Should ivy be pulled off a tree?
Yes, and NC State Extension says to start at the ground: cut the English ivy stems there, then remove the vines from the tree as high as can easily be reached. The vines left higher up die and eventually fall off, so the crown clears itself instead of being pulled down.
Ivy shades a tree out and weighs it down
English ivy, left unchecked, climbs and kills trees, NC State Extension says; the NC Forest Service describes vines enveloping the branches and cutting off photosynthesis until the tree dies. Two county Extension centers say the added weight breaks limbs more readily, and the Forest Service says trees heavily draped with vines are "more susceptible to blowing over during storms."
Cut low and carefully, and mind the sap
A caption in NC State Extension's ivy publication warns to "be careful not to damage your trees" when cutting stems that climb them. For vines already in the canopy, the NC Forest Service says to "cut each stem as close to the ground as possible," cut again a little higher, and remove the piece between.
The sap is a hazard too: the Plant Toolbox says contact with it causes severe skin irritation, and Madison County's Extension center says to protect skin and wash it well afterward.
A bare trunk can be read again
Union County's Extension center says ivy obscures a tree's trunk and branches, so a structural defect stays out of sight until it is too late. A cleared trunk can be looked over against the hazard tree field guide, and a mushroom at the base gets its reading on mushrooms at the base of a tree.
Union County's center says ivy in the canopy kills entire large limbs by shading out the tree's own foliage, and for that deadwood, tree trimming and pruning takes over.
What did the trunk show once the ivy died back?
Questions people actually ask.
How fast do the crown's vines die once cut?
Months for the leaves, years for the wood, by Union County's Extension center: the vine's leaves brown and drop within a couple of months, and the dried vines turn brittle and fall over a couple of years.
Why not pull the vines down from the crown?
Union County's Extension center says ripping the vines out from the ground will probably snap off tree limbs unless they have dried and gone brittle, and even then it may be best to leave them.
Does ivy on the ground around a tree matter too?
Yes. Union County's Extension center says the ivy's ground carpet outcompetes the tree's roots for soil moisture and nutrients, yet another way English ivy kills trees.
Can ivy come back from what is left?
Many vines do: NC State Extension says they regrow from the roots, so the spot needs regular visits, and the NC Forest Service says pulled pieces left on the ground can keep growing.