"Spring and all" relates to the conventions of pastoral poetry because:
The poem describes nature in a certain scenario but it complements it by idealizing it with a value judgment.
The two reasons that back the answer are that in the first place description is good and objective. However, when it is complemented with believes, and opinions, they gain a grade of value judgment. In our case the association between pastoral poetry and the poem of William is that he idealizes certain aspects of the scenario he describes like:
But now the stark dignity of
entrance—Still, the profound change
has come upon them: rooted, they
grip down and begin to awaken
Providing dignity to a natural characteristic that is improper of it. Showing his idealizations.
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