What is the difference between scrub and shrub?

Shrubs are woody plants that always grow in shrub form, usually less than 6 meters tall and 3 in diameter at breast height, while scrubs are trees that are in "shrubby" form due to stunting, usually as a result of poor soil conditions or soil saturation. Some example communties include "scrub/shrub" wetlands, barrens and savanahs.