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.
Silver can conduct electricity. So if he can fuse silver with the wood within him, it will allow wood to conduct electricity instead of being destroyed by lightning.
Unfortunately, his goal won't be achieved until he can confirm that the fusion is capable of forming a foundation and is stable after forming a formation. This is why he needs the silver trees to reach the third mortal coil.
This is a precaution that he must take since he will be converting his entire body into a vessel made from the fusion of wood and silver in order to reach the third mortal coil.
Once he reaches the third mortal coil, there is no going back for him. So he has to make sure that the hybridization of wood and silver is stable and capable of growth.
These are just the minimum requirements that he has to achieve before he can settle on the feasibility of his foundation establishment.
The second reason he is fixated on silver trees and not the ironwood is because silver is the metal with the highest conductivity of electricity.
That means if he can get the hybridization of silver and wood to work, his foundation will have a high compatibility with both lightning and wood.
He encouraged himself and said, "I'm only 42 anyways. I still have time."
Foundation establishment is not something that can be rushed. It is too dangerous and has to be addressed with the utmost caution. This is why he is willing to spend time to work out the kinks.
Fortunately, he is not alone in his endeavor. Some people in the Agriculture Hall are also working on improving the silver trees. They were the ones who figured out how to make the silver trees breed sexually.
At first, the silver pine trees could only grow asexually. They didn't produce gametes, which would fuse to create seeds. Their main means of reproduction was through cloning, which limited what breeding practices could be used on them.
It was the Agriculture Hall that figured out how to change that. They also made other advancements by changing silver with other metals and alloys of silver that can advance to rank 3 with enough energy.
He didn't take the same route as them since what he wanted to do with the silver tree was different from what they wanted to do with it. But their research has also been useful for him.
Now he has three options in order to achieve what he wants to achieve. He can use a second metal to enhance the silver trees up to the third mortal coil.
The second way is to find a way to make the plant base of the silver tree advance to rank 3 on its own. This is difficult, but it is easier than his third option, which is to convert silver into its superalloy and make it the basis for the fusion with wood.
Of the three options, he is more inclined to the third. The third is the most difficult because mithril is poisonous and corrosive to living things because it is highly reactive with mana.
He doesn't want to give up on the third option, but he knows that it won't be easy to achieve. This is why he has decided to be patient with his research.
He shook his head and said, "If it is already difficult to help a plant to advance to the third mortal coil, how difficult is it for humans to advance?"
From his limited experience, he knows that it is very difficult. Fortunately, he is not hopeless.
Even though he picked the third option, odds are that he will be able to achieve the second option because the two options are not mutually exclusive.
In his attempt to achieve the fusion of wood with mithril, he needs wood that can withstand mithril and won't break down once it comes into contact with it. Odds are that the plant that can withstand mithril can also reach the third mortal coil and beyond.
So the answer to the two options is the same. The difference is that the requirement for the second option is lower than that of the third option. This is because it is easier for a plant to advance to rank 3 than to withstand mithril.
But he still picked to work on achieving option three because his research won't go to waste even if he fails to find a plant capable of withstanding mithril. This is because he can still go the mithril route after he creates his foundation and grows stronger.
Silver can be converted into mithril. So if one day he grows strong enough and can bear the intense reaction with mithril, he will always be able to fall back on the third option.
As for the first option, he considers that a shortcut with a detrimental effect on his goal of highest electric conductivity. He doesn't want to bring in a second metal into the fusion that will compromise his goal just so that he can advance to the third mortal coil.