There are a wide range of different factors that can cause physical damage to trees. Effectively, most of the biological and chemical factors could also be considered to be physical damage. Most sources of physical damage are relatively easy to spot and identify; these may include:
Damage from strimming
Poor fencing practice (wire stapled to trees etc.)
There are a number of other factors that can damage trees which are harder to distinguish. These include, frost, ground compaction and site waterlogging / drought. The best way to determine the cause of damage is to look at the site conditions over a period of time and to examine specific species characteristics (such as a lack of frost tolerance for example.) If you are still unsure, and there is significant damage to a number of trees across the site, we recommend speaking to an expert. There is a list of useful organisations available here.