I have seen the dead deer that tried living at far fewer then 40 over winter deer per square mile even when they had better browse then beech, birch and striped maple, so in know first hand that any report you read that nonsense in is full of bologna unless they were only evaluating the ability of those deer to survive during one winter of limited snow conditions before they were removed to better habitat or fewer deer.
Without a doubt you can find winter killed deer at DDs under 40 DPSM when weather conditions such as exceptionally deep snow or or a heavy ice crust on the snow prevents deer from accessing the available browse for extended periods. But the history of the herd in the NC counties shows the herd increased to over 40 DPSM after severe over browsing from 1929 to 1939.
Here is the link.
http://www.deerandforests.org/resour...ainability.pdf