Could it be lack of water? Don't know where you are but its certainly been an exceptionally dry Winter, Spring and Summer here. I'm sure someone on here will be able to help.

when you say mature how mature? they are not very long lived [50 or so years so I have read]. perhaps it is suffering from stress after the last 2 winters. Are there any toadstools around the trunk? scrape a small section of bark off, is it still green [alive] or brown [dead] or does it have a white fungal layer [fungal infection - similar to white mould on cheese].

my young rowan has not been happy this year. so I a hoping for a better winter for it.

Seaburn, does it take young rowans a few years to establish? I have 3 which I planted 2/3 years ago and they've not grown much - like you say though, tne last two winters have not been kind

I cant remember as I grew mine in a pot for 5 yrs to keep it small. Then when we moved here I put it in the garden.

Thanks Seaburn. Think mine haven't done too well due to 2 hard winters and no rain.