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Root growth responses to defoliation were observed in the field with an improved root periscope technique, which is described. the grazing tolerant, Eurasian bunchgrass, Agropyron desertorum, was compared with the very similar but grazing sensitive, North American bunchgrass, A. spicatum. Root length growth of clipped A. desertorum was about 50% of that of intact plants, while root elongation of clipped A. spicatum continued relatively unabated during ninety days of regrowth following severe defoliation. The reduced root growth in A. desertorum was correlated with the allocation of relatively more resources to aboveground regrowth, thus aiding reestablishment of the root:shoot balance. This balance was apparent in cimilar root mortality patterns of clipped and control A. desertorum plants in the season following defoliation. In clipped A. spicatum, however, root mortality increased in the winter following the season in which the clipping was done and continued into the subsequent growing season. Reduction of root growth following defoliation appears to be an effective mechanism to aid reestablishment of the photosynthetic canopy and the root:shoot balance. As such it contributes to both herbivory tolerance and maintenance of competitive ability.