Here is an article I found online that explains a lot.
Kristy :)

Here is a link that might be useful: About vines

Here is a useful site to check out: www.noivyleague.com
The Nature Conservancy also has useful management articles on their web site about many invasive plants including English Ivy.

The article mentioned above covers some of the basics. English Ivy is bad stuff for trees. If the ivy is thick enough the damage could be happenning at all parts of the tree. English Ivy in the canopy can shade out your trees. The extra weight the vines add can bring down branches and even whole trees, especially if loaded with ice. The really thick vines create micro climates underneath that help rot the trees. Like all weeds, english ivy also competes for light, air, space and nutrition.
Some vines, like oriental bittersweet, can spiral around the tree and choke it. I have cut large bittersweet vines off of trees and seen spiral shaped indentations in the trunks.

Some vines don't seem to do much harm. I haven't seen virginia creeper hurting my trees much. I haven't seen poison ivy do much damage to trees either which is just as well since sawing at it is likely to make you feel miserable later. The stuff will grow back from below where you cut so ripping it out altogether is best but even cutting several years of growth away from your trees will create a very fast and very dramatic improvement. Good luck.

Here in Washington State Ivy is on the Noxious Weed list so the State, City parks and Botinacial gardens are removing run away ivy along road ways and native areas. The Ivy can get vary big and weights the tree down, so when we get High winds it creats a "sail" out of the tree canopy. When trees fall people lose power and damage to home and lives. Also the Ivy provides a freeway for pest, rats, racoons, ants many more. Here is a great web site. I think that Ivy should be only used in flower pots where it can be "Controled"! www.aphis.usda.gov/ppq/weeds
Good luck!

I don't disagree with the idea of removing non-native, overwhelming vines like English Ivy, Kudzu, Oriental Bittersweet, etc., but keep in mind that vines are an important part of native forests, at least in the eastern part of the country. Vines add a lot of bird habitat to a forest by creating a unique type of feeding substrate (vines along tree trunks and thick tangles of vines), flowers (trumpet creeper, cross vine, carolina jessamine, etc.) fruit (grapes, virginia creeper, poison ivy, etc), and supporting a lot of interesting insects which in turn support birds (look at the leaves of your local vines - those holes are made by live bird food). I think too many people think that all vines should be removed from a woodland to create a "healthier" woodland. Removing all of the vines creates a neater-looking woodland, but destroys important habitat that is a natural part of the environment. The earliest european explorers reported thick tangles of vines hanging from trees along the coast and along rivers, and the abundance of wild grapes was one of the attractive features of the new world that was reported back to Europe. It is unusual for a native vine to seriously harm a healthly tree. In some instances vines may either add too much weight, causing a tree to fall or excessively shade a tree, causing it to grow slowly or even die. In most cases, however, vines do not harm trees. Rather than try to kill all of your vines, try to appreciate them for what they are - an important part of the forest.

Its usually the added weight of the vines or their addition to the winter snow loads that eventually brings down trees.A small wood lot that existed between me and a newly constructed cabin had been heavily colonised by wild grape vines which had grown up with the trees for decades.Almost all the trees were hevily laden with their weight and many had been toppled.The resultant removal of the canopy alowed the establishment of invasive shrubs,honeysuckle and barberry,on the ground.Wanting to protect the remaning trees,and my buffer with my new neighbors,I cut thru the vines at grown level around the largest trees.The trees,mostly black cherries and sugar maples were probably about 70 ft tall and as old.The vines were thicker than an arm.I relise that this action could be considered destructive by some and went against my policy of not harming plants older than me,but it was carefully considered,and I had a fantastic scource of holliday decorating material for years.Be cause the vines no longer had the sunlight at ground level they never regrew,and in two years time,their dried remenents were shed from the trees.
I alway kill oreintal bittersweet,and english ivy on sight.Know thine enemy!

Some time ago a tree-killer without any roots-to-earth appeared in my Mango-tree. Looks to me like a cancer seeded by bird-excrements. Avoiding the use of chemicals the infected parts of the branches had to be cut down.
Photos can be seen in my album.

Here is a link that might be useful: JAPANESE DODDER, ka faag? (Thai), CONVOLVULACEAE JUSS (CUSCUTA JAPONICA)

growing up on a farm in mass when the red sox had a chance (1967)We had a pear tree at the end of a hay field with a concord grape growing in it. It was a bartlet pear by memory about 15" trunk. the tree must have been 30' tall and the grapes seemed to cover a lot of it. After the june hay cut we could use that field for base ball and the tree shaded home plate.a lot of kids were in that tree for a lot of years. We picked grapes in baskets and lowered them down for jelly only the brave would eat them out of hand. the pears were very good but the tree was so prolific that it gave a a home court advantage late when we would tell other neighborhood teams about the yellow jackets. if you feel a tickel on your neck don't move fast= strike one. lol