Arboriculture & Urban Forestry 40(4): July 2014 —higher fine-root mortality concurrent with increased root growth (Meier and Leuschner 2008). In wet soils, the growth of roots tends to be con- fined towards the soil surface. In dry soils, root growth can be shiſted downward due to water depletion in surface soils (Torreano and Morris 1998). When urban soils limit rooting depth, the ability of tree root systems to respond to periods of drought and high soil moisture may be very limited. Flooding of soil usually leads to greatly reduced root growth, and death of many of the fine absorbing roots. The small root systems of flooded trees reflect the combined effect of reduction in root initiation and reduced growth of existing roots, as well as decay of the origi- nal root system. Because root growth is usually decreased more than shoot growth by high soil moisture, drought tolerance of flooded trees is reduced after the flood waters recede. This change reflects the inability of the small root systems to supply enough water to meet the transpirational requirements of the crown (Kozlowski 1985). Responses of tree species to flooding vary widely (White 1973; Bell and Johnson 1974; Whitlow and Harris 1979). Tolerance can vary from only a few hours to many days or weeks, depending on the species, the organs directly affected, the stage of development, and external conditions, such as temperature. Roots are oſten more susceptible to oxygen deficiency than shoots (Vartapetian and Jackson 1997). Broadleaved trees as a group are much more flood-tolerant than conifers. Older trees usually tolerate flooding better than seed- lings or saplings. Flooding during the dormant season is much less harmful than flooding during the growing season (Heinicke 1932). The greater injury and growth reduction by flooding dur- ing the growing season are associated with high oxygen requirements of growing roots with high respiration rates (Yelenosky 1963; Koslowski 1985). Aeration Respiration by plant roots and other soil organisms consumes oxygen and produces carbon dioxide. In unsaturated soils, the soil air connects directly with the aboveground atmosphere, but diffusion of gasses through the soil is slowed by water and soil particles. Oxygen concentrations decline and carbon dioxide concentrations increase with depth due to the oxy- 195 gen demands of the roots, the soil fauna, fungi, and microbes. Oxygen deficiency in roots will be more likely to occur in warm soils than in cooler soils when reduced respiration is more balanced with diffusion rates (Yelenosky 1963; Armstrong and Drew 2002). For most species, approximately 10%–12% oxy- gen in the soil atmosphere is needed for adequate root growth (Stolzy and Letey 1964; Tackett and Pearson 1964; Stolzy 1974; Valoras et al. 1964; Gilman et al. 1987; Mukhtar et al. 1996), and growth may cease at 5% oxygen (Stolzy 1974). Soil carbon dioxide concentration can be damaging to roots when it reaches 0.6% (Gaertig et al. 2002). For most species, root growth is reduced or stopped when the oxygen diffusion rate (ODR) drops below 0.2 µg/cm2 /min. Most plants are severely stressed between 0.2 and 0.4 µg/cm2/min. Above 0.4 µg/cm2 /min, plants grow normally (Stolzey and Letey 1964; Valoras et al. 1964; Lunt et al. 1973; Stolzy 1974; Erickson 1982; Blackwell and Wells 1983). Redox potential can also be used as a mea- sure of the oxygen status of the soil. Soil redox potentials of 400–700 mV are generally consid- ered well aerated. Root growth of most species is stopped at a soil redox potential of 350 mV, though roots of more water-tolerant species (e.g., Taxo- dium distichum) are able to grow until the redox potential reaches 200 mV (Carter and Rouge 1986; Pezeshki 1991; Stepniewski et al. 1991). Soil aeration is impacted by urban landscape features. In undisturbed, well-drained soil, oxygen and carbon dioxide contents can be near atmo- spheric levels close to the soil surface, decreasing most rapidly in the first 30 cm (Yelenosky 1963; Brady and Weil 1996). When not paved, vegetated and nonvegetated urban sites can be as well-aerated as forest stands (Gaertig et al. 2002). However, if topsoils are sealed or compacted, gas exchange between the soil and the atmosphere is inter- rupted (Gaertig et al. 2002). Oxygen content was reduced to 14.5% and carbon dioxide content was increased to 6% at 15 cm depth under an unpaved parking lot. The same levels were not reached until 90 cm depth in the adjacent undisturbed forest soil (Yelenosky 1963). In another study, there were minimal differences in soil oxygen between pavement and turf in the top 45 cm (Hodge and Boswell 1993). However, soil oxygen measure- ments were made only 75 cm from the edge of the ©2014 International Society of Arboriculture
July 2014
| 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
