ABSTRACT
Although the physiological processes in urban trees are the same as in trees growing in forests, the ambient conditions under which these processes occur in cities are more variable and extreme. Environmental conditions in urban areas are tremendously diverse because of differences in climate, building density and structure, building material, and fraction of green and blue space. Generally, urban areas are warmer than their surroundings by up to 1–3°C because the dominating stone structures absorb radiation more than vegetated surfaces and lack the cooling effect from evapotranspiration (Arnfield, 2003). This phenomenon is known as the urban heat island (UHI) effect, which is usually larger at night than during the day and in winter than in summer (Gaffin et al., 2008). In addition, UHI can alter regional precipitation and increase the frequency of severe weather events (Changon, 2001). Water and nutrient availability might be decreased compared to forests or pastures due to increased runoff (on impervious surfaces such as roofs, streets, sidewalks, parking lots, etc.) and soil compaction (during construction, by vehicles and people). More specifically, soil compaction can lead to aeration problems, as well as increased mechanical damage. In addition, urban forests are often grown on former rubble or garbage dumps and thus might be exposed to soil contaminants (Craul, 1991). Furthermore, leaves are often removed especially in parks and residential yards altering the natural cycling of litter and nutrients between plants and soils (Kjelgren and Clark, 1992), whereas to compensate for low soil fertility, the vegetation is sometimes fertilized.
