Indirect carbon reduction by residential vegetation and planting strategies in Chicago, USA.

Concern about climate change has evoked interest in the potential for urban vegetation to help reduce the levels of atmospheric carbon. This study applied computer simulations to try to quantify the modifying effects of existing vegetation on the indirect reduction of atmospheric carbon for two residential neighborhoods in north-west Chicago. The effects of shading, evapotranspiration, and windspeed reduction were considered and were found to have decreased carbon emissions by 3.2 to 3.9% per year for building types in study block 1 where tree cover was 33%, and -0.2 to 3.8% in block 2 where tree cover was 11%. This resulted in a total annual reduction of carbon emission averaging 158.7 (+/- 12.8) kg per residence in block 1 and 18.1 (+/- 5.4) kg per residence in block 2. Windspeed reduction greatly contributed to the decrease of carbon emission. However, shading increased annual carbon emission from the combined change in heating and cooling energy use due to many trees in the wrong locations, which increase heating energy use during the winter. The increase of carbon emission from shading is somewhat specific to Chicago, due in part to the large amount of clean, nuclear-generated cooling energy and the long heating season. In Chicago, heating energy is required for about eight months from October to May and cooling energy is used for the remaining 4 months from June to September. If fossil fuels had been the primary source for cooling energy and the heating season had been shorter, the shading effects on the reduction of carbon emission would be greater. Planting of large trees close to the west wall of buildings, dense planting on the north, and avoidance of planting on the south are recommended to maximize indirect carbon reduction by residential vegetation, in Chicago and other mid and high-latitude cities with long heating seasons.