Eruvin, Chapter 10, Mishnah 8. The Mishnah discusses a tree whose branches are broad and droop down all around it, to the point that they form a covering over the ground surrounding the trunk, a kind of canopy (the term used is related to the laws of sukkah), and in this way they supply a partition for the area around the trunk.
The branches of the tree as a partition:
If the branches are no higher than three tefachim above the ground, as in a weeping willow, whose branches come down almost to the ground, they are considered halachically to be an actual partition, and therefore it is permitted to carry beneath the tree. Even though the tree stands in a public domain, it creates a private domain for itself, since the branches serve as a partition for it.
The Gemara's conditions for validating this partition:
Filling the gaps: Every gap between the branches that is larger than three tefachim must be filled in with some material; otherwise the partition is ineffective.
Stability: The filling must be stable enough that it will not be blown away by an ordinary wind. Under these conditions it counts as a partition.
The size of the area: The area may not be larger than beis sasayim (five thousand square amos), which in any case is a rare situation. If it is larger than that, these partitions cannot be relied upon, since they were not made for that purpose: they are not "hukaf l'dirah", enclosed for the sake of dwelling, and they were not made for the needs of people living there.
The roots of the tree:
The Mishnah continues: if the roots of the tree are more than three tefachim above the ground, one may not sit on them. The Sages prohibited making use of a tree on Shabbos, meaning physical use, lest one come to pluck a branch from it. Its sides may be used as a partition, but one may not come into direct contact with it, lean on it or rest on it on Shabbos.
If, however, the roots are less than three tefachim above the ground, they are considered part of the ground and not part of the tree, and therefore it is permitted to sit on them. Only when they are higher than three tefachim is sitting prohibited.
Temporary closures - "delet shebamuktzeh":
The Mishnah goes on to list items that are used to close off an opening in a temporary manner:
"Delet shebamuktzeh" - a muktzeh is a spot in the back yard designated only for storage and not for living. The door there is not a permanent door, since it is not usual to install a door there on real hinges; they would take a board and stand it up temporarily against the opening, and when they wanted to open it they would move it aside.
"V'chedek v'siach" - thorns, thornbushes or branches of thornbushes. They would use these to close off a breach in a wall, something like setting up barbed wire in a particular area.
"U'machtzalos" - mats made of reeds.
Regarding all of these the Mishnah rules: "ein no'alin bahen" - they may not be used as a door, that is, they may not be set up in the opening on Shabbos, because this involves "mosif al habinyan", adding to a structure on Shabbos.
"Ela im ken gavo'ah min ha'aretz":
When they are raised off the ground and do not touch it, it is evident that they are not functioning as a sealing of the opening and a closing up of the structure, and there is no element of adding to the structure in them; rather, they serve as a temporary door or as a temporary means of blocking off the open space.
In summary: in this Mishnah we learned that tree branches that droop down to within three tefachim of the ground are considered a partition and create a private domain, provided that the gaps between them are filled in and stable in an ordinary wind and that the area is not larger than beis sasayim; that roots higher than three tefachim have the status of the tree itself and may not be sat upon, while those lower than that are subsumed in the ground and are permitted; and that a temporary door, thorns and mats may not be used to close an opening on Shabbos because of "mosif al habinyan", unless they are raised off the ground.