the emphasis,<<hard times>>

Re: the emphasis,<<hard times>>
Hi, I'm imagining a man who is almost completely bald, but has hair in an ear-to-ear circle around his head.


The relative pronoun which refers to hair. The hair bristled on the skirts (around the sides) of his bald head.


The plantation of firs is all the hairs in this "ring" and the shining surface is either the bald top area, the skin under his hairs or both.


This one's kinda tuff. This could mean the speaker's hair is all covered with knobs, his bald head is all covered with knobs, or its shining surface is all covered with knobs.

I would rule out the second one because relative clauses can generally be eliminated without changing the meaning (and if we did that, we better hope there's not something else in the sentence which refers to it).

So if we eliminate it:
The emphasis was helped by the speaker's hair, a plantation of firs to keep the wind from its shining surface, all covered with knobs, like the crust of a plum pie, as if the head had scarcely warehouse-room for the hard facts stored inside.

a plantation of firs to keep the wind from its shining surface is in apposition to the speaker's hair (see here). Appositions can also often be eliminated. However, since all covered with knobs comes right after it, it could refer to its shining surface.

It makes more sense to me that its shining surface is all covered with knobs rather than the speaker's hair. You can decide for yourself. But whatever you choose, that thing is all covered with knobs like the crust of a plumb pie.

Finally, the "as if" clause specifies how the thing you chose is all covered with knobs. If you choose its shining surface, then the surface is all covered with knobs as if the head had scarcely....

Just remember that the main clause is just The emphasis was helped by the speaker's hair. Everything else is just adding detail to that statement.

There might be some other interpretations too. But this should be a start for you.