Roots usually keep feeding "dead" trees moisture because the roots aren't dead yet. One exception occurs whenever a tree is killed by multiple indirect lightning strikes. Then the roots are fried also, die immediately, and feed no moisture to the tree. So, a lightning-killed tree will dry out in a year because it was literally cooked. Branches from such a tree when cut down after standing dead for a year after being hit will practically shatter as they impact the ground. It's freakish, but it does occur.I believe it. The one I took down had 4 main trunks to it. 3 were pretty dead and dry. The 4th was so wet water came out of some of the splits while splitting.