So... What does, or did, the skier responsibility statute say about inherent risks?
Colorado's statue says the following:
(3.5) "Inherent dangers and risks of skiing" means those dangers or conditions that are part of the sport of skiing, including changing weather conditions; snow conditions as they exist or may change, such as ice, hard pack, powder, packed powder, wind pack, corn, crust, slush, cut-up snow, and machinemade snow; surface or subsurface conditions such as bare spots, forest growth, rocks, stumps, streambeds, cliffs, extreme terrain, and trees, or other natural objects, and collisions with such natural objects; impact with lift towers, signs, posts, fences or enclosures, hydrants, water pipes, or other manmade structures and their components; variations in steepness or terrain, whether natural or as a result of slope design, snowmaking or grooming operations, including but not limited to roads, freestyle terrain, jumps, and catwalks or other terrain modifications; collisions with other skiers; and the failure of skiers to ski within their own abilities. The term "inherent dangers and risks of skiing" does not include the negligence of a ski area operator as set forth in section 33-44-104 (2). Nothing in this section shall be construed to limit the liability of the ski area operator for injury caused by the use or operation of ski lifts.
My interpretation of Colorado's statute is indeed that tree-wells are an inherent risk- if it isn't covered by "snow conditions as they exist or may change," I'd be hard pressed to not include tree wells in "surface or subsurface conditions such as... forest growth... and trees."
At the moment the state of Colorado is hashing out whether avalanches are an inherent risk. By my interpretation, avalanches are an inherent risk, involving "snow conditions" "steepness or terrain" and "changing weather conditions." This is also the argument of the defense, pointing out that an avalanche involves all of those. However, I was not aware until reading the article that the Ski Safety Act referred to the list of conditions as "including but not limited to" and in 1990 the "but not limited to" language was removed. That may move the needle for me.
But, I am still inclined to lean towards the defense in these cases. You can't have an avalanche without snow and steeps, and changing weather conditions is the primary factor in determining whether a snowy steep slope is going to release. Likewise, you can't have a tree well without snow and trees, both of which are explicitly named as inherent risks.
Even ignoring what the statute defines and looking at the meaning of "inherent risk" I lean defense. "Inherent risk" is the idea that certain activities by their very nature have certain risks that cannot be mitigated. We cannot with any failsafe degree of certainty predict whether a slope will slide- hence the number of backcountry (and inbounds) deaths from avalanches. If you can't predict with certainty, the only way to avoid the risk is to permanently close any inbounds pitch of 25* or more and blast all of the steeper slopes down to bare rock to prevent them from releasing onto the shallower terrain.
By the same token, one can't know which trees harbor tree wells, so to avoid the risk, one would have to:
A. Clear cut every single tree in the resort boundary, OR
B. Permanently close all treed areas and run a ropeline and impenetrable fence several feet inboard of the edge of every cleared run.
Those risks look pretty inherent to me. You can't avoid the risk without avoiding the situation (snow on steeps or trees) entirely.