This is the 17157th most frequent Russian word.
ствольной
Relating to a barrel (of a gun or similar object).
In this sentence, 'ствольная' describes something related to the 'ствол' (trunk) of the tree.
У дерева была сильная ствольная структура.
The tree had a strong trunk structure.
Here, 'ствольная' is used to refer to the 'ствол' (barrel) component of the firearm.
Мастер проверил ствольную часть ружья перед охотой.
The master inspected the barrel part of the gun before hunting.
In this case, 'ствольные' describe shades or patterns reminiscent of the 'ствол' (trunk) in appearance or effect.