His fixation on the silver pine trees is for two reasons. The first reason is that he has a lot in common with silver trees. They are both living things that have to fuse with something dangerous in order to become stronger.
Just like wood fused with silver, he needs the wood element of the Everlasting Wood Art to fuse with the lightning element of the Divine Lightning Armament.
What he wants to achieve is very dangerous. Lightning destroys wood on contact, so it is nearly impossible for him to fuse the two elements.
His goal for foundation establishment is so difficult that Elder Storm Hawk advised him to give up on one of the elements. But despite this difficulty, he is not entirely hopeless.
He believes that he can ameliorate the irreconcilable differences between the two elements if he has an intermediary like a metal. The metal he has in mind is silver.
