What do we mean by urban green?

When do we define a piece of nature in the city as a park? And when is something a tree or shrub? It may seem obvious, but in the scientific literature the definitions vary quite a bit. That makes comparisons difficult. Environmental scientist Joeri Morpurgo looked at the differences and designed a general classification system for urban green spaces.

Urban greenery is a popular topic in scientific research on ecosystems and biodiversity. “If a researcher wants to reduce the heat in the city, parks play a big role,” says Morpurgo. “If someone else looks at urban water regulation, the well-being of residents or urban biodiversity, parks are often a crucial part as well.”

The definition of a park varies quite a bit. “We have a general idea about urban green: we think it’s positive. But if we keep adjusting the definition according to our own research, we don’t have a benchmark at all to measure and compare the effects.”

When is something a tree?

Morpurgo analyzed 143 scientific articles on topics such as climate, human health and biodiversity. In it he looked at what researchers defined as urban green in their publications. “The biggest difference I found is the term tree. As a biologist, the definition is pretty clear to me: if it’s a tree species, then it’s a tree. But that’s not what we saw in the study.”

Some researchers used the definition that all vegetation above two meters is a tree. Others then used very different measurements, Morpurgo said. “But of course a twenty-meter tree has a completely different effect on the environment than a two-meter shrub. Policymakers often say: ‘Research shows that trees are good for the city or for well-being.’ But then we don’t even know what they are talking about exactly. That is actually quite crazy. And you also see the same with other terms. For example, I found 27 different descriptions within the theme of afforestation.”

A new tool for vegetation

To avoid confusion in the future, Morpurgo created a classification system based on existing literature. “This division first provides general definitions: what do we mean by agriculture, a park, wetland or a green roof, for example. Then we added a deeper layer.” With questions about cover, height, stratification and, in the case of forests, the dominant tree species. A specific designation will then appear. “With these definitions, we can be sure that we are talking about the same thing in the future.”

The next step is to use that classification to create a new tool. “The idea is that you can type in a location and get a map with information. On it you can see what vegetation there is, at what height and under which category it falls.” With a few clicks you immediately have all the information. “So not just whether it’s a forest, but also what kind of plants are in it and how many. Forest A, for example, may be completely overgrown with hardy trees, and forest B may have a patch of grassland or riparian vegetation here and there. .”

An untouched gold mine of data

To create the tool, Morpurgo combines existing data. “In the Netherlands, for example, we have the Current Height File Netherlands (AHN). This digital map shows the height of the vegetation everywhere in the Netherlands. The Sentinel-2 satellite is available worldwide to estimate how ‘green’ an area is. We then combine that with data on land use and land cover.”

The ambition is to be able to use the tool worldwide. “The data may not be as detailed everywhere as in the Netherlands, but we can still get a lot of information from, for example, satellite images and Google Streetview. That is a goldmine of data that we are not yet tapping into properly.”

Companies can give better advice

The tool and the classification system are interesting for both researchers and companies. “There is a general demand for standards and definitions that we can use. Not only in the Netherlands, but on a global scale. That way we are clearly talking to each other about the same thing. The system is based on what we already use in the literature. Therefore, I assume the terms are easy to use.”

Several companies have already expressed interest in Morpurgo’s classification. “As a result, a company can, for example, make much better calculation maps of urban green areas. Or companies that advise on urban development with issues in the field of biodiversity and well-being can do so in a much more targeted manner.”

Morpurgo hopes that scientists and companies will use the classification as a standard in the future. “It feels good to be able to offer an alternative to the ‘everyone-do-something’ system. These insights will be very valuable for global urban planning.”

The work is published in the magazine Landscape and Urbanism.