Hi Marc, do you mean wood occurring in those habitats in general or frogs being associated with wood in those habitats? It is an interesting question, either way. I am not aware of any data.

Hi, Marc. I presume that you're looking at R. pretiosa overwintering sites but I'm not sure what you mean when you say "...freeze in the water column." . The last time you and I talked about winterkill in RAPR we agreed that this is uncommon and seems to be limited to rare circumstances when the frogs get trapped beneath ice at shallow sites and can't escape to safer habitat as the ice thickens. Do you have observations that indicate higher survival in sites with woody structure? Although one might postulate that wood is somewhat better insulation than water and might reduce the rate of heat loss in the winter, I doubt that it would substantially change the rate of freezing unless there was a lot of floating wood at the surface. On the other hand, it's well known among swimming pool owners who don't heat their pools in the winter, that floating a log in the pool helps prevent ice from expanding and breaking the walls of the pool. This is purely mechanical as it allows the ice to buckle and push up rather than expanding outwards only.

Nancy, I mean frogs associated with those habitats, and Jay, you guessed it, I am talking RAPR overwintering because we are working with some data that suggest the likelihood of mortality with frogs either using wood overwintering structures or use wood overwintering structures for more of the overwintering period is less that with those that do not. Of course, I have always viewed the use of beaver dams as the near ideal overwintering habitat because of the triumvirate of adequate oxygenation, protection from freezing and protection from predators, so the freezing option is clearly not the only basis for using wood.  However, some of the overwintering wood structures we found used were not beaver dams, in fact, they were artificial structures placed for habitat enhancement (but not originally with the overwintering purpose in mind), so as frequently occurs, the issue is likely more complicated than we perceive.  And Jay, I am wondering if your log in the pool is really purely mechanical.  Anyway, I did some digging, probably not enough, into literature on the thermal properties of wood....as you can expect, a lot of stuff from the construction angle that is not very useful in the context I suggest here, but I have not been able to find anything linked with wood linked to water that might provide insights.

I agree that beaver ponds represent not only a great overwintering site but probably a habitat feature with which RAPR co-evolved but which is now often lacking from many sites. You indicated "beaver dams" but I'm guessing that included not just the dam structure itself but the pond and perhaps the lodge. We have some tracking data showing a variety of overwintering sites, including use of beaver features such as lodges, runways, and bank burrows. But you're right about logs seeming to be attractive. We did follow one frog that left a pond and spent about half the winter somewhere back under some logs recently placed for bank reinforcement along the Deschutes in Bend. It eventually left this apparently completely safe site and moved out into the river for a couple of weeks before coming back to shore and entering a beaver burrow on the bank about 40m downstream of the original site.However, among the telemetered frogs we've been able to follow for extended periods, they have opted for a variety of protected sites, including a beaver lodge, beaver burrow, muskrat burrows, and rock piles. It seems reasonable to me that logs likely offer protective cover from winter predators that might include mink, otters or fish.

Jay, I agree with you.  What is needed is an assessment of whether you have differential survivorship relative to predators of frogs that spend more time at protected sites versus less protected sites.  However, I also suspect that a great deal of the vulnerability from predators arise during any movements between locations, and if movements make frogs vulnerable, what drives the movements during overwintering is of particular interest as well. One thing that appears to drive movements at Conboy during cap ice, is localized oxygen depletion.

The oxygen question is an important one. There is evidence the RAPR are more tolerant of low oxygen conditions than most other (perhaps all other) amphibians, but they clearly can't go indefinitely at severely hypoxic sites under ice and, as Tattersall's work with R temporaria points out, the frogs easily detect oxygen gradients and preferentially move to sites of lowest temperature and highest oxygen during winter conditions. But in the case of the frogs we tracked a year ago, they still seemed to make periodic moves even when occupying predator-safe and oxygenated sites, often making multiple moves over several months, all within the context of flowing water of the open Deschutes. 

To paraphrase from the old radio show The Shadow--"Who knows what evil lurks in the minds of frogs."

The thermal properties of wood, when assessed against those of water - specific heat capacity, volumetric heat capacity, thermal diffusivity and thermal conductivity - all suggest that wood is likely to delay/reduce the freezing process in a stillwater environment. The volume of wood present is going to be a critcal factor here, and bear in mind that the heat capacity of wet wood is greater than that of dry wood. There are some online resources where specific data can be found for the aforementioned thermal properties of wood - let me know if you can't find anything after searches using the specific terms mentioned above. A lot of the information you want will be cryptically embedded in some truly awful physics!

Thanks much, Aleksandra. I have strongly suspected that something will be buried in physics literature, but have not really found what I want yet and it probably relates to a combination of me not hitting the just right search terms and some of what I am looking for is pseudo-common knowledge that has little work focused on it.

This, and some references within, might be of use. It's old, but perhaps a good start.

Thank you much.  Marc