The pattern of urban development may be the key to determine the amount of CO₂ emissions and the ability of cities to reduce them. CO₂ sequestration and storage are not only affected by the amount of green areas and trees, but also by their stress tolerance and maintenance needs. Choosing the right tree in the right place can have important consequences on CO₂ sequestration and storage and a wrong choice might convert trees in a net carbon source. Trees, especially when stressed, emit a variety of substances many of which are reactive volatile organic compounds (VOCs). These compounds disperse into the air and, when mixed with certain toxins, can contribute to air pollution.

Management choices related to species composition and tree maintenance activities can also directly affect the overall carbon benefits from urban forests. Therefore planting trees requires special consideration regarding potential adverse factors to ensure the health of the trees, including a proper watering regime and adequate space.

Limited water resources can be a big issue in the future, especially in the drier areas where trees will continue to provide many benefits only if they are chosen according to the present and future conditions and are properly maintained.

In this presentation results from some research projects carried out to determine how different tree species under stressful conditions differ in net carbon benefits will be shown and some suggestions for tree selection strategies that optimize the net impact of urban forestry on reducing atmospheric carbon dioxide will be given.