the only real safe thing to do with living trees is to cut coppice, hazel and sweet chestnut are very commonly coppiced and you can take the odd sneaky pole without worry of any impact on the environment.
also many fallen but live trees from vigorous species such as willows, sweet chestnut and poplars produce suckers all along the stem. these can be cut in the same way as coppice without doing much harm as long as you only take a few from many trees rather than striping a whole tree. also when harvesting this type of timber make sure you prune the suckers off close to the trunk without leaving stubs or cutting them too flush. both of these bad practices can lead to fungal attack. if pruned correctly the tree can grow over the wound to protect itself.
species such as oak and sweet chestnut take a long time to rot, especially the heart wood. often old fallen trees are still perfectly solid with the very hard durable heart wood still preserved.
if you take whole trees (obviously you need permission!) the best way to put something back is to plant several trees (at least five is sensible) for every tree you fell. three year old bare root trees are very cheep (in bulk its usually under a quid per tree) and a tree guards (essential) are also very cheep.
with some species such as willow you can stick wasted branches in the ground and they may root and become new trees.
you could help out with your local volunteer conservation group which is not only putting something back but you will almost certainly be able to obtain wood and other products through waste (often trees are felled if they are causing hazards such as dead trees overhanging footpaths). also you will learn new skills and broaden your knowledge.
or if your lucky enough to know a tree surgeon like me we can usually sort you out
pete
