Sunset Song refers symbolically to the twilight of an older way of life. It is clear that the community of Kinraddie is breaking up in the aftermath of the war and through the introduction of new farming methods.
The landscape is altering, too, as in the cutting down of the trees. Chris' favourite song
the Flowers of the Forest reinforces the elegiac mood of the story, anticipating the deaths to come during the war.
The pride o' oor land lie cauld in the clay suggests the death of Ewan and the others in the war but also reflects the view of Will, Chris's brother, that
Scotland is dead or it's dying.
The connection between this theme and the Scots language is discussed by Long Rob and Gordon at Chris' wedding:
And Rob said
You can tell me, man, what's the English for sotter, or greip, or smore, or pleiter, gloaming or glunching or well-kenspeckled? And if you said gloaming was sunset you'd fair be a liar; and you're hardly that, Mr. Gordon.
But Gordon was decent and reasonable,
You can't help it, Rob. If folk are to get on in the world nowadays, away from the ploughshafts and out of the pleiter, they must use the English, orra though it might be.
Rob wishes to preserve the Scots words while Gordon realises that to 'get on' in the modern world the Scots must use standard English.
Grassic Gibbon was perhaps acknowledging the truth of Gordon's viewpoint by using the word 'Sunset' in the title of the book, rather than calling it Gloaming Song.