Effectiveness of urban tree planting for city cooling varies between European regions

This study in Global Change Biology reports differences in cooling efficiency trends of urban trees between European regions, and highlights issues that affect cooling benefits.

Planting trees in urban areas is a popular strategy for reducing temperatures in cities under global warming. understood. This study revealed some of the specific conditions that determine the relative effectiveness of this strategy across regions to inform decisions about urban tree planting programmes.

The study modelled ‘tree cooling efficiency’ (TCE) of 806 cities around the world between 2000 and 2015 using satellite data on tree cover and land temperature. TCE is the reduction in temperature achieved by increasing tree cover by 1%. The most important factor found to increase TCE was high leaf area index – a measure of the density of the leaves in the tree canopy. The second most important factor was a low ‘reflection coefficient’ or ‘albedo’ of the city, as cities with low albedo have many dark materials such as asphalt absorbing a lot of heat from the sun and benefit more from additional trees shade.

The study also reports that TCE increased globally by 45% over the study period, indicating that the effectiveness of tree planting to reduce urban temperatures is rising. Cities in west-central Europe, were among those that experienced greater increases in TCE. Meanwhile, cities in southern Europe experienced substantially lower increases in TCE, which coincided with lower TCE in general and less pronounced increases in tree cover and leaf area index in those regions.

The study notes that in dry soil conditions, trees are less efficient in cooling. This highlights the importance of tree maintenance (to increase the leaf area index) and irrigation in maximising the cooling benefits of urban trees. Also, TCE decreases with increasing tree cover – so that it is recommended to plant trees in an urban area that has very few trees produces a greater benefit than planting in an area with lots of trees.