300 



ROOT DIAMETER INSIDE BARK, millimeters 



Figure 1 7. — Tensile resistance of roots of Rocky Mountain Douglas- 

 fir as a function of age and size of root (from Burroughs and 

 Thomas 1977). 



Burroughs and Thomas (1977) also determined the tensile 

 resistance of roots for various size classes and various elapsed 

 times after cutting as shown in figure 1 7. Tensile resistance was 

 only measured for Douglas-fir roots in the size interval to 0.4 

 inch (0 to 1 cm). The tensile strength computed as tensile 

 resistance divided by root cross sectional area (as distinct from 

 resistance plotted in fig. 1 7) decreased from 3,280 lb in' (22 600 

 kPa) for a 0.08-inch (2 mm) diameter root to 2,1 50 lb in' (1 4 800 

 kPa) for a 0.4-inch (10 mm) root. The average tensile strength 

 for roots in this size interval, that is, to 0.4 inch (0 to 1 cm), was 

 approximately 2,720 lb in' (19 800 kPa). This variation in root 

 tensile strength with root size has been observed by other 

 investigators as well (Gray 1978; Wu 1976). This type of in- 

 formation makes it possible to compute the average tensile 

 strength per unit area of soil for a Douglas-fir root-soil system. 

 By using a simple root or fiber reinforcement model described in 

 the next section, these data can in turn be translated into a 

 shear strength increase or pseudo "root cohesion." 



Unfortunately, Burroughs and Thomas (1977) only looked at 

 the concentration of lateral roots. It is the vertical roots (tap and 

 sinker roots) that will contribute most to sliding resistance of 

 soils on steep, inclined slopes. Studies by Curtis (1964) on 

 ponderosa pine indicate deep penetration of tap roots and 

 sinker roots in a cylindrical zone around each tree in granitic 

 soils of the batholith. The extent of vertical root penetration and 

 concentration across the slope surface as a whole is not known 

 precisely. 



One way of estimating this concentration is simply to use root 

 area ratios that can be calculated from root distributions mea- 

 sured by Burroughs and Thomas (1977). Those authors cau- 

 tion, however, that their measurements were restricted to lateral 

 roots of Douglas-fir crossing a vertical plane midway between 

 trees. On the other hand, they also state that the finer roots (0.4 

 inch [1 cm] and smaller) are found throughout the rooting zone 

 of each tree root system. Furthermore, they note that these 

 same roots "... will penetrate a shallow soil overlying a weath- 

 ered or well-fractured bedrock or glacial till to anchor the soil- 

 root mass." 



More direct data on critical root distribution in granitic soils 

 can be obtained from the landslide study by Megahan and 

 others (1978). The size and frequency of roots exposed in a 

 slide shear plane in a forested slope are summarized in figure 

 18. The dominant size class (modal value) of roots in the shear 

 plane was 0.20 to 0.40 inches (0.51 to 1 .02 cm) in diameter. The 

 dominant (modal value) root density class was 6.5 to 1 0.5 roots 

 per square foot (70 to 113 roots per square meter). This in- 

 formation yields root area ratios ranging from 0.14 to 0,93 

 percent using data from the modal class. Burroughs and Tho- 

 mas (1977) found the same class to be dominant for Rocky 

 f^ountain Douglas-fir in their study, i,e., most of the roots they 

 counted fell into the to 0,4 inch (0 to 1 cm) size range. 



30 - 



Dominant Size I centimeters ) Density (number / sq m ) 



Figure 1 8, — Landslide occurrence by sheared roots at slide site (from 

 Megahan and others 1978), 



In contrast, the root area ratio (0,045 percent) calculated from 

 their study for this size class was much lower than those com- 

 puted from the data of Megahan and others (1978), The differ- 

 ence is most likely caused by the fact that Burroughs and 

 Thomas (1 977) counted only Douglas-fir roots in vertical planes 

 midway between tree stumps, whereas in the other study all 

 roots were counted in shear planes parallel to the slope, regard- 

 less of species, Wu (1976) measured root area ratios ranging 

 from 0,05 to 0,17 and averaging 0,08 percent in his study of 

 vegetative influence on landslide occurrence in Southeast Alas- 

 ka, His root area ratios were measured at the contact between 

 the B and C soil horizons, in soils developed on glacial till slopes 

 supporting a spruce-hemlock forest, 



Kozlowski (1971) observed that root structure as well as 

 depth and rate of root growth are markedly influenced by the 

 rooting medium or environment. The water-holding capacity or 

 water availability in a soil is particularly important in this regard. 

 Roots tend to avoid regions of high moisture stress and invade 

 or permeate moist zones. In the case of shallow, coarse- 

 textured soils of the Idaho batholith, the most favorable region 

 for roots to exploit from a moisture standpoint is the zone close 

 to the contact between the soil and the underlying fractured, 

 disintegrated bedrock. In other words, one should expect a fairly 

 high concentration of vertical or sinker roots across this contact. 



This expectation is partly confirmed by McMinn (1963) in his 

 extensive study of the characteristics of Douglas-fir root sys- 

 tems. Hydraulic excavation of root systems in Douglas-fir 

 stands revealed a pronounced tendency towards steeply in- 

 clined or downward root penetration, not only from the central, 

 deep root system, but also from semivertical roots (sinkers) 

 from laterals. This trend was most pronounced in older tree 

 stands. 



9 



