It seems that 寄らば大樹の陰 is an idiomatic saying that literally means 'If you need shelter, a big tree is better than a little one' or maybe 'If you need shelter, choose a big tree', but is actually a metaphor for 'If you need protection, find an influential man.'
Well, that's what the dictionary says anyway, but, how did they get from 寄る to 寄らば? The sentences looks to me like it says, "If you're getting close (to an unspecified thing or place), there is the shade of a large tree', excepting that vowel change is wrong for a ば conditional.
I can understand 'shade of a tree' is a metaphor for 'in the shelter of a tree', but beyond that I don't see where the supposedly literal meaning of this sentence is coming from.
Can anyone clarify this for me?