We have an area in a relatively sparse shrubland that Oxalis pes caprae, Bermuda buttercup, is invading. Shrubs like  Artemisia californica, CA sagebrush, appear to me to be resistant to oxalis’ allelopathic effects.

Does anyone know anything about above?

I would be very dubious that this is true.  How long has it been under observation?  The sagebrush grows in poor, often thin, soils, which slows oxalis down considerably.  It needs soil of a little depth; it struggles in thin soils.

Jake Sigg