Complex sentence: Amongst many Druids, there is a system of tree lore, through which different associations are attributed to different species of tree, including particular moods, actions, phases of life, deities and ancestors.
Split into simpler sentences: Amongst many Druids, there is a system of tree lore, through which different associations are attributed to different species of tree. Those things which are associated with trees include particular moods, actions, phases of life, deities and ancestors.