sorry for the long post, but it is not simple:
http://www2.mnbar.org/benchandbar/2002/mar02/tree.htm
Negligence: Hazard Trees and Limbs
The trend across the country is to hold tree owners legally responsible for damage caused by unsound or "hazard trees."11 A hazard tree is a tree with a defect plus a target, such as a sidewalk, a car, or a house in the path of an unstable or decaying tree.
Minnesota cases involving negligence in tree law tend to fall into two categories: damage caused by trees or damage done to trees. Foreseeability is the common thread that runs through both types of claims. In both instances, courts will look at what should have been obvious to the tree owner about the tree's condition.
Damage Your Client's Tree Causes. If a neighbor's tree is unsound and threatens your client's property, the neighbor may be liable for any damage that occurs. The test is whether the tree owner knew or should have known that damage was likely. A tree owner is not expected to be a tree expert, but she is expected to recognize obvious symptoms of a problem, such as the unseasonal lack of leaves, a dead limb, visible decay, or a tree leaning dangerously to one side. If the potential for damage is foreseeable and if the tree owner fails to take corrective action, the courts will likely hold the owner legally responsible for damage caused to people or property.
In an unpublished opinion, the Minnesota Court of Appeals found that a landowner was not liable in a personal injury case where the landowner's tree did not pose an obvious danger.12 In that case, a tree trimmer was injured when a decaying branch broke. Liability was not imposed, because the branch appeared to be sturdy and showed no signs of decay. In another case, a landowner was found to owe no duty to protect a pedestrian from a low-hanging branch that was clearly visible.13
What's Entropy Got to Do With It? A Georgia case that reaches the same conclusion about foreseeable danger is worth quoting. Taking judicial notice of the Second Law of Thermodynamics, the court said,
This law tells us that all in the universe, trees, human beings, plants, animals, buildings, and all else are headed downward from complexity to simplicity toward decay, deterioration, decadence, and death. Everything heads towards decay; for example, a tree decaying, which is an increase of entropy, or uselessness. We are specifically limiting liability to patent, visible decay, and not the normal, usual, latent, micro-nonvisible, accumulative decay. In other words, there is no duty to consistently and constantly check all pine trees for non-visible rot, as the manifestation of decay must be visible, apparent, and patent so that one could be aware that high winds might combine with visible rot and cause damage.14
Act of God. A frequently heard excuse is that damage caused by a fallen tree was an act of God. Not every tree that falls over in a strong wind and causes damage is the result of an act of God.17 To qualify as an act of God in negligence cases, all of the following elements are needed: 1) the accident must have happened from a force of nature that was both unexpected and unforeseeable; 2) that force must have been the sole cause of the accident; and 3) the accident could not have been prevented by using reasonable care.18 A bolt of lightning is an act of God, if it is the sole cause of an injury. However, a person is liable if his own prior negligence combined with the act of God to cause the injury.
Notes
1 Holmberg v. Bergin, 172 N.W.2d 739 (Minn. 1969).
2 Minn. Stat.¤561.01
3 Holmberg v. Bergin, supra.
4 Michalson v. Nutting, 275 Mass. 232, 175 N.E. 490 (1931)
5 Richmond v. General Engineering Enterprises Co., 454 So. 2d 16 (Fla App D3, 1984).
6 Holmberg v. Bergin, 172 N.W.2d at 744.
7 Booska v. Patel, 24 Cal. App. 4th 1787, 30 Cal. Rptr. 2d 241 (1994).
8 Michalson v. Nutting, supra, 175 N.E. at 490.
9Smith v. Holt, 174 Va. 213, 5 S.E.2d 492 (1939)
10 See, e.g., Skinner v. Wilder, 38 Vt. 115 (1865).
11 "Hazard tree" is a term of art used by arborists and tree scientists.
12 Allison v. Olson and Mauer, filed December 12, 2000, C0-00-942 (unpublished). http://www.lawlibrary.state.mn.us/archive/ctapun/0012/942.htm
13 Sperr by Sperr v. Ramsey County, 429 N.W.2d 317 (Minn. App. 1988).
14 Cornett v. Agee, 143 Ga. App. 55, 237 S.E.2nd 522, 524 (1977).
15 Rector v. McCrossan, 235 N.W.2d 609 (1975)
16 Guide for Plant Appraisal, 8th Ed. 1992.
17 Swanson v. LaFontaine, 238 Minn. 460, 57 N.W.2d 262 (1953)
18 VandenBroucke v. Lyon County, 301 Minn. 300, 222 N.W.2d 792 (1974)