Answer. Let's brush up on Oregon's hazard tree law as it relates to landlord-tenant responsibilities:
- "DBH" means the diameter at breast height, which is measured as the width of a standing tree at four and one-half feet above the ground on the uphill side.
- "Hazard tree" means a tree that:
- Is located on a rented space in a manufactured dwelling park;
- Measures at least eight inches DBH; and
- Is considered, by an arborist licensed as a landscape construction professional pursuant to ORS 671.560 and certified by the International Society of Arboriculture, to pose an unreasonable risk of causing serious physical harm or damage to individuals or property in the near future. (Italics and underscore mine.)
- Resident Duties re Trees Located on Space. A resident shall maintain and water trees, including cleanup and removal of fallen branches and leaves, on the rented space for a manufactured dwelling except for hazard trees.
- "Maintaining a tree" means removing or trimming a tree for the purpose of eliminating features of the tree that cause the tree to be hazardous, or that may cause the tree to become hazardous in the near future.
- "Removing a tree" includes:
- Felling and removing the tree; and
- Grinding or removing the stump of the tree.
- Landlord Duties re Hazard Trees.
- Landlord shall maintain a hazard tree that was not planted by the current resident if the landlord knows or should know that the tree is a hazard tree;
- Landlord may maintain a tree on the rented space to prevent the tree from becoming a hazard tree;
- Landlord must provide residents with reasonable written notice and reasonable opportunity to maintain the tree themselves.
- Landlord has discretion to decide whether the appropriate maintenance of a hazard tree is removal or trimming.
- Landlord is not responsible for:
- Maintaining a tree that is not a hazard tree; or
- Maintaining any tree for aesthetic purposes.
- A landlord must comply with the access provisions of ORS 90.725 before entering a resident's space to inspect or maintain a tree. [Generally, 24-hour notice. - PCQ]
- Subject to the preceding, a resident is responsible for maintaining the non-hazard trees on the resident's space at the resident's expense.
- The resident may retain an arborist licensed as a landscape construction professional pursuant to ORS 671.560 and certified by the International Society of Arboriculture to inspect a tree on the resident's space at the resident's expense;
- If the arborist determines that the tree is a hazard, the resident may:
- Require the landlord to maintain the tree as a hazard tree; or
- Maintain the tree at the resident's expense, after providing the landlord with reasonable written notice of the proposed maintenance and a copy of the arborist's report.
The first question is whether this is a "hazard tree"? You say it is not, but based upon measurement, it is. Note that the statutory definition above says it is a hazard tree if it poses '_ an unreasonable risk of causing serious physical harm or damage to individuals or property in the near future."
My take is that this is a hazard tree, since it meets the physical specs, and not only is capable of causing serious damage, but its root system already has. At the risk of oversimplification, botanically speaking, the tree is the entire plant, not just the above-ground portion. While it is does not sound like it is in danger of toppling on anyone or anything, it does sound as if its root system is interfering with the resident's sewage system.
Again, at the risk of oversimplification, I don't believe the issue is who actually owns the resident's lateral sewer line. Why? Because I suspect that the sewer system was not installed by the resident - it was likely installed at the time the park was developed. It is in the ground, and the ground is owned by the landlord.
Moreover, I believe ORS 90.730 (Landlord's habitability duties) applies in this case. It provides:
(2) A landlord who rents a space for a manufactured dwelling or floating home shall at all times during the tenancy maintain the rented space, vacant spaces in the facility and the facility common areas in a habitable condition. The landlord does not have a duty to maintain a dwelling or home. A landlord's habitability duty under this section includes only the matters described in subsections (3) to (6) of this section.
(3) For purposes of this section, a rented space is considered uninhabitable if it substantially lacks:
(a) A sewage disposal system and a connection to the space approved under applicable law at the time of installation and maintained in good working order to the extent that the sewage disposal system can be controlled by the landlord; (Emphasis added.)
The roots of the tree are interfering with the operation of the resident's sewer system, and you as a landlord have a maintenance obligation '_to the extent that the sewage disposal system can be controlled by the landlord."
And if the tree is deemed to be a hazard tree because of the damage to the lateral servicing the resident's space, then ORS 90.730(4), would arguably apply, which provides that the failure to maintain it can constitute a separate habitability violation by the landlord.
Conclusion. Unfortunately, it appears to me that: (a) Absent some language in the hazard tree statutes indicating an intent to exclude that root systems, and (b) your statutory habitability duty to maintain the sewage disposal system, a strong case could be made that remediating the damage caused by the tree roots (regardless of whether it's a "hazard tree" and regardless of whether the resident technically "owns" the lateral - which I doubt) the cost of keeping the system clear of tree roots, is on your shoulders.
 Since I was involved with John VanLandingham and others in the drafting of this legislation, I can only speak for myself, but admittedly, it seemed our focus was on the above-ground risks, e.g. toppling trees or branches falling on residents, their homes and automobiles.