Arboriculture & Urban Forestry 38(2): March 2012 Table 7. Mean leaf area, and leaf dry weight measured in 2003 and 2004 showing responses to two pre-plant nitrogen (N) rates. Year: Pre-plant N ratez 2003 Standard rate (n = 69) Low rate (n = 71) 2004 Standard rate (n = 37) Low rate (n = 38) z difference HSD). The ρb ues in these NC plots were also restrictive to plant growth in their studies (Day et al. 1995). Alberty et al. (1984) found that early forsythia (Forsythia ovata Nak.) plants had a re- duced root and shoot growth at a ρb lower than the mean ρb of 1.2 g·cm-3 readings achieved in the compacted treatments were similar to those found by other researchers to be restrictive to woody plant (Chiapperini and Donnelly 1978; Day and Bassuk 1994). Some researchers found that the same ρb val- , which is of the NC soils in the current study. tion in Ks of 60% in the clay loam soils, but no effect on Ks compared to soil between tracks, in the seedbed. In their work in north Germany, Gebhardt et al. (2009) found texture played an important role in Ks plots over the three years. Coutadeur et al. (2002) found that com- paction caused by tractor-wheel tracks reduced Ks in compacted soils. They found a reduc- due ential response to stress, in this case compacted soil, between the red oak and red maple species (Chapin et al. 1993; Grime 1977). loams and silty-clay textures, the differences were of a much greater magnitude. Although many studies focus only on soil parameters, Donnelly and Shane (1986) found no reduction in growth of Quercus rubra (red oak), but Acer rubrum did show growth decreases over their five-year study with an increase in bulk density and a decrease in Ks to compaction in the sandy and sandy loam soils. While results of the current study also show a decline in Ks . This may be related to a differ- in compacted clay Compaction activities reduce air-filled pore space and hydrau- lic conductivity because they disrupt the continuity of pore spaces and decrease overall pore size. Researchers found that mean AP in compacted soils was 7% lower than in the NC soils (11%), and while significantly different, values around 10% are common for clay-type soils (Scott 2000). Poor aeration does not always pro- vide a clear explanation of plant performance in compacted soils. Taylor et al. (1974) found that for cotton seedlings, low aeration porosity did not restrict root growth. Voorhees et al. (1975) found that root elongation rates decreased with an increase in bulk aera- tion porosity. Greenwood (1968, as cited in Eavis 1972) suggests that the “spatial distribution and area of the gas/liquid interface in the soil surrounding the roots” has a greater influence over root aeration than simply the reduction in the percent air-filled pore space due to compaction. Often, plants will develop shorter, thicker root systems in response to highly compacted or saturat- ed soils, exhibiting low aeration porosity as a means of “adapt- ing” to compacted soil and maintaining some level of growth (Eavis 1972; Voorhees et al. 1975; Shierlaw and Alston 1984). Ks in the compacted soils was 98% less than in the NC soil by 40% when 13.2 ± 1.2 a 10.5 ± 1.2 b 1008 ± 87 a 787 ± 78 b Means ± standard error followed by different letters indicate a significant differ- ence within each biomass measure for pre-plant N treatments. Standard rate [H: 100 mg·L-1 ] or Low rate [L: 25 mg·L-1], at the P ≤ 0.05 (Tukey’s honestly significant Leaf area (m2 4.2 ± 0.4 a 3.1 ± 0.3 b ) Leaf dry weight (g) 378 ± 31 a 288 ± 27 b 71 Ares et al. (2005) found no reduction in the growth of Douglas fir. In work done by Gomez et al. (2002), after similar compaction efforts, loamy soils had higher soil strength than clay soils. Com- paction, however, had no effect on tree growth in the loamy soils, but in the clay soils biomass was reduced. This may be due in part to higher water content held throughout the growing season in the loamy soils. Studies by Donnelly and Shane (1986) and Ares et al. (2005) found that compaction increased available water con- tent, whereas the study authors found a decrease (Table 1). These differences may be due to textural variation. Researchers fo the current study did find that NC soils had less water available at per- manent wilting point (Table 1), which means that the compacted soils held water more tightly at the higher tension. Water held in compacted pores could then become available during portions of the growing season as weather and soil fauna affect density. It is apparent that the hydraulic properties of soil, their tempo- ral nature (Ares et al. 2005), and soil texture interact to determine restricting bulk densities for plant growth. In the clay-based soils in this study, hydraulic conductivity was significantly reduced due to compaction, and the higher density soils led to a reduction in aboveground biomass for the majority of the maples used. The results are similar to others that found variable responses across species and soil textural types. In a 1985 study, Pan and Bas- suk found that Ailanthus altissima (tree-of-heaven) root growth was considerably more restricted in a sandy loam soil with a bulk density of 1.64 g·cm-3 than in mason sand with a bulk density of 1.67 g·cm-3. Ferree and Streeter (2004) found that in a silt- loam, peat, and perlite soil mix with a bulk density of 1.5 g·cm- 3 certain cultivars of grape showed a reduction in leaf area and shoot length. In a sandy loam soil, avocado tree roots were lim- ited to the upper few soil centimeters and clumped in soil areas with lower bulk densities (Abercrombie 1990). Day et al. (2000) found that the effects of compaction were mitigated when the soil was wet for Acer saccharinum (silver maple), but not for Cornus florida (flowering dogwood). The roots of silver maple were able to penetrate the compacted soil matrix when the water content was near saturation. However, the roots of flowering dogwood were not able to take advantage of the wet soils to elongate their roots, and showed dieback and in many cases death due to the high water content and limitations of the high bulk density. This may provide some explanation for the adaptability of bottomland species such as silver maple, Celtis occidentalis (hackberry), and Platanus occidentalis (sycamore) to compacted, urban soils. Despite the small sample of excavated roots, researchers not- ed anecdotal root growth that supports some of these findings (Figure 3a; Figure 3b). In Figure 3a, roots of ‘Celzam’ grown in a compacted plot seem to have produced a large number of roots in the top few inches of the soil, directly beneath, and in the mulch. Roots escaped the original root ball mainly at the sur- face level. Figure 3b illustrates a broader spread of roots, with no distinct pattern evident when grown in uncompacted soil. There was a significantly different response for the major- ity of biomass measures between cultivars. In general, most trees had reduced biomass growth as bulk density increased, but this was not consistent from year to year. Clearly, ‘Celzam’ Freeman maple out-performed all other cultivars, despite a re- duced growth at higher bulk densities (Table 3; Table 4; Table 5; Table 6). ‘Fairview Flame’ was one of the best performing red maples when basing assessment on caliper growth, leaf area, and response to pre-planting N rates (Table 4; Table 5; Table 6). ©2012 International Society of Arboriculture
March 2012
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