It has always been my private conviction that any man who pits his intelligence against a fish and loses has it coming.
I am sure it depends on what species you are hanging from, but I think trees like Maples and similar species will be fine with just webbing straps. Even Pines and Spruces and Firs if you don't mind sap. As for prolonged, once in my Plant Ecology class we were surveying a plot of woods and during a lecture our professor found a forgotten chain link about 70% wrapped in bark. The tree was still alive! But I am sure that ability lies within a species ya know. In other words, I am willing to bet your trees will be just fine As long as your webbing strap isn't their for a really long time.
"In seed time learn, in harvest teach, in winter enjoy." -D'Signore's, Tide Mill Farm, Edmunds, Maine.
I think you have to take into account the type of tree and the bark type. Most oaks, pines, hickorys and hardwoods have pretty stout park and hanging would not affect the cambium layer. Trees like aspen, poplar, birch and other soft woods have a thinner softer bark that would be affected. I hung my hammock on pine trees with my one inch strap and noticed not damage at all. One of my scouts hung his with the supplied rope on an aspen and I could see damage to the bark and massive compression on the tree and he was less than a 100lbs. If you hang on soft barked trees use a wider strap or do what arborists do and run it through a rubber hose. I wouldn't worry about using 1" straps on the ponderosa's I have cause the bark is usually 3" thick but would never hang on poplars or aspens with them.
In Indiana,,and elsewhere as well,,,deer completely ring a soft wood tree during the pre-rut,,building his neck muscles to fight,,and scraping the velvet off,,sometimes same trees year after year,,and they get penciled down sometimes depending on the size of the rack on the buck,,and the tree survives. I don't see a tree being abused by hammock hanging no matter how many times we hang from it,,others have stated barbed wire,,I have seen cable buried completely around trees,,old metal tree stands left hanging that bark and trees have grown around for years. I really don't think its a major concern as using tree straps of any size is definitely doing your best to protect a tree. Trees are amazingly rezillant and will grow over anything that is drilled or driven in them. I was hiking on a Indian Res. once, and even they know how to take the birch bark off a tree and keep it growing to where they don't kill the trees and some of the trees I have seen are very old where they go back and take more using a knowledge of when to do this practice in not harming their envioment. That is not to say,,the white man uses the same practice as he may not be as astute as the American Indiana.
You can always do what climbers/rappers do: bring along a cut-to-fit piece of closed-cell-foam to insert between the tree and the strap, instead of using twigs or small branches. The wide area helps distribute the weight and impact. I would recommend this if you plan on using the same pair of trees for more than a night.
Author and illustrator: The Ultimate Hang: An Illustrated Guide To Hammock Camping