194 for temperate zone species is between 2°C and 25°C (Lyr and Hoffman 1967). Root elongation of many temperate species is severely limited when soil temperatures fall below 10°C (Harris et al. 1995; Harris et al. 1996). In contrast to their shoots, which have a dormant period that can only be overcome by chilling, the roots of many temperate zone trees do not exhibit an easily identified period of innate dormancy (Richardson 1958; Taylor and Dumbroff 1975), and can respond quickly to warming soil. However, Arnold and Young (1990) found evidence with several Malus (apple) species that an innate root dormancy satisfied by low temperature exposure may exist in some tree species. Lack of moisture suppresses root growth in two ways: first by restricting water uptake that drives cell expansion, and second by increasing soil strength (see Compacted Soil as a Permeable Impediment). For trees in tropical areas, water availability is the main environ- mental determinant for periodic root growth patterns (Borchert 1994), and root biomass is strongly correlated with soil moisture across tropical moisture gradients (Green et al. 2005; McGroddy and Silver 2009). In temperate species, soil moisture dynamics influence root growth periodicity within the confines of tem- perature controls (Tesky and Hinkley 1981; Kuhns et al. 1985). ROOT RESPONSE TO PHYSICAL CONSTRAINTS The ability of roots to explore the belowground environment in urban settings influences tree health, stability, and longevity. However, few studies have addressed rooting response of urban trees to specific characteristics of the belowground environment (for a general view of root architecture in urban settings, see Day et al. 2010). In a study encompassing seven German cities, 20- to 40-year-old Tilia spp. (lindens, species not identified) were excavated in an attempt to identify belowground factors that in- fluenced root penetration and proliferation (Krieter 1986). One unusual facet of this large-scale study was the excavation of potential rooting spaces under streets and sidewalks. Root pen- etration and fine root proliferation were influenced by soil type. Both pure sands and gravel layers (no fine materials) as well as highly compacted loamy and clayey soils restricted or prevented root penetration (see also Soil Compaction). Greater fine root proliferation was observed within irrigated areas, around utility and irrigation lines, in areas with coarse gravel and debris mixed with finer materials (clay and silt), and at curb interfaces and similar structures where a physical “dam” was created that may have collected water. Even with this large-scale study, however, variation was considerable, and the root responses observed may have been unique to German street tree installation practices, to the northern European climate, or to the particular tree species. As this study demonstrates, there are multiple physical constraints that dictate root exploration of the subterranean urban environment. These constraints can be broadly clas- sified into two types: solid impediments such as building foundations, roads, and rocks; and permeable impediments, such as compacted soils. Root exploration of these physi- cal obstructions may further depend upon moisture content. Urban Infrastructure as a Solid Impediment In urban conditions, tree root systems may be confined by be- lowground infrastructure that is essentially impenetrable unless seams, cracks, or other openings are present. Studies in urban and landscape settings documenting tree root growth in and around ©2010 International Society of Arboriculture Day et al.: Tree Root Ecology in the Urban Environment this infrastructure are extremely limited. Nonetheless, the follow- ing examples illustrate the potential for roots to navigate min- ute fissures in the urban underground complex. In a case study describing management of root–infrastructure conflicts, Schro- eder (2005) published a photo of Acer pseudoplatanus (sycamore maple) fibrous roots penetrating through mortar joints into an underground utility room and extending 1 m or more through the air inside the chamber. Root interactions with sewer pipes have been reviewed by Randrup et al. (2001), who documented numerous intrusions by roots into unsealed pipes. Although tree roots may successfully explore belowground urban infrastruc- ture, this does not necessarily mean that adequate nutrients and water can be obtained, and spatial availability of these resources can have a profound effect on root distribution (Mou et al. 1997). Because research in urban settings is limited, we must rely on studies in analogous situations to provide additional insight into root response to physical constraints. For example, trees adapt- ed to arid, rocky conditions may grow roots through very small cracks (less than 0.3 cm wide) in rock up to 9 m deep in order to access the water table (Saunier and Wagle 1967). In southwest- ern Oregon, U.S., roots were found in rock fissures as small as 100 µm (Zwieniecki and Newton 1995). While the stele retains its regular shape under such confined conditions, the root cortex may become flat, creating wing-like structures on the sides of the stele (Saunier and Wagle 1967; Stone and Kalisz 1991; Zwie- niecki and Newton 1995). These structures have been measured at up to 0.75 mm across with root hairs only occurring on the edg- es of the structures (Zwieniecki and Newton 1995). These studies illustrate how roots might penetrate minute fissures in concrete, masonry, or other urban infrastructure and adapt anatomically to the space. Documented observations in urban environments are few, and the conditions necessary for this adaptive growth are unknown. In some cases, tree roots will grow around physical ob- stacles. For example, Platanus × acerifolia roots were observed to partially or completely encapsulate 2 cm limestone gravel that was a component of a structural soil mix (Bassuk 2008). Compacted Soil as a Permeable Impediment Soil compaction arising from urban land development and use is a more pervasive cause of root restriction for landscape trees. Compaction occurs as soil is compressed, which de- grades structure, diminishes porosity, and increases strength— the soil’s physical resistance to penetration. Soil compaction in urban areas is widespread. In a study of 48 sites in Moscow, Idaho, and Pullman, Washington, recently developed sites were found to have higher soil bulk densities than older sites (Scha- renbroch et al. 2005), presumably due to more stringent engi- neering standards and more effective compaction equipment. Site development practices often entail removal of upper soil horizons (especially O and A) during grading (Jim 1998), leav- ing denser subsoil at the surface, and the soil underlying pave- ment is typically compacted to provide structural support. Thus urban tree root systems are likely to encounter compacted soil. These restricted root systems are commonly shallower, confined by dense soil underlying pavement or planting pits, or exhibit less extensive soil exploration than would be possible in uncom- pacted soil. Root systems in compacted soil are more highly branched and consist of thicker, stubbier roots, which often re- sults in shallower rooting depth (Tackett and Pearson 1964; Voor- hees et al. 1975; Gilman et al. 1987; Materechera et al. 1991).
September 2010
| Title Name |
Pages |
Delete |
Url |
| Empty |
Search Text Block
Page #page_num
#doc_title
Hi $receivername|$receiveremail,
$sendername|$senderemail wrote these comments for you:
$message
$sendername|$senderemail would like for you to view the following digital edition.
Please click on the page below to be directed to the digital edition:
$thumbnail$pagenum
$link$pagenum
Your form submission was a success. You will be contacted by Washington Gas with follow-up information regarding your request.
This process might take longer please wait
