All of these deterimentally influence other soil properties such as water infiltration and permeability, water-holding capacity, aeration status and root penetrability, especially of the upper soil layers where roots are concentrated.Poor vigor and decline in general well-being of trees and shrubs follow from the lack of water and oxygen and poor root development. Mortality is the final result under the stressful conditions of the urban environment.Soil compaction and loss of pore space arises from forces exerted on the soil surface compressing and crushing the aggregates into smaller sizes (Patterson and Mader, 1982). Foot and vehicular traffic exert the forces. Soil with high silt or very fine sand components coupled with low organic matter content tends to naturally compact under certain moisture conditions and the presence of vibrational forces (Craul and Klein, 1980). This phenomenon is one reason for the compaction of the lower soil layers beyond the influence of surface compactive forces. Urban soils may have bulk densities that occur within the range of natural soils, but most often they are at or exceed the higher limit of the bulk density range. Patterson (1976) found average values ranging from 1.74 to 2.18 Mg/m3 in four profiles of the Washington, D.C. Mall. Root penetration is highly restricted at values exceeding 1.70 Mg/m3. Craul and Klein (1980) found a range of 1.54 to 1.90 Mg/m3 with most centering on 1.82 Mg/m3. Values from New York’s Central Park (unpublished data) range from less than 1 .OO to 1.34 Mg/m3 for undisturbed surface soil and 1.52 to 1.96
Mg/m3 for subsoils.