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• Rug 05 99 01:35p Nathanei1 Sperry 6 Teresa (541) 461 -0091 p .1
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<br /> c y From: Torrespt©aoi.com
<br /> Sent: Thursday, July 22, 1999 6:48 PM
<br /> To nsperry@uswest.net
<br /> Subject: Re: doug fir water tolerance reference
<br /> Soil atmosphere
<br /> To grow and develop, roots require a minimum amount of oxygen for
<br /> aerobic respiration (Wedenroth, 1993 1107]. The diffusion of oxygen into the
<br /> soil, and of carbon dioxide and other gaseous products of metabolism out of
<br /> the soil, steadily decline as bulk density and soil moisture content
<br /> increase. Oxygen depletion in the soil, which is considered to be any value
<br /> less than 15%, promotes anaerobic respiration In roots. In this condition,
<br /> roots are unable to absorb water and nutrients, and cease to elongate
<br /> (Greenly, 1995 #1011 The elevation of carbon dioxide pressure can change the
<br /> internal pH of cells to toxic levels. In addition to compaction, back - filling
<br /> around trees can have this effect. Increasing the depth of fill material can
<br /> reduce the oxygen content below that needed to oxidize organic compounds,
<br /> which results in the build up of toxic aldehydes [Roark, 1982 #79].
<br /> A slow rate of oxygen diffusion to roots causes a stressful condition
<br /> that predisposes roots to successful pathogen challenges [MacDonald, 1993
<br /> #100]. Physiologic effects of poor aeration and impedance are exacerbated by
<br /> these root pathogens. This has been demonstrated with some agricultural
<br /> plants [Grew, 1980 #78]. As root metabolism continues by fermentation some of
<br /> the nitrogen - containing hydrocarbons that result stimulate pathogen activity
<br /> tAllmaras, 1988 #73]. Increased impedance brings about more ethylene
<br /> synthesis, radial swelling of cortical cells, and accumulation of osmotic
<br /> solutes in the root apices [Atwell, 1993 #108]. These changes make a root
<br /> more susceptible to pathogens, including weak pathogens like Rhizoctonia and
<br /> Pythium, especially when the protective tissues are breached by abrasion, and
<br /> exudates are evaitabie to activate spores. Strained roots become more
<br /> susceptible to exopathogens as welt. When neither a parasitic pathogen, nor
<br /> physiogenic causes can be found for a diseased condition. an exopathogen
<br /> should be suspected. An exopathogen does not parasitize the suscept, but
<br /> creates a diseased condition in the roots by toxins released into the
<br /> rhizosphere (Woltz, 1978176].
<br /> Development, including agriculture, can impair the deepest, most
<br /> fertile soils. Compaction can induce pans (compacted layers) by collapsing
<br /> the macropores. A hard surface layer can develop by the cementing of fine
<br /> organic matter (Raney, 1995 031]. in addition to foot traffic, the impact of
<br /> rain causes crusting. In a study of campsites, Legg found that once the
<br /> surface litter was depleted, such a surface formed quickly. This reduced
<br /> infiltration of water and exchange of gases (Legg, 1977 #11]. Soil strength,
<br /> soil atmosphere, soil moisture, and the genetic control of the root system
<br /> are all important parameters for root system characteristics, but there are
<br /> others. Sod temperature, pH, nutrient status, mycortfiizae, photosynthesis,
<br /> and reeproductive growth stage should also be considered [McMichael, 1993
<br /> #109].
<br /> Fluctuations in the water table can result in periodic dieback of
<br /> roots if they are killed by waterlogging when the soil water rises above
<br /> field capacity for too long. Gadgil found that the deliberate waterlogging of
<br /> Douglas-fir seedlings for two weeks caused young root tips to change In color
<br /> from a healthy white, to a purplish black. After four weeks the root tips
<br /> were black, and the mycorrhizae were brown, the color changes coinciding with
<br /> the gradual death of the fungal mantle and the root cortex. (Like other
<br /> members of the family Pinaceae. Douglas -fir roots are ectomycorrhizal.)
<br /> Waterlogging the soil of Douglas - fir seedlings for two weeks adversely
<br /> affected root physiology, but even after sixteen weeks, the roots tested by
<br /> Gadgil were not entirely dead [ Gadgil. 1971 *5]. Some air exchange can occur
<br />
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