Arboriculture & Urban Forestry 35(5): September 2009 The mean Δ across all attribute categories was greater (p = 0.1044) for backfill compared deep surface organic application. The type of organic material had significant impacts across all attribute categories and on physiological attributes (Fig- ure 4). Studies that used compost materials had significantly (p = 0.0012) greater Δ values compared to studies with mulch or a mixed materials. Physiological Δ values were significant- ly (p = 0.0326) greater for compost and mulch studies com- pared to studies using a mix. Shoot growth, root growth, and soil physical Δ were greater (p = 0.1282, 0.3237, and 0.1171, respectively) for compost studies compared to mulch and mix. DISCUSSION Summary of Literature Search Trends and conclusions from the 143 studies not includ- ed in the meta-analysis were compiled and are summa- rized in the following section. Although only selected refer- ences are given for the general trends, a full bibliography of these studies can be acquired by contacting the author. Studies published in the Journal of Arboriculture in the 1970s were often qualitative tree evaluations. These studies suggest: 1) inorganic fertilization is required for establishment and main- tenance; 2) organic material may be beneficial for tree estab- lishment; 3) organic material may have benefits for controlling weeds and root rots; 4) plastic under mulch should be avoided; and 5) organic mulches are alternative uses of arboricultural waste (e.g., Hoitink et al. 1975; Schulte and Whitcomb 1975; Swisher 1976; Walker 1977; Smith 1979; Whitcomb 1979). In the 1980s, the Journal of Arboriculture published more quantitative data on tree and soil responses to organic materi- als. Studies published in the 1980s: 1) increased awareness of the importance of soils and site factors for urban tree growth; 2) showed that the effects of inorganic mulches on trees and soils are different compared to organic mulches; 3) demonstrat- ed that the environment under organic mulches is quite differ- ent from under turf-grass (e.g., Whitcomb 1980; Hamilton et al. 1981; Peck 1981; van de Werken 1981; Whitcomb 1981; Funk 1983; Craul 1985; Dyer and Mader 1986; Kozlowski 1987). During the 1990s, the Journal of Arboriculture continued to Figure 3. Mean % response relative to control (Δ) for surface mulches (0–10 cm), deep surface mulches (>10 cm), and backfill organic amendments across ecological attribute categories. Sig- nificant (p ≤ 0.05) differences for each are identified by different letters. Errors bars indicate the 95% confidence interval. publish more quantitative data on tree and soil, as well as, envi- ronmental responses to organic materials. Research in the 1990s suggested that: 1) organic materials have many benefits as soil surface covers; 2) urban soil organic matter cycling is unique from other systems; 3) organic materials may be useful to offset inorganic fertilization; 4) biological inoculants and plant growth hormones may have benefits for urban soils and trees (e.g., Dixon and Johnson 1992; Wager and Barker 1993; Craul 1994; Cregg 1995; Burch et al. 1996; Close et al. 1996; Marx et al. 1997; Smiley et al. 1997; McPherson 1998; Perry and Hickman 1998). From the years 2000 to 2008, studies relating to organic materials published in Journal of Arboriculture and AUF de- tailed: 1) chemical and physical properties of mulches; 2) depth and placement mulches; 3) pathogens and flammabil- ity of mulches; 4) reviews of N availability and tree fertiliza- tion; 5) effects on soil biology and urban ecological function (e.g., Jin et al. 2002; Nowak et al. 2002; Struve 2002; Steward et al. 2003; Koski and Jacobi 2004; Scharenbroch and Lloyd 2004; Jacobs 2005; Wells et al. 2006; Day and Harris 2007). The Arboriculture & Urban Forestry literature search showed that organic materials are a relevant AUF topic. Some recurring themes pertaining to organic materials in AUF literature relate to specifics of type of organic, how it is placed (i.e., mode), and quanti- fication of impacts on trees, soil, and the environment. The following sections discuss these themes in the context of this meta-analysis. Figure 4. Mean % response relative to control (Δ) for compost, mulch, and mixed organic materials across ecological attribute categories. Significant (p ≤ 0.05) differences for each are identi- fied by different letters. Errors bars indicate the 95% confidence interval. Type of Organic Material In general, mulches in these studies were coarser organic ma- terials from tree trimmings, wood chips, hay, bark, etc. (Table 1). Compost was finer, more stabilized organic material, and mixed material contained some of both types. Most studies in ©2009 International Society of Arboriculture 225
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