Stumps discovered at the Liverpool East site in Indiana (Schneider and Hansel 1990; Panyushkina et al, accepted) appear to have been snapped off (sheared) at a fairly uniform height about 30-60 cm above the surface in which they are rooted.  A high energy agent is required, even if the trees were already dead.  Below is a collection of images of modern trees that might be possible analogs.
Spruce stumps, Liverpool, Indiana.
[Photo credit: I.P.Panyushkina & S.W. Leavitt]
Wind
glacier-modern
tsunami
snow avalanche
volcanic eruption
beavers
humans
glacier-ancient
(Schneider, A.F., Hansel, A.K., 1990. Evidence for post-Two Creeks age of the type Calumet shoreline of glacial Lake Chicago. Geological Society of America Special Paper 251, pp. 1-8. Panyushkina, I.P., Leavitt, S.W., Thompson, T.A., Schneider, A.F., Lange, T., accepted. Environment and paleoecology of a 12 ka mid-North American Younger Dryas forest chronicled in tree rings, Quaternary Research Sept. 2008)
Ice floes-
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