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The two species are illustrated here: http://www.elisan...a/strongy/

I have collected S. pictipes a few times, always on the trunk of a recently fallen large aspen. The flies look and behave like clusiids; males run rather slowly around and flash their wings to flies of similar size. On a suitable tree males can be numerous.

S. ustulata is definitely not an aspen-only species. It is locally common in humid, rich forests, especially along forest streams (exactly as you describe). Unlike S. pictipes, I have never found this species on aspen trunks. I've often caught it on the leaves of Prunus padus.