Summary: Urban areas are growing in size so less trees are found in the city. According to report released in 2000 about , 2.8 percent of tree canopy is covered in the United States. City trees are dealing with very different types of growing conditions than tress in rural areas. The cities are notorious heat islands which makes nights hotter which has more evaporation which drains soil of moisture. Don't forgot about pollution run off from cars and fertilizers.
Reflection: They did tested seedling growth, and noticed that the baby plants were more sensitive to soil moisture. I think this causes it to be more prone to dehydration cause of increased evaporation under the warm climates. Adding Nitrogen it will kill the seedlings. So it is harder to grow plants in cities than rural areas.