental science moves towards an entirely modelled world.