Wageningen Researcher wins first Leendert Koudstaal Talent Award

Around 70% of the world's population lives in cities, and a strong green infrastructure is essential for a livable urban environment. Urban greenery provides a range of benefits, such as rainwater retention, capturing particulate matter, regulating perceived temperatures, promoting biodiversity, and reducing noise pollution. These benefits, also known as ecosystem services, are widely recognised and have been thoroughly researched.

However, due to accelerating climate change, many of the tree species currently found in cities are under increasing pressure. It is estimated that two-thirds of urban trees will struggle to grow or even die in the coming decades. This is because these species were not selected for extreme climate conditions, such as heat, drought, and soil salinisation. This not only affects tree health but also leads to increased replacement costs and a decline in the wellbeing of urban residents.

CSI Trees: searching for the trees of the future

The research project CSI Trees: Climate-Smart Trees for the Future focuses on identifying new tree species that are more resilient to changing conditions. These trees must be able to withstand multiple forms of stress to continue growing healthily in both urban and rural areas.

The approach of CSI Trees is unique: researchers model how above-ground and below-ground conditions will change over the next 60 years. They then look to areas of the world that already have tree species capable of thriving under these future conditions.

Using a database of 28,000 tree species, they compile a list of suitable candidates. This list is then further analysed in collaboration with the Netherlands Plant Eco-Phenotyping Centre (NPEC). The trees' responses to stress factors are carefully measured, allowing species to be classified based on hard data for specific locations. Before new species are introduced, researchers also ensure that they pose no risk of uncontrolled spread and that they can contribute to biodiversity.

New winter hardiness maps

One of the key criteria for future trees is their winter hardiness: the lowest temperature a tree can tolerate without dying. The winter hardiness maps for Europe and the Netherlands were significantly outdated (last updated in 1984), making them unsuitable for selecting tree species.

Lucas Hulsman, a junior researcher at the Meteorology and Air Quality group of Wageningen University & Research, has made a major contribution to updating these maps as part of the CSI Trees project. Following his master’s degree at the Royal Netherlands Meteorological Institute (KNMI), Hulsman began calculating shifts in winter hardiness zones for the period 1980–2020. For the Netherlands, he created maps based on the four KNMI climate scenarios, projecting winter hardiness zones into the future.

This new map, with a grid size of 100 by 100 metres, reveals for the first time that a single city can contain three different winter hardiness zones. This allows urban planners to select tree species more precisely, tailoring choices to the specific conditions of a street, square, or neighbourhood. The method Hulsman developed can also be used in the future to calculate other important variables for tree selection.