In article ,
Jeff Layman wrote:

On 05/05/2014 22:49, David Hill wrote:
....
You might find this item of interest
http://www.arborecology.co.uk/article_forf.htm

An interesting article, although it doesn't mention ivy on conifers.

Yes - interesting, and provides answers (presumably accurate ones) to
all the questions I had (as well as the one I actually asked, which was:
does mature ivy morph into parasitic growth on trees? Answer: No.).

Jeff also said:

Although I would never expect ivy to act like a strangler fig, I
sometimes wonder if excessive coverage inhibits tree growth. ...

It does. I have dealt with ivy in a very large hawthorn hedge, where it
had been allowed to grow unrestricted for many, many years. Some of the
ivy stems were 4 to 6 inches thick; where they had twisted round the
hawthorn trunks, the trees were dying; further, the thick masses of ivy
"undergrowth" in the hedge tops had caused the host wood to rot, through
debris accruing year, on year, on year, getting wet, and rotting down: a
hedge bottom in the hedge top, in fact.

I would never eradicate ivy (you couldn't, anyway!), because of the good
that it does (listed in that article); but it's an awful bully, and
needs keeping strictly in check. In my humble opinion.

J.