Small but powerful? The seasonal and diurnal cooling potential of urban green spaces in Munich, Germany, is moderated by size and vegetation complexity
Sophie Arzberger, Barbara Brunschweiger, Monika Egerer, Michael Suda, Dominik Thom, Peter Annighöfer
Abstract
Urban green spaces are becoming increasingly important under climate change as they significantly reduce local air temperatures during hot days. The cooling potential of urban green spaces largely depends on the size and the composition of the vegetation. Yet, how the size-dependent differences in the cooling potential relate to the influence of the vegetation structure is not fully understood. To investigate the cooling effect of vegetation structures and green space size, we continuously measured air temperature in 36 parks across Munich, Germany, and in their urban surroundings. The parks covered a gradient in size (0.25 to 375 ha) and vegetation structural complexity, ranging from sparse to forest-like. The vegetation structure was recorded using mobile laser scanning. On hot days, cooling effects of up to 3.3 °C were observed in parks with high structural complexity, while parks with low structural complexity were partially warmer than their surroundings. While the cooling effect generally increased with green space size, the vegetation structural complexity could partially compensate for size differences. Small green spaces (0.25 – 2 ha) with high structural complexity provided significantly more cooling on hot days than large green spaces with low structural complexity. During the hottest time of the day, canopy cover was the most important variable for reducing temperature. An increase of 23% points in canopy cover led to a temperature reduction of approximately 1 °C. We highlight the importance of large parks with mature tree populations for urban cooling, but we also conclude that where space is limited, establishing small parks with high structural complexity is a viable option to mitigate extreme heat in cities. Newly established parks require time until their local climate regulation function has reached its full potential. Consequently, farsighted urban planning is essential to protect people in cities from extreme heat in the future.