Oh nice, a poem! I’m not very knowledgeable in matters of poetry or the literary language but I think あした / ゆふべ here were meant to be “morning / evening”. It’s a classic oxymoron pair. IMHO he is not thinking positively about what comes next but reflecting on the beauty of nature as something more permanent. That’s just my interpretation, though.
By the way, this particular poem isn’t too hard, but for beginners looking at the grammar, here are some notes.
Like pretty much all traditional poetry, this looks like it was written in the literary language. ゆふべ (ゆうべ) shows old kana usage, which is as good an indicator as any.
山あれば Note あれば in the classical / literary implies existence and usually causality of the condition, so either “when(ever) there are” or “because there are” ; it is subtly different from あれば in the modern though in this case, it doesn’t impact us much I think. “When” is probably a good translation here.
よろし We can see the ending is し and not しい as in the modern language. For those who might not know, this is the conclusive (“sentence ending”) form of adjectives in the classical language, while しき (which gave us しい) is used before nouns.
Knowing Santoka’s particular style, I’ve always read this last portion of the poem, in the context of the first two ideas, as returning to and emphasizing the present.
Lots of Santoka’s poetry is about what is going right here, right now. This particular poem almost seems like a response to something else, maybe something he was reading or a conversation. Maybe it’s a comment on Haiku in general, since he was and is still considered a haiku anarchist. It’s unusual, and is one of his longest pieces.
He’s kind of saying, there’s joy and delight to be found right here. In the Mountains, in the rain, in all the seasons, but especially right here this evening or tomorrow. Day by day, we discover joy.