put salt around the tree,then water the soil with water and salt mix (mix the weakest soulition so 1/2 a tespoon of salt in a jug) and also introduce bilogical controll(eg: birds like thrushes so put out large rocks so thrushes can smash the snails shells on and put perches and birdfeeders,mabye even bird houses)the reson for the salt is that if u put salt on a snails skin it melts them and same with slugs!

A layer of sharp grit on the soil around the trunk will stop them as they won't cross it to climb up the tree.

Please don't put salt on the soil - it is poison to all living plants and the myriads of organisms within the soil, and won't do your tree any good at all. It is far, far worse than most chemical treatments.

Slug pellets sprinkled around and about - the snails are climbing the tree because they can - reduce the snail population and the problem goes away. If you want something organic, then try Myron's suggestion, so long as the grit is sharp, or Natural Slug blocker granules or a similar product - these form a sort of mat around susceptible plants which snails and slugs do not like to crawl over, but it won't kill them.