On fallen, decaying braches of Populus tremula. Up to 0,7 mm tall; stalk K+ reddish brown; spores 1-septate, mostly 12-13x4-5 um.
I used this key: https://www.sekj.org/PDF/anbf33/anbf33-205.pdf
On dead alder (I think) in a snowmelt area. Yuba pass, 2045MASL