The halacha of “karpaf” requires that any area to be enclosed by an eruv that is a Beis Se’asayim (5000 square amos, the shiur of the chatzer in the Mishkan, from whence most parameters of Meleches Shabbos are derived) or greater, must be mukkaf l’dira, i.e., it must be enclosed for the express purpose of habitation, not enclosed merely by happenstance of natural growth. For example, if the foliage of a tree or several trees droop down to the ground in the form of a “wall,” that enclosure forms a “natural” eruv. If, however, the trees grow to encompass a Beis Se’asayim, even if the foliage forms a wall, it cannot serve as the enclosure of the area, as it is not mukkaf l’dira.
Similarly, natural riverbanks or cliffs cannot comprise an eruv of an area large than a Besi Se’asayim unless some portion of their circumference is artificial.
The Yerushalmi here goes further. In discussing the parameters of the rectification required at the entrance to a mavui – an alley that is enclosed on three sides, but open on its fourth side to a reshus ho’rabbim – which Chazal decreed to require either a vertical post or horizontal beam (lechi or korah), the Yerushalmi rules that a grapevine or gourd tree can only serve as a lechi or korah for an area under the size of a Beis Se’asayim. If the area is larger, then despite the fact that three of its sides are enclosed by man-made walls, the lechi or korah must also be man-made, i.e., mukkaf l’dira (see R’ Chaim Kanievski shlita’s Biur!
This is very interesting in and of itself, but, even more so, because in a classic four-sided eruv, only a small portion of the enclosure need be artificial. Yet here, even though the vast majority of the enclosure is artificial, this does not suffice! Clearly, the Yerushalmi regards the lechi or korah as a separate category unto itself with its own specific parameters. So far as I know, this severity of the Yerushalmi is not brought down l’halacha.