Tilia cordata trees don’t transpire to the same extent in all environments based on a study by Mohammad Rahman, Chair for Strategic Landscape Planning and Management, Technical University of Munich (TUM). During the summer heat,
transpiration — the loss of water from the leaves — from those trees grown in open green squares cools us down more effectively than grown in narrow, paved squares. This is caused by local differences in the meteorology and the surface covers. Street canyons, roads and squares get particularly hot in summer. Trees cool the asphalt under their crowns by up to 30F and the air by up to 3F as demonstrated in the TUM studies.
TUM’s tests were conducted on two squares in the heart of Munich: the green Bordeaux Platz and the paved Pariser Platz. “Local meteorological conditions vary greatly and affect how the trees transpire,” Mohammad explained.
Urban trees growing in open green squares provide an optimal cooling effect on summer days compared to narrow and paved squares. The cooling power of these “green air conditioners” is at least 20% lower on narrow, paved squares with small cut-out pits for trees.
Plants release water vapor when they absorb CO2 for photosynthesis via stomata. At Bordeaux Platz, the researchers measured a sap flow rate of up to 2 gal./hour in the vascular system. In terms of energy loss, this means that Tilia cordata achieves a cooling power of up to 2.3 kilowatts. “The trees’ output is comparable to that of an air conditioner for a single room,” he said.
The study measurements show that small-scale differences in the immediate environment around the plants affect transpiration. Wind blows across open green spaces at a higher speed, the air is less saturated with water and the trees are exposed to more sunlight as compared to a narrow paved square that’s surrounded by buildings on all sides. Furthermore, the ground of the green spaces at Bordeaux Platz is cooler and can retain soil moisture longer than the completely covered Pariser Platz. “These conditions influence transpiration and hence the trees’ cooling effect.
“In order to reduce the amount of heat in cities, it would make sense to create more open spaces and squares—this would allow us to directly influence the cooling potential of the trees,” Mohammad recommends. In addition, the plant ecologist advises that more grass lawns might have added benefit in terms of reducing the effect of ground heat storage and increase boundary layer cooling compared to those planted in narrow paved squares. —University of Munich press release in English.
November 15, 2016 NewTerrain.