232 Arnold and McDonald: Groundcovers, Mulches, and Surfaces Affect Tree Establishment Arboriculture & Urban Forestry 2009. 35(5): 232–240 Groundcovers, Organic and Inorganic Mulches, and Masonry Surfaces Differentially Affect Establishment and Root Zone Characteristics of Urban Trees Michael A. Arnold and Garry V. McDonald Abstract. Three experiments investigated the effects of various groundcovers on establishment of redbuds [Cercis canadensis L. var. texensis (S. Wat- son) M. Hopkins ‘Alba’] and baldcypress [Taxodium distichum (L.) Rich.]. The first experiment involved eight surface treatments. Controls were bare soil. Remaining treatments were pine bark mulch; Asian jasmine [Trachelospermum asiaticum (Siebold & Zucc.) Nakai]; St. Augustinegrass [Steno- taphrum secundatum (Walt.) Kuntze]; decorative gravel; recycled paper mulch; decorative brick pavers; or seasonal rotations of herbaceous annuals. Other experiments compared brick-on-sand treatments ranging in color from light blonde to dark charcoal with bare soil on establishment of redbuds or baldcypress. Most organic and living soil surface covers were preferable to bare soils, however, some inorganic surface covers were detrimen- tal to tree growth. Paving surfaces adversely affected survival, shoot or root growth, but differences were species dependent. Soil moisture, pH, and bulk density did not appear to be limiting under pavers, but substantial seasonal fluctuations in soil temperatures were observed. Light and medium bricks reflected more photosynthetically active radiation than dark bricks or bare soil. Atmospheric temperatures were greatest above dark and medium bricks. Root growth decreased as darkness of brick color increased. Redbud survival and growth were more adversely affected than with baldcypress. Key Words. Baldcypress; Brick-on-Sand Paving; Cercis canadensis var. texensis; Groundcovers; Herbaceous Annuals; Inorganic Mulch; Organic Mulch; Plant Competition; Redbud; Taxodium distichum; Transplant Establishment; Tree Establishment. In situations that permit the space, many arboriculture practitio- ners recommend mulching trees to the extent of the drip line or beyond (Carlson 2002). However, tree establishment in urban environments often occurs in limited rooting volumes, such as planting pits in sidewalks or in parking lot islands. Soil sur- face covers can be highly variable, depending on the pedestrian traffic and intended use in the area, in addition to the level of maintenance desired. Inorganic groundcovers, such as decora- tive gravel or brick-on-sand pavers may be used where heavy pedestrian traffic is anticipated while areas with lighter foot traf- fic may involve the use of organic mulches or living groundcov- ers such as turfgrass or vines. Areas with high visibility may be covered with decorative organic groundcovers, including pine bark; or rotations of seasonal herbaceous annuals (seasonal color changes) may be planted cyclically throughout the year in beds beneath the trees. In other cases, environmentally friendly soil surface covers, such as recycled paper products, may be used. While studies have addressed various characteristics of mulch- es relative to their durability (Litzow and Pellett 1983; Skroch et al. 1992; Duryea et al. 1999), attractiveness (Ashworth and Har- rison 1983; Skroch et al. 1992; Duryea et al. 1999), effects on soil physical or chemical characteristics (Stephenson and Schuster 1945; Fraedrich and Ham 1982; Watson 1988; Hartz and Gianni- ni 1998; Duryea et al. 1999), and to a limited extent their absorp- tion or reflectance of sunlight or heat (Kjelgren and Montague 1998), information is limited about their relative effects on tree establishment. In most cases, information on tree establishment is limited to studies investigating nonliving organic groundcovers, such as composted or chipped mulches, or plastics based inor- ©2009 International Society of Arboriculture ganic groundcovers such as polyethylene or polypropylene plas- tic or woven mats (Litzow and Pellett 1983; Green and Watson 1989). Likewise, limited information is available on the impacts of living groundcovers on the establishment of trees, but some reports of allelopathic influences (Walters and Gilmore 1976; Todhunter and Chan 1979) or at least competitive effects (Green and Watson 1989; Downer and Hodel 2001; Arnold and McDon- ald 2008) of turfgrass has been reported on tree establishment. Taxodium distichum and C. canadensis were selected as the test species since they are commonly planted and have con- trasting tolerances to urban sites. Cercis canadensis represents a species native to woodland edges and areas with well-drained soils (Arnold 2008). Cercis canadensis is popular for its or- namental foliage, growth form, and spring flowers, but tends to be susceptible to mechanical bark injury, grows poorly on compacted soils, and is often short-lived in urban landscapes in southern U.S. (Gilman 1997; Arnold 2008). Alternately, T. distichum is native to wet sites with poorly drained soils, but also tolerates drier sites, and is known to perform well in many urban sites with slightly alkaline to acidic soils with atten- tion to appropriate provenances (Gilman 1997; Arnold 2008). The objectives of the experiments presented herein were to: 1) compare effects of a range of commonly utilized inorganic and or- ganic mulches, living groundcovers, and seasonal rotations of her- baceous annuals on establishment of C. canadensis; and 2) com- pare the effects of various colors of brick-on-sand pavers on the establishment of two tree species, T. distichum which is tolerant of urban conditions and C. canadensis which is less tolerant of urban conditions, high root zone temperatures, and reflected irradiances.
September 2009
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