What does roland mean in mediaeval landscaping context?

Solution 1:

It's clearly explained on the page that this roland is a tree with symbolic importance.

Pagans occasionally clustered their dwelling around great single trees, but more often...The pine was often confused with the cedar, cypress..

Observing that the olive tree is always associated with Saracens in the Song of Roland and the pine --except in..., which may be an error ..., Karl-Josef Steinmeyer suggests that the pine may be a symbol of Faith.
...
In the Song of Roland trees must be studied in their own context, since they do not have a uniform, symbolic significance. ref.

And still at eve, when shades deceive,
Amid the doubful light,
The peasants see, in Roland's tree,
The stature of a knight. "The Lady of the Oak".

See also Roland statues/Roland columns Encyclopedia of Symbols.

Solution 2:

From Wikipedia:

A Roland statue is a statue of a knight with a drawn sword, signifying the town privileges of a medieval city.

And from an article on Roland Fountain Statues...

Statues of Roland appear in numerous cities of the former Holy Roman Empire, as emblems of city liberties, (Stadtrechte). The Roland statue at Bremen is the oldest surviving example.

The specific text cited by OP refers to "hewn" rolands, which could well imply that sometimes such a "statue" might be carved in-situ from an actual tree growing at that spot (but stone as well as wood can be hewn, so maybe not).

As implied by my second link, I suspect both the statues themselves and the terminology might be more familiar to mainland Europeans (particularly, Germanic) rather than to Brits. It's not really much of an "English" term or concept, and probably never was.