Pacific N.W., BC to N. Calif, N. Idaho (PNW native)
"Casara buckthorn is a wide-spread but not abundant shrub found primarily in forested mountains. It has been listed as a browse species for mule deer in Oregon and for elk in northern Idaho. Casara buckthorn was found to be a winter browse species for mule deer in northwestern Oregon. Sixty-eight percent of available Casara buckthorn shrubs were browsed during the winter. In summer, however, Casara buckthorn was less desirable; only 27 percent of available Casara buckthorn shrubs were browsed. Other mammals that browse Casara buckthorn include the Olympic black bear, Oregon gray fox, raccoon, and ring-tailed cat. Casara buckthorn drupes are eaten by five species of birds including the Oregon ruffed grouse and band-tailed pigeon. Casara buckthorn is of no value as forage for livestock. How the purgative characteristics of Casara buckthorn bark and drupes affect wildlife are not known." 
"The Kootenai and Flathead tribes of western Montana used Casara buckthorn as a laxative, consuming it in the form of a tea brewed from the bark. These Indians believed that it would be a purgative when the bark was stripped downward. If stripped upward, the drug would act as an emetic. Casara buckthorn bark contains anthraquinare derivatives, tannin, resins, starch, glucose, and other compounds. When the bark is chewed, it tastes extremely bitter, and may temporarily numb the taste buds.
The flesh of some animals which have consumed the drupes is said to retain some of the purgative properties. The juice pressed from the berries is used to prepare a 'syrup of Casara buckthorn'. The bark and dried berries have been used as a source of yellow- and saffron-colored dyes. The berry juice. when combined with alum, produces a green dye once used by artists.
Apparently, if Casara buckthorn is handled for a long time, the laxative effects can even be transferred through the skin. For maximum effectiveness, bark collection is recommended from mid-April to the end of August, and bark should be stored as long as possible before being used." 
"The greatest known value of Casara buckthorn is its purgative properties. In a single year, five million pounds of dried Casara buckthorn bark from the Pacific Northwest was processed by pharmaceutical companies in the manufacture of laxatives" 
 Habeck, R. J. 1992. Frangula purshiana. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Available: https://www.fs.fed.us/database/feis/plants/shrub/frapur/all.html [2020, July 1].