196 Watson et al.: Management of Tree Root Systems in Urban and Suburban Settings pavement and oxygen could have diffused laterally from the nearby exposed soil. While it is commonly accepted that stone pavement with gaps allows for aeration of the soil, there was no difference in gas diffusivity between completely sealed surfaces (asphalt) and areas with flagstone or cobblestone with gaps in between (Weltecke and Gaertig 2012). A water table less than 50 cm deep can reduce oxygen below levels considered sufficient to sustain vigorous root growth to within 5 cm of the soil surface (Callebaut et al. 1982). Ele- vated berm soils can be more aerated than sur- rounding soils at grade (Handel et al. 1997). Measurement Assessment of soil oxygen can be helpful in choosing the appropriate plant (Carnell and Anderson 1986; Hodge and Knott 1993; Hodge et al. 1993) and has been related to fine-root development of trees (Watson 2006a). Fine-root density in soils, where rust was present on over 60% of the steel rods, was generally three times greater than in soils with less than 25% rust- ing. This method can provide an indication of soil aeration over a period of months and up to a depth of 60 cm without the use of expensive equipment. for the site, or under- standing whether site modifications, such as improved drainage, may be necessary. However, measuring oxygen levels in the soil can be chal- lenging: equipment can be expensive and suited primarily for research applications. Measurement at any moment in time may not reflect sustained conditions, and not all measurements provide the same information related to root growth. Oxygen content, expressed as a percentage, is the amount of oxygen in the soil gases (the aboveground atmosphere contains 21% oxygen). ODR measures the rate at which oxygen can move through the soil to replace oxygen that is used by the root. ODR can be a better indicator of soil aeration (i.e., oxygen availability to roots) than oxygen content because it is possible to have a high soil oxygen concentration, but very low diffusion rate (MacDonald et al. 1993). The oxy- gen concentration in the soil atmosphere may not vary substantially at monitoring sites over time, or in response to changes in soil moisture. In contrast, ODR is strongly influenced by soil moisture and bulk density. Oxygen concentra- tion was not consistently low enough to severely inhibit root function at sites where trees were declining. At the same time, ODR values within the root zones of declining trees were invariably in a range considered injurious to roots, while ODR values around vigorous trees were favor- ably high (Stolzy 1974; MacDonald et al. 1993). Rusting pattern on steel rods can be used to assess soil anaerobism over an extended period ©2014 International Society of Arboriculture Effect on Root Growth Growing root tips have high oxygen requirements, and fine-root density is oſten reduced when oxy- gen availability is low (Koslowski 1985; Gaertig et al. 2002; Weltecke and Gaertig 2012). In older parts of the root, the oxygen demand can be approxi- mately half that of the tip (Armstrong and Drew 2002). Root dysfunction as a result of inadequate oxygenation can modify plant growth and devel- opment through interference in water relations, mineral nutrition, and hormone balance (Kramer and Kozlowski 1979; Armstrong and Drew 2002). Species vary in their root system tolerance to low soil aeration. For example, loblolly pine (Pinus taeda) grew better at low aeration conditions (either high compaction or high water content) than pon- derosa pine (Pinus ponderosa var. scopulorum) or shortleaf pine (Pinus echinata) (Siegel-Issem et al. 2005). Lists of species’ tolerance to flooding, which reduces soil aeration, are available (White 1973; Bell and Johnson 1974; Whitlow and Harris 1979). In some trees, such as willow (Salix), alder (Alnus), poplar (Populus), tupelo (Nyssa), ash (Fraxinus), baldcypress (Taxodium), and birch (Betula), oxygen can move down to the roots internally through intercellular spaces. This oxygen- transporting tissue within roots is called aeren- chyma. It is not uncommon in the subapical parts of wetland plant roots for as much as 60% of the root volume to be gas space for diffusion of oxygen from the shoot (Drew 1997; Armstrong and Drew 2002). Enough oxygen can be transported so that some is released into the soil immediately surrounding the roots (Hook et al. 1971; Armstrong and Read 1972). Mechanical Resistance Bulk density is a measure of dry mass per unit volume and used to describe limits to root growth in compacted soil. Soil strength, expressed as penetra-
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