“Subtle effects on trees”….does subtle mean very little or does it have anything to do with the number of trees?
(background: it is about the effects of oil which is deposited on the sea-bed)
You are correct in thinking that subtle in this sentence means very little or slight.
The adjective “subtle” is modifying the noun “effects” in the sentence “subtle effects on trees”. So if there are any effects or consequences on the trees, they are very slight effects and a dendrologist may not be too concerned about the connection between oil in the sea and the condition of trees.
I would conclude that the writer is stating that the deposition of oil into the sea has very little side-effects on tree life.
