It is a slightly unusual use of the word 'punctuated', but there's only really one way that this could be understood. The far edge of the field (the edge furthest away from the perspective of the author as they look at the field before them) has a line of trees running along it. The variety of the trees is unspecified except that the furthest left is a spruce and the furthest right is a maple.
'Punctuated', in this kind of a context, usually means that a sequence is interrupted, or broken up by something. By that definition, you might expect the spruce and maple to appear somewhere in the middle of the line of trees, but that would more likely be phrased this way:
A line of trees marks its far edge, which is punctuated by a spruce and a maple.
However, your text says "a spruce on its left side and a maple on its right". The pronoun can only refer to "the line of trees", and the left and right of a line can only mean the ends.
There are other definitions of 'punctuate' that could apply:
- to give emphasis to
- to mark the beginning and end (of text)
This is still a slightly unusual use of the word in context. A better word would be 'flanked', for example:
A line of trees marks its far edge, flanked by a spruce on its left side and a maple on its right.