These were in dense mounds, about 4-6 inches high by 8-12 inches across. The branches were comprised of ovate leaves, rising along the stem. The branches were 1-2 mm across.
Yup. Climacium dendroides or C. americanum.
That looks very similar, but those tree mosses are reported in the far, northestern part of the US only, per USDA. These were mounded on the ground in Arkansas.
USDA data for mosses is severely lacking. C. americanum definitely occurs in AR.
Here is a link that might be useful: Specimen data for Climacium americanum
Very cool. I've been using MOBOT, but did not know you could search the specimens list in that way. Are there any keys online for the mosses, or good descriptions of the way families are divided, etc.?
Not that I know of. The Bryophyte Flora of North America is a work in progress and most treatments are not yet available online. The definitive work for the northeast seems to be Mosses of Eastern North America by Crum & Anderson.
I checked the prices - oh my. If anybody has the 2-volume set collecting dust, and wants to donate it, let me know.
Thanks for that ID lycopus.