To investigate the vulnerability of the roots by exogenetic forces

the anti-fracture force and the anti-tension force of lateral-root branches and adjacent upper straight roots diameters of 1～4 mm of 4 shrub species at the age of 3~4 years were measured with the testing machine in vigorous growing period. The 4 species were Caragana Korshinskii Kom.

Salix psammophila C. Wang et Chang Y. Yang

Hippophae rhamnoides Linn. and Artemisia sphaerocephala Krasch. The results showed that the anti-fracture force of lateral-root branches was higher than anti-tension force for C. Korshinskii and S. psammophila

while the anti-fracture force of lateral-root branches were lower than the anti-tension force for H. rhamnides and A. Sphaerphala. The same tendency was observed for the adjacent upper straight roots of 4 plants. The anti-tension strength and anti-fracture strength of lateral-root branches were as follows: C. Korshinskii were (23.70±3.97＜33.66±7.74) MPa

S. psammophila were (14.86±1.28＜17.31±1.91) MPa

H. rhamnides were (10.61±2.40＞3.97±1.23) MPa

and A. Sphaerphala were (5.07±1.25＞2.18±0.39) MPa. The anti-tension strength and anti-fracture strength of adjacent upper straight roots were as follows: C. Korshinskii were 28.02±4.40＜47.06±4.41) MPa

S. psammophila were (20.33±1.76＜27.54±3.82) MPa

H. rhamnides were (15.86±3.90＞8.75±1.71) MPa

and A. Sphaerphala were (8.80±1.74＞6.15±1.01) MPa. For C. Korshinskii and S. psammophila

the roots were be liable to be damaged by anti-tension force in vigorous growing period. For H. rhamnides and A. Sphaerphala

the roots were liable to be damaged by anti-fracture force. The anti-tension strength and anti-fracture strength of lateral-root branches were lesser than those of adjacent upper straight roots with the same diameter. The anti-tension strength and anti-fracture strength between lateral-root branches and adjacent upper straight roots for the 4 species followed the sequence of C. Korshinski＞S. psammophila＞H. rhamnides＞A. Sphaerphala. The roots with 1 to 1.5 mm played a leading role in the soil-fixing of 4 kinds of plants.