150 Day et al.: Contemporary Concepts of Root System Architecture of Urban Trees and Pallardy 1997). Anatomically, roots can be fundamentally classified as woody or nonwoody (e.g., Lyford and Wilson 1964). Woody roots are those that have undergone secondary growth, resulting in rigid structure and perennial lifespan. Functionally, these roots are often referred to as structural roots (see Sutton and Tinus 1983), acknowledging their role in anchoring the tree and creating a framework for the root system. Typically, a tree has 5–15 (or more) primary structural roots that emanate from the root collar and descend obliquely into the soil before becoming hori- zontal within a short distance of the trunk, although the pattern of root development can vary considerably. The area within 1–2 m of the trunk on larger trees is frequently referred to as the zone of rapid taper because structural roots found there often exhibit considerable secondary thickening not present on roots farther from the trunk (see Wilson 1964). Wilson (1964) additionally re- views the development of this zone and its relation to mechanical stability. Near the trunk juncture, structural roots on large trees may become thickened eccentrically in the vertical plane and are thus termed buttress roots, reflecting their shape and stabilization function (Sutton and Tinus 1983). These roots may have smaller diameter vessels than those found in the more rope-like roots found farther from the trunk (Wilson 1964). Although reach- ing phenomenal proportions on tree species in tropical forests, buttress roots are more modest on temperate tree species. The presence of pronounced buttress roots has been associated with soils that offer poor anchorage and trees that lack tap roots (see Henwood 1973), but other studies have found taproots on both buttressed and unbuttressed tropical trees (Crook et al. 1997). It is generally believed that the eccentric shape of buttress roots more effectively distributes mechanical stress on the root system [see Mattheck (1991) for a theoretical discussion; and Clair et al. (2003) for an empirical study of buttress roots and mechanics] and serve both tension and compression roles in stabilization (Crock- ett et al.1997). In some tree species, horizontal structural roots near the trunk produce sinker roots that plunge vertically into the soil, providing supplemental anchorage (Ghani et al. 2009). Beyond the zone of rapid taper emanates a framework of woody structural roots that provide additional anchorage and serve as conduits for long distance transport of water, nutrients, and metabolites. The size of these roots may be influenced by mechanical stresses, with more large roots forming in the wind- ward and leeward directions in trees subjected to winds from one direction (Stokes et al. 1995). Tree stability in urban settings is critically important. In Singapore, for example, 20% of tree fail- ures have been attributed to uprooting (Rahjardo et al. 2009). Limited information is available about how urbanized sites af- fect root anchorage, although it can be expected that whenever root architecture is altered, such as by an urban growing envi- ronment, there is the possibility that tree anchorage could be af- fected. Physically confined planting holes must necessarily lim- it the development of buttress roots, for example. In addition, the wide variety of specialized soil mixes used in urban settings undoubtedly have different shear strengths, further altering the behavior of root systems as tree anchors. For example, Rahardjo (2009) found that an 80:20 mix of soil and granite chips, akin to a structural soil, enhanced tree resistance to uprooting. Although structural roots comprise most of the root biomass, they account for a small percentage of total root length and root surface area. Root surface area is instead dominated by an extensive net- work of “nonwoody” roots, so called because they have not un- ©2010 International Society of Arboriculture dergone secondary growth, proliferating from the structural root framework. Functionally, these roots are often referred to as fine or absorbing roots, acknowledging their primary role in water and nutrient uptake. These roots are generally small in diame- ter (<2 mm), have high metabolic rates, and have a lifespan that ranges from a few days to weeks (Black et al. 1998; Pregitzer et al. 1998; Pregitzer et al. 2002). In addition to uptake, nonwoody roots are the primary location of root hormone synthesis, nutrient assimilation, root exudation, and symbiosis with soil microorgan- isms (Smith 1976; Marschner 1996; Guo et al. 2008). Among these fine roots, function is variable and is often determined by position on the root system hierarchy (Pregitzer 2002; Pregitzer et al. 2002; Guo et al. 2008). First-order roots (the ultimate root tip) are the most likely point for mycorrhizal colonization and consistently have higher nitrogen (N) levels than higher order roots (Pregitzer et al. 2002). These fine roots require the least investment of carbon (C) to grow, but are the most metabolically costly for trees to maintain on a mass basis. Nonetheless, they provide the most plasticity for trees in responding to nutrient and water resources in the soil (Pregitzer et al. 2002). Despite their diminutive stature, fine roots can account for as much as 90% of total root system length (Roberts 1976). Indeed, first- order fine roots may have considerably greater root length den- sity than other fine roots (Wang et al. 2006). Some nonwoody roots eventually undergo secondary development to become woody, structural roots and contribute to the root system frame- work, but most perish and are replaced (Fahey and Hughes 1994). ROOT SYSTEM DEPTH AND SPREAD “Where are the roots?” is a fundamental question in arboriculture and urban forestry. Estimating root depth and spread is a prereq- uisite for many arboricultural practices, such as tree preservation, and guides a wide range of research decisions. Although advanc- es in remote detection technologies, such as ground-penetrating radar (e.g., Nadezhdina and Cermak 2003; Hirano et al. 2009) may enable accurate determination of root location in the future, rules of thumb are typically relied upon for estimating root extent and depth. Typical rules found in texts and educational materials estimate root spread as up to 3 × canopy spread (e.g., Elmendorf et al. 2005) or 1–1.5 × tree height (e.g., Mariotte, undated.) The exact origins of these rules are unclear or multiple, but may origi- nate from studies with young nursery trees (e.g., Gilman 1988) and from early studies at Harvard Forest on four Acer rubrum (red maple) trees (Wilson 1964), respectively. Tree protection zones for sensitive older specimens are prescribed as a ground radius of 0.18 m per cm of trunk diameter (Harris et al. 2004); presum- ably this is intended to encompass the vast majority of the root system. Depth, described less consistently in educational pub- lications, is sometimes vaguely described as being primarily or concentrated in the upper 0.3 m of soil, or as having the majority of fine roots in this region (Gilman 2003; Elmendorf et al. 2005). Root Depth Inconsistencies in descriptions of root depth may reflect vari- ability in soil profiles across landscapes (Coile 1937), as well as differences among tree species. Crow (2005) provides a concise review. There is certainly a tendency for roots to exploit upper soil regions (Wilson 1964; Crow 2005; Wang et al. 2006), and roots of deeply planted trees have been observed to quickly rise
July 2010
| Title Name |
Pages |
Delete |
Url |
| Empty |
AI Assistant
Ask anything about this document
AI is thinking…
Ai generated response may be inaccurate.
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
Enter a description for this bookmark
Your form submission was a success.
This process might take longer please wait
