Arboriculture & Urban Forestry 38(5): September 2012 stability, earthworms, and micronutrients) to be important secondary inclusions to indicate differences in soil quality. Despite differences in parent material, climate, organisms, re- lief, time, and human influences, the MDS derived for these urban soils appears to be similar to MDS from other systems. Table 6. Principal component scores based on nine tree response variables.zy Chicago, IL. Data from 84 trees in western suburban Principal component PC1 ≈ size Eigenvalue Proportion Cumulative proportion Age (yr) Trunk diameter (cm) Height (m) Crown area (m2 Leaf N (%) ) ) Tree condition indexx Leaf chlorophyll (SPAD) Trunk diameter growth rate (mm yr-1 Tree height growth rate (m yr-1 ) z Only principal components (PC) with eigenvalues >1 and that explain >5% of the total variance were retained. y component. x Identifies parameters with significant loadings on the within column principal Tree condition index is a qualitative score of seven factor scores of growth, structure, pest, trunk, crown, root, and life. Urban Tree Size, Soil Quality, and Landscape Age Tree size increased with urban landscape age and also across the urban soil quality gradient (Figure 1). It is reasonable to expect tree size to increase with age and also improved soil quality with time; however, this study was unable to distinguish if either of these two mechanisms were more important. Urban soil quality is linked with urban landscape age (Figure 1). The authors pro- pose two mechanisms for the increase in soil quality with time: 1) advances in construction equipment and compaction technology increasing the soil impact on more recently developed sites, and 2) biogeochemical processes increasing soil quality with time. Major advances in earthmoving and soil compaction equip- ment in the last century include the standardization of the in- ternal combustion engine, the sheep’s foot roller (c.a., 1920s), and the vibratory compactor (c.a., 1960s) (Harris 2006). It is likely that the progression in construction technology over the past century has influenced the extent and degree of ur- ban site disturbance and impact on soil quality. However, the significant linear relationship with soil quality and age of site disturbance in the most recently disturbed sites (within the last 30 years) (USQI = -5.66 + 0.318 * age; R2 = 0.43, P < 0.0001) (data not shown) suggests that technological advanc- es may not play the only role in explaining the observations. Time is one of the five soil formation factors (Jenny 1945). There are many biogeochemical processes that may increase soil quality over time (Buol et al. 2003). The predominant ex- amples relevant to urban soils include: littering (organic accu- mulation on soil surface), desalinization (removal of soluble salts), dealkalization (removal of sodium carbonate), lessivage 4.60 50.09 50.09 PC2 ≈ growth 1.82 20.25 70.34 Scores of two rotated eigenvectors 0.96y 0.89y 0.85y 0.80y 0.65y 0.26 0.19 0.08 -0.04 0.38 0.37 0.47 0.07 0.59y 0.67y 0.84y -0.57y 0.38 223 (migration of mineral particles from A to B horizons), pedo- turbation (biological or physical churning of soil materials), decomposition (breakdown of mineral and organic materials), synthesis (formation of new mineral and organic species), hu- mification (transformation of raw organic materials to humus), mineralization (release of oxide solids through organic matter de- composition), and loosening (increase in void volumes through biological and physical processes or by leaching). A number of processes may also contribute to decreases in urban soil qual- ity with time: salinization (accumulation of soluble salts), alka- lization (accumulation of sodium carbonate), and hardening (de- crease in void volume by collapse, compaction, and in-filling). Others studies have found improvements in urban soil quality with landscape age. An urban soil study in the U.S. Pacific North- west by Scharenbroch et al. (2005) found increased SOM con- tents, increased nutrient availability, increased biological activity, increased microbial efficiency, and decreased bulk density with ur- ban landscape age. Smetak et al. (2007) found greater earthworm biomass and abundances in older urban soils compared to young- er ones. Beyer et al. (1995) report increased SOM and microbial efficiencies with landscape age in urban soils in Kiel, Germany. Studies in non-urban systems confirm these studies showing soil recovery after site disturbance progresses via organic matter accu- mulations, increases in microbial activity and nutrient availability (e.g., Pastor et al. 1987; Zak et al. 1990; Diquelou et al. 1999). Figure 1. Relative tree size, urban soil quality index, and urban landscape age for 84 plots in western suburban Chicago, IL,U.S. ©2012 International Society of Arboriculture
September 2012
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