In article ,
David Hill wrote:

My question: is there any truth in the old saw that ivy, once securely
established on a tree, will become genuinely parasitic, and will draw
sustenance from the tree itself?

None at all. The greatest danger is that the ivy will increase windage
on the tree and may cause its downfall during gales, esp. following wet
weather.

On the other hand tie ivy was providing shelter and food for a number of
birds and insects
€Holly blue caterpillar feeds on the flowers buds.
€Wasps, hornets, hoverflies, bumblebees, red admirals, small
tortoiseshells and peacock butterflies, and other late-flying insects,
drink the nectar.
€Many birds, such as blackbirds and thrushes, eat the berries.

Sure David - I am acutely aware of the beneficial effects. However we're
talking "balance of nature" here, and I want to restore some of the
balance lost when agricultural workers left the land. [1]

John

[1] Most of them for the cities, and the rest, for their tractor cabs! I
don't begrudge them their tractors one whit, in particular as their
numbers are a tiny fraction of what they used to be. However there are a
million small jobs that used to be done on the land which now never even
get considered: keeping the ivy off major trees is one such. And there's
***plenty*** of ivy elsewhere, by golly!