Arboriculture & Urban Forestry 34(4): July 2008 nus occidentalis × P. orientalis) were severed by simulated dig- ging and harvested 4 years later. There was no decay or discoloration in the trunk from root severing and no decay more than 7 cm (2.8 in) from the cut (Santamour 1985). Santamour (1985) also reported differences between species in their ability to resist trunk decay and discoloration after root severance. Four years after severing roots within 0.5m(1.7 ft) of the trunk, there was no discoloration or decay in trunk tissues in red maples (Acer rubrum) and 6 cm (2.4 in) maximum in the roots. Discoloration and decay were present in trunk tissues of two of ten black oaks (Quercus velutina) and four of ten white oaks (Q. alba) after similar root severance. All of these indi- vidual trees were weak Wall 2 compartmentalizers, but not all weak Wall 2 compartmentalizers developed discoloration and decay in trunk tissues. The objective of this study was to investigate whether sever- ing deciduous hardwood tree roots of any size or at distance from the trunk could lead to defect development that could threaten the health or stability of the tree. METHODS The trees available for this study were planted as blocks of seedlings in 1956 on an Askum silty clay loam soil at the Morton Arboretum in northern Illinois, U.S. Supplemental irrigation was not provided at any time after the plot was established. Four species of trees included in the study were honeylocust (Gledit- sia triacanthos var. inermis), pin oak (Quercus palustris), tulip- tree (Liriodendron tulipifera), and green ash (Fraxinus pennsyl- vanica). The individual trees selected for this study were the more vigorous individuals in a moderately crowded block (4.5 m [14.9 ft] centers). All exhibited typical twig growth for healthy trees of their species and age. Average diameter at breast height of trees used in the study was 30, 30.7, 35.4, and 34.3 cm (12, 12.3, 14.2, and 13.7 in), respectively. Roots were severed at four distances from the trunk in No- vember 1998: on the root flare where the root becomes visible at the soil surface and 1, 2, or 3 m (3.3, 6.6, and 9.9 ft) from the trunk. Roots were traced to the appropriate distance without excavating using a tile probe. All roots were no more than 30 cm (12 in) deep. There were five roots severed at each distance. In most cases, different roots were severed at each of the four distances on a single tree. Occasionally, when not enough roots could be found on a single tree, a second nearby tree had to be used to complete the set, most often for the root flare cut. No roots could be successfully traced to 3 m (9.9 ft) on pin oaks because of the small diameter of the roots of this species. A chain saw with carbide teeth was used to cut the flare root. A stump grinder was used at the other three distances. The diameter of the severed root end was measured from the face of the cut. Two perpendicular diameter measurements were aver- aged. To facilitate finding the roots later, locations of the cut roots were mapped and a steel nail was buried approximately 10 cm (4 in) below grade to mark the exact location of the severed root end. Root wounds were exposed to natural decay fungi present in the soil, but no inoculations were performed. After 5 years, the severed ends of the roots were relocated using the maps and a metal detector. All discolored and decayed portions of the cut roots were excavated with an air tool and removed. The excavated roots were cut into sections. The first 10 cm (4 in) was split longitudinally and sanded to measure the extent of discoloration and decay. If discoloration and decay 261 extended beyond the first 10 cm (4 in), additional 5 cm (2 in) sections were cut and sanded until discoloration and decay were no longer present in the cross-section. The cross-section of the stump (approximately 10 to 15 cm [4 to 6 in] above grade) was examined when the tree was cut down. If any discoloration was present, additional cross-sections of the trunk were cut at 30 cm (12 in) intervals until discoloration was no longer visible. Stumps were excavated and a longitudinal section of the severed flare root was removed with a chain saw and sanded smooth. Discoloration was quantified by measuring the proximal ex- tent of visibly discolored wood from the severed root end. Decay in living trees has been described as a softening or weakening of the woody xylem tissues (Hartman et al. 2000). Methods used in similar situations have involved noting color change (Whitney 1997) along with simple techniques to estimate mechanical re- sistance such as probing with a sharp object (Tokuda et al. 2007) or drilling (Costello and Quarles 1999; Fraedrich and Smiley 2002). A similar method of detecting decayed wood in the dis- sected roots was devised using change in color and mechanical resistance of the wood tissue on the longitudinal sections of roots. The sample was placed on a Homs Model 40 spring scale (Douglas Homs Corp., Belmont, CA) and a blunt dissecting needle was pressed into the wood until the scale read 1.8 kg (4 lbs) to develop consistent pressure each time. In unaffected and discolored wood, the depression made by the blunt tip was barely detectable. At the border between discolored and decayed wood (identifiable by color change), the tip abruptly began to penetrate deeper. When the tip penetrated more than 1 mm (0.04 in), the wood was considered decayed. The transition zone was always less than 10 mm (0.4 in) and often less than 5 mm (0.2 in). The extent of decayed wood from the severed end was recorded. If some wood had completely decayed away, the position of the original cut was estimated as accurately as possible from the new sound wood formed around the decayed wood. This method was much more precise than 5 cm (2 in) resolution obtained by methods using culture and identification of decay fungi (Lind- berg and Johansson 1992; Garboletto et al. 1997). Each species was considered a separate experiment. One-way analysis of variance (P 0.05), with separation of means by the Student-Newman-Keuls Method (SigmaStat 3.0; SPSS Science, Chicago, IL) was used to compare extent of discoloration and decay among root-severing distances. RESULTS AND DISCUSSION After 5 years, the furthest extent of decay development ranged between 4.5 cm (1.8 in) in green ash and 10.8 cm (4.3 in) in honeylocust. The furthest extent of discoloration development also varied from 6.3 cm (2.5 in) in green ash to 77.1 cm (30.8 in) in honeylocust. Typical examples are shown in Figure 1. The cut location with the greatest decay or discoloration varied widely among species (Table 1). The annual rate of decay development was very slow at 0.9 to 2.2 cm/yr–1 (0.4 to 0.9 in/yr–1), compared with rates of 10 to 53 cm/yr–1 (4 to 21.2 in/yr–1) reported after inoculation of aggres- sive root rot fungi in conifers (White and Kile 1993; Morrison and Redfern 1994; Garboletto et al. 1997; Piri 1998). Santamour (1985) observed an even slower maximum rate of decay pro- gression of 1.5 cm/yr–1 (0.6 in/yr–1) resulting from natural in- fection in response to severing roots of hardwoods 0.5 m (1.7 ft) from the trunk and also with variation in species response. ©2008 International Society of Arboriculture
July 2008
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