Collection location: Howarth Park, Santa Rosa, Sonoma Co., California, USA [Click for map]
Powdery pileus, browned bruises (although apparently not instant), slightly decurrent gills. Associated with oak and bay. 10 cm across. Solitary
R. brevipes is one of the sub-genus Compacta, which usually you can tell by the large size, the really stiff and hard stipe (stalk), and the real clue is the sub-gills between the main gills. In compacta in between pretty much every gill will be a shorter sub-gill.
In these photos the gills are all even and go from the stipe to the edge of the cap. You can see what I mean in the second photo of this one:
Or some other from the compacta group:
Where if you look close you can see next to each gill that runs from the edge of the cap to the stipe is a short gill that only goes half way to the stipe, and often between those are other really short gills.
This happens in other Russulas not in compacta but not to such an extent, the short gills will be the exceptions, and not always there. Or like in this one, not there at all.
I think with the smaller size here, and the white cap, and also these gills seem to be starting to turn yellow, this points to either R. albida or R. albidula, depending if it tastes acrid or mild. Also there are some other white capped Russulas so these are just a guess.