Vilem Duha writes:
The BlenderKit database is constantly growing with new items uploaded by users, but we also try to do our best to make your life easier by creating the free assets ourselves.
We just released a set of tree bark materials. These should help you when creating trees!
There are some really nice free tree generators for Blender like MTree or Sapling(directly in Blender).
But getting the right bark material might take time, especially when you go the procedural way. Most of the materials provided by us are all PBR texture based, and should cover you as long as you create pines, oaks, and beeches, or some trees with similar barks. There are 3 extra procedural materials - 2 for pine, 3rd for plane tree. You can potentially blend between the materials, since a tree bark is always different on different stages of the trunk and branches.
Are you missing some material types? Create yours and upload them! All materials in BlenderKit library are free and created by the community.