Here's a question that I know was asked previously, but I couldn't find the answer on the forum: At what temperatures do the limbs of the various conifers begin to fail?

It's a lot colder than average here in the southern Appalachians, with the morning lows for the upcoming week projected to be about 20-25 degrees below the seasonal norm (the National Weather Service predicted it to be much warmer than normal from December to February in my area, so maybe the meteorologists haven't gone outside lately...).

I want to climb in something that's still green, and the yellow pines outside the den windows are pretty inviting despite the chill. I know that the eastern white pines around here are stronger in cold weather than the yellows, but there are none on the farm property -- I'd have to drive about 20 miles or so to get to a higher elevation where there are some good ones.

Comments? Suggestions?

Everything I've heard from northern U.S climbers is there's no perceptible strength difference. The reason of course that more conifer limbs break in the winter is because of winter storms combined with snow and ice weight on the limbs, I don't think there's any inherent weakness in cold limbs. I've climbed all over white pine in winter conditions and never noticed small diameter branches popping off any more than they already do in the summer. It may be that they're actually weaker in spring and summer because of sap flow.

In rec climbing in general we tend to already err on the side of choosing very solid TIP's, that practice alone should protect against any small percentage strength change if there actually is one for any conifer species.
-AJ

This is the hallmark of predicted climate change scenarios, more severe temperature swings, it's likely that in the northeast we'll be seeing sub-zero f. days followed by mid to high 50's in the deepest days of winter. We've been seeing that already (in recent winters), one of the reasons I can climb all winter no problem, just a wait a few days and it will be warm enough.
-AJ

I think about this a lot, given where I am but I haven't found any good research on it.

Here's my current best SWAGs (Scientific Wild-Ass Guesses):

Branches do get more brittle at low temperatures (direct but unsystematic observation), especially when they have a high moisture content (small twigs up to smallish limbs). This seems to be due to the frozen water making the limb less flexible and possibly in some cases ice expansion pushing out - although the latter doesn't happen much because cold-hardened trees get generally drier as they get ready for winter. In rare cases, a trunk crack might result from freezing, but this seems to only occur when there is already a problem with heart rot (raises the water content of the heartwood). In my experience (direct but unsystematic observation again), you have to have temperatures in the single digits F or below before you notice much difference. Don't suppose you get much of that in Georgia.

The strength of dry wood is basically insensitive to cold over any range of temperatures you'll find outside anywhere.

For the bigger limbs that you're likely to rely on for climbing, Moss probably has it about right - winter damage has more to do with wind + snow/ice loading.

A note on "strength": This is a bit of a vague term. Sometimes, people measure "stiffness" - how hard you have to push the limb to get it to bend a bit. Cold twigs are pretty stiff, but very brittle. A better measure of strength might be "work to failure", where work is force X distance. A more flexible limb might have a large "work to failure" and therefore be much stronger in the way that a climber wants.

All of this needs some real testing. It's getting A LOT more interesting to me these days, as I'm thinking of working with a new grad student on winter cold-hardening gradients within really tall white pine trees.

I was hoping the foremost rogue expert on all things northern and frozen would chime in, thanks Mark!
-AJ

I happened to be sitting at the same lunch table yesterday with the Extension Service director for the U.S. Ag Department in my area of north Georgia, so I posed the question to him. He is primarily a forester, as is a lot of the Extension Service people in the southern Appalachians, but he quickly admitted that the correct answer was well beyond his expertise. He planned to e-mail it to a colleague at a place that I think is called "Tall Pines Research Station" near Tallahassee, Florida.

He did offer a thought that many of the yellow pine species could retain more water in winter than the whites, cedars and spruces found a little farther north or at higher elevations. Temps would have to fall to about zero or lower, though, before the moisture begins to freeze in any limb that would be large enough for climbing. He said this is probably one of several reasons why yellow pines break sooner than other species when weighted with ice/snow.

If I ever hear back from him or the researcher at Tall Pines, I will pass on the info or link.

[url=http://www.mortonarb.org/tree-plant-advice/article/691/winter-injury-to-trees-and-shrubs.html]The Morton Arboretum:
Frost cracks, sometimes called freeze cracks, appear as shallow to deep longitudinal cracks in the trunk of trees. They are most evident in winter at temperatures below 15oF. Frost cracks occur on the south or southwest sides of trees because this area experiences the greatest temperature fluctuations between day and night. A sudden drop in temperature causes the outer layer of wood to contract more rapidly than the inner layer, which results in a long vertical crack at weak points in the trunk. Once a freeze crack occurs on a tree, it is likely to appear annually. Trees most susceptible to frost cracks include London plane, oak, Norway and red maple, horsechestnut, crabapple, walnut, linden, and willow.[/url]

[url=http://pubs.ext.vt.edu/426/426-500/426-500.html]Virginia Cooperative Extention:
A sharp temperature change between day and night may freeze the water within the trunk of a tree, causing it to explode or split open in a symptom called frost cracking. If not severe, these cracks seem to close when warm weather arrives, although the wood fibers within may not grow back together. This is sometimes called southwest injury because it is commonly found on the southwest side of shade trees where warm afternoon sun creates further extremes in the day and night temperatures.[/url]

Yep I've heard trunk cracks booming through a grove of European Beech as the sun "warmed" them on a 0 degree f. day. I think this is a different issue than limb or branch strength on cold days.
-AJ

Agreed, but they're related. The salient relation the articles imply is the rate of temperature change. It's just a suggesion that it is more complicated than temperature alone. Rate of change is simply another important factor to take into consideration. Stresses and the associated risks are greatly increased when temperature changes quickly.

I imagine the dominant scenario is there is already a crack or area of rot in the tree that has collected water, freezing and thawing acts like a vertical wedge and splits the trunk. I guess that's why I see it as more of an assessment problem than a cold problem, ie: the same flaws that make a tree vulnerable to freeze cracking should be observed and assessed any time of the year.
-AJ

In general, fast freezing is much harder on plants than slow chilling. Most temperate and boreal plants have some ability to cold harden - which includes among other things decreasing the water content of tissues. The genetic ability to do this is definitely a major factor in northern range limits of our trees. But these changes take a fair amount of time, on the order of days, while a hard frost can happen in hours. Along the same lines, temperature fluctuations are bad news, for many of the same reasons already mentioned.