Frost Cracks

Fresh snow highlighted every twig, and more flakes floated down as we hiked through the Rainbow Lakes Wilderness Area on the North Country Trail (NCT). This intrepid group of women calls themselves the NCT Navigators, and they meet almost every Monday (when the mosquitoes aren’t out) to hike a section of trail. Most of them have completed the NCT’s 100-mile challenge. Most of them are retired, too, and I count myself lucky to fit in a hike with them even a few times a year.

The NCT Navigators hike a section of the North Country Trail almost every Monday. 

They are well on their way to hiking another 100 miles this year! Photo by Emily Stone.

Today the fresh snow was making the ski trails slow, and I was looking for inspiration, so I gave my friends an assignment: find me something to write about! Well, this artistic group had already gotten in the habit of noticing patterns in nature, and it wasn’t long before someone keyed in on an adorably squiqqly line snaking up the length of a tree.

The crack in this sugar maple may have been widened by freeze-thaw action in winter, but it likely got its start in the decaying wood where a branch fell off. Photo by Emily Stone.

A frost crack! Of course. What a perfect topic for a winter day. Long ago I learned that these cracks burst open with a noise like a rifle shot as a sunny day plunges into a frigid night. I couldn’t remember, though: was it the contraction of cooling wood, or the expansion of ice that caused the trunk to split? Both make sense. I pondered this as we hiked along, and also tried to spot more cracks throughout the forest. It wasn’t hard. While not on every tree, I could see a long, vertical welt from just about any point on the trail. Quite a few of the cracks had their upper terminus at a scar where a branch had fallen off, or some other blemish on the tree.

Back at the office, I sought a more complete explanation. My initial research was frustrating, though, since the sources all gave multiple explanations of frost cracks. Yes, it seems to involve the low-angled winter sun shining heavily on the southwest side of a tree. The bark and inner wood both warm up and expand. When the sun sets and the temperature drops, the outer bark contracts more quickly than the inner wood, and suddenly becomes too small a sheath for its core. The bark and first layer of wood split.

Another part of the explanation is that the heat causes the tree’s cells to break dormancy. Trees winterize their living cells in a few ways (dead cells, which compose most of the wood, just freeze). Cell membranes become more flexible, which allows water to migrate out of the cells and into the intercellular spaces where it can freeze without harm. Trees also fill their cells with sugar in order to lower the freezing point of their remaining liquid. And, like magic, the syrup inside the cells supercools to a glass-like phase where it is so viscous that it appears to be solid, while not forming sharp crystals. In a sun-warmed tree, water may begin to move into places it shouldn’t. As the tree re-freezes, water shifts around and freezes quickly and unevenly, causing stress on the wood and opening the crack.

A few sources mentioned that frost cracks are often associated with previous damage to the tree, but the process didn’t fully make sense until I read an interview with a plant pathologist in Northern Woodlands magazine. Walter Shortler and his mentor, Alex Shigo, did a sort of forensic pathology for trees. They used a chainsaw to slice up thousands of logs to get to the bottom—or the top—of the frost crack issue.

The result? Shigo found that “All radial shakes [cracks along the radius of the tree] were associated with wounds, branch stubs, or basal sprout stubs, and with ring shakes [cracks that follow tree rings] at some point in the trunk.” Almost anything can create an area of weakness: fire, damage during logging operations, poor pruning techniques by humans, gnawing rodents, rubbing deer, root rot, basal sprouts, or branch stubs that didn’t heal. Trees that are damaged when young seem to be much more prone to later frost cracking than older trees that become damaged.

A photo from Radial Shakes and ''Frost Cracks'' in Living Oak Trees by Heinz Butin and Alex L. Shiga https://www.nrs.fs.fed.us/pubs/rp/rp_ne478.pdf
Original caption reads: "Figure 14.-Many small radial shakes often start from wounds. It is not known why they start from some wounds and not others. Some of the shakes in this sample have split out to the bark. Note the curved primary shake that opened wide after the sample dried."  

In any case, damage to the bark and underlying sapwood provides an entry for decay. Bacteria and fungi move in, and as they decompose the wood it absorbs extra moisture. The defective wood doesn’t expand and contract as well as healthy wood, and rapid freezing finally bursts open a significant crack. Repeated freeze-thaw cycles, coupled with stress from wind, can enlarge a crack, even as the tree tries to heal it.

So, a combination of all the forces I read about likely play a role in opening and widening cracks, but none of them alone would do the job without a prior injury. This all made sense to me, especially in light of my observation that many frost cracks we saw topped out at an old branch scar.

The tree will try to heal the wound, and the growth of new wood around the crack is sometimes called a frost rib. Indeed, we admired the raised scar that highlighted our squiggly crack.

Foresters, of course, are concerned with reducing damage to young trees so that frost cracks can’t gain a foothold in what could have been valuable timber. Naturalists might appreciate a different perspective. A little ways down the trail we spotted another tree with a convoluted scar. Nestled inside were two brown acorns. I suspect that naturally occurring frost cracks may provide valuable habitat for critters in their cavities and decay. In the words of Leonard Cohen, “There is a crack in everything. That's how the light gets in.”

Even damaged trees can be valuable members of a forest! This scar in a smaller tree became a hiding place for some little critter’s acorns. Photo by Emily Stone.

For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: "Better Together--Celebrating a Natural Community" is now open!

MuseumMobile

Three times a year I get to spend a few days at the elementary school in Drummond, WI, teaching kids my favorite nature facts using my favorite nature props. Once each season, in Fall, Winter, and Spring, I load seven plastic tubs filled with skulls, furs, bones, rubber scat and other oddities into the Museum’s mini-van for a visit to each classroom in grades pre-k through six.

A selection of MuseumMobile props from my Winter visit.

The pre-k kids in Ms. Bonney’s class are one of my favorite visits. Their enthusiasm is too ripe to be contained within tiny, squirming bodies, and their endless stories wander all over the map. We point to our eyes, ears, nose, tongue, and fingers, and practice exploring nature with our senses.

In one of the small, cloth, “touch bags” are a pair of short-tailed weasel skins. It’s fun to watch their little faces light up in pleasure when they touch the soft furs. It’s even more fun to demonstrate why the weasels—both the brown summer fur and the white winter fur—have black tips on their tails. “Imagine you’re a hungry red-tailed hawk,” I instruct them, “and you’re watching this weasel run across the snow. What part of him would you see first and try to grab?” The black tip of course. But the bony talons of a hawk can’t grasp the tip of a weasel tail, so the sneaky critter runs off to safety. 

The first thing I pull out of the first grade tub is a dried specimen of a little brown bat. Projected up on the white board through a document camera, its larger-than-life details capture the children’s attention right away. Their favorite part? The fact that you can match your own fingers to the bones in the bat’s wings, including a tiny, upward pointing thumb. This year I was also gratified at the number of oohs and ahhs I elicited when showing off a delicate bat skeleton cast in resin.

During my fall visit, second graders learned how the eyes and teeth of herbivores, omnivores, and carnivores are adapted to their specific lifestyles. Since herbivores are prey animals, they need eyes located on the sides of their heads to watch for danger in all directions. Predators have forward-facing eyes that give them binocular vision for catching prey. Herbivores have flat teeth for grinding up plants, and the wolf skull is full of sharp teeth for shearing meat off of bones. Omnivores, like us, have both sharp canines and flat molars.

For the winter lesson I take second graders a little farther down the alimentary canal. We use a handy identification key and rubber replicas of animal scat to learn more about herbivores, omnivores, and carnivores. Did you ever think about the fact that most herbivores make small pellets like our familiar deer scat? Or that all carnivores tend to make long, ropy scats due to the presence of hair? Or that omnivores find the average of those two by shaping their scats into long cylinders? This lesson gets lots of laughs, but it’s valuable information for little naturalists just getting their start at animal tracking.

The thick pelts of a wolf, a coyote, and a fox grab the attention of third graders, as does a wolf skull familiar to them from our second grade look at carnivore teeth. With this lesson, we also observe the ridge of bone sticking up along the back of the wolf’s skull. This is where its jaw muscles attach. The bigger the fin, the more muscles, and the stronger the animal’s jaw. Wolves can crack the upper leg bone of an adult elk.

Fourth graders migrate on to talk about birds. In the fall, we dress up a kid with all the adaptations of an owl and then dissect owl pellets. In the winter, we compare the adaptations of owls and loons. It is amazing how different two birds can be. Looking at talons vs webbed feet really drives home the concept of being adapted to your specific habitat and lifestyle. A loon could never perch on a tree branch, and an owl couldn’t swim very far. We end the lesson by listening to the territorial yodels of male loons and matching them with sonograms. Of course, I also want to connect the loon calls to owl hoots. Luckily, the fourth grade teacher, Mrs. Thurn, was once the Education Director at the Cable Natural History Museum. She has the best imitation barred owl call I’ve ever heard, and right on cue she’ll give us a perfect “who cooks for you?”

We take a break from life science in fifth grade, and the kids compare the electricity use and heat output of three types of lightbulbs. Once we’ve done the math to figure out how much more expensive it is to use incandescent bulbs over compact fluorescent bulbs—and especially over LED lightbulbs, I have the kids tell me what they’d rather do with that money. Video games I’ve never heard of generally top the list.

With the sixth graders, I get to do a wonderful progression of lessons about symbiotic relationships—one of my favorite topics. Every autumn I gather a couple gallon bags of goldenrod galls and tuck them in the back of a freezer. The kids get pry open galls to see who’s living inside. Mostly they find the gall fly larva who created the gall, but occasionally they discover the larva of a parasitic wasp or a predatory beetle. Some galls are empty—with a telltale woodpecker hole in the side.

I’ve been teaching these MuseumMobile lessons at Drummond since the sixth graders were in kindergarten. By now the kids are accustomed to my tubs of animal parts and my unbridled enthusiasm for weird stuff. The most common behavioral problem I deal with is kids interrupting class with random nature facts and nature stories they just can’t wait to share with me.

Over the years, this part of my job has shrunk as other responsibilities have grown, but I just can’t bring myself to give it up completely. After all, I never grew out of the stage where I blurt out cool nature stories to anyone who will listen!

Special Note: Columnist Emily Stone is publishing a second book of her Natural Connections articles as a fundraiser for youth programming at the Cable Natural History Museum. Since kids in the community are often the inspiration for her articles, the Museum is conducting an art contest for kids to illustrate each chapter with a black-and-white line drawing. Find out more at http://cablemuseum.org/connect/.

For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: "Better Together--Celebrating a Natural Community" is now open!

Skiing Through History

Up hills, around corners, and swooping through the woods I went as my skis swooshed beneath me and poles gripped the trail. A pink sunset glimmered wistfully between the trees, and a pair of ravens gave sonorous croaks as they pushed their wings against chilly air. What a joy it was to kick and glide over perfect grooming on the North End Trail’s main 12-kilometer loop.

Earlier, on my first long ski of the season, some parts of the trail felt strangely unfamiliar. Logging operations had altered the scenery by turning patches of dense forest into open woodlands and fields. It was a good reminder that the North End Trails are situated on a working forest owned and managed by Bayfield County.

Just over 4 kilometers in, not far beyond intersection #76 (I put a map at the very bottom of this post), I paused at the top of a long slope to take a breather. More than just a necessary break, this peaceful spot was too pretty to zoom on through. A dense grove of large evergreen trees hugged the trail and seemed to silence the wind.

Looking out, a striking pattern of light and dark stretched as far back as I could see. In their own dense shade, the lower branches of these trees were dying, and their bare arms held up bright snow. Looking up, graceful branches arched up to embrace the sky while their deep green fingers dangled like the fringe on a shawl.

Those pendulous branchlets are the signature form of Norway spruce, Picea abies, and another reminder that this is a working forest. In its native range of Northern, Central, and Eastern Europe (including Norway), this hardy, fast-growing tree is a commercially important source of wood for lumber. It was also used by Stradivarius to make instruments, its cones were once employed as weights in grandfather and cuckoo clocks, and it stands elegantly as the Christmas Tree in Rockefeller Center each winter. European immigrants first planted it in Massachusetts in 1860, but its heyday in the Midwest came later.

The “cutover,” when most of our original forests were logged to build our rapidly growing country, left the land denuded. Hopeful immigrants tried to homestead the land and soon found that the soils were better suited for trees than crops. When the farmers couldn’t manage to pay their property taxes, their lands forfeited back to the county. The federal government purchased some land, and the county kept some, too. On his way out of office in 1933, President Herbert Hoover created both the Nicolet and Chequamegon National Forests. When President Franklin D. Roosevelt swept in with the New Deal in 1933, a new (badly needed) era of conservation began.

The Civilian Conservation Corps (CCC) is a well-known product of the New Deal, whose mark on the land has endured. We still appreciate the log and stone buildings, trails, and bridges that those hardworking boys constructed on all sorts of public lands. Planting trees also formed a huge part of their endeavors. A crew of 200 men might plant 150,000 trees per day, five days per week. Their work was essential to reestablishing forests in Wisconsin and most other Midwestern and Northeastern states as well.

With the goal of stabilizing the damaged soil of abandoned farms as quickly as possible, foresters experimented with having the CCC boys plant non-native species in the hopes that they wouldn’t be susceptible to the insect and disease problems that slow growth in our native species. Scotch pine, Austrian pine, and Norway spruce were all given a try. When I asked Jason Holmes, a Bayfield County Forester, about the success of these foreigners, he admitted that “unforeseen pests weren’t very welcoming to these European tree immigrants. Scotch (or Scots) pine was a classic example of this kind of failure.” But then he added, “I’m sitting here now wondering if Norway spruce is an exception.”

Indeed, it seems to have thrived. When young, Norway spruce can grow up to 3 feet a year. It tolerates shade, drought, and acid soils, and is not a preferred snack for deer, insects, or mice. Squirrels will nibble on the seeds, but those seeds are still Norway spruce’s main route to reproduction, and it has become naturalized in many states.

The calm air I felt while skiing through the Norway spruce grove has been noticed by others, too, and this species is often recommended when planting windbreaks. It’s a common street tree, too. My neighbor growing up in Iowa had a huge one in her backyard. Once I began looking, I found Norway spruce all over the town of Cable.

Out on the trail, though, I didn’t see much Norway spruce regeneration. The mature, almost 80-year-old trees, which currently represent 40 out of about 170,000 acres of the Bayfield County Forest (a whopping 0.024%), will be thinned and harvested according to the same management principals as the white pines growing nearby.  As opposed to aspens, birches, and red maples, Norway spruce doesn’t sprout back after you cut it, so these stands will likely be easy to replant in native species that have come back into favor.

Despite the grove’s calm air, the chill started to seep in and I kicked off down the hill, enjoying the gentle grade of what must be an old logging road. No matter how much I might try to focus on the present—elegant trees, proper technique, keeping warm, and raven’s calls—I’ll still be skiing through history.

Special Note: Columnist Emily Stone is publishing a second book of her Natural Connections articles as a fundraiser for youth programming at the Cable Natural History Museum. Since kids in the community are often the inspiration for her articles, the Museum is conducting an art contest for kids to illustrate each chapter with a black-and-white line drawing. Find out more at http://cablemuseum.org/connect/.

For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: "Better Together--Celebrating a Natural Community" is now open!

If you want to know exactly where I was on the trail, here's the map! The colored rectangle in the background is a stand map from Jason Holmes, the Bayfield Country Forester. The darker red polygons outlined in turquoise are the Norway spruces. You can also see the spruces from the North End Classic Trail. 

If it helps to see the stand map without the colored ski trails, here you go! Thanks Jason!

Life in the Snow

Lately, the snow has been making me chuckle. Clumps from the first big storm still cling to branches and twigs, but they are slowly slipping off. In the process, the tufts have slumped, curled, dripped, twisted, and oozed into almost organic forms. It’s like looking for shapes in the clouds. I see a sloth, a ghost, an amoeba, snakes, and Silly Putty. The luxurious way that some clumps stretch along a branch reminds me of a cat or a lizard draping itself in the sunshine for a nap. This snow has taken on a personality all its own.

Of course, that’s just my overactive imagination. The snow clumps are simply responding to temperature and wind, and the crystals’ natural metamorphosis as they age. Or are they? Snow is more alive than you may realize.

A recently rebroadcast Radiolab episode on NPR briefly mentioned a scientific discovery that I’d missed. I’ve known for a while that snowflakes and raindrops have a speck of dust at their centers. You see, perfectly clean water won’t freeze until it reaches -40 degrees. Dust particles in the water are needed to act as nucleators that initiate crystal formation in super-cooled water. The dust forces water molecules to assemble in a structure around them. Once a bit of ice with the correct angles has formed, the molecules will continue to crystallize more easily. The crystals can then melt and fall as rain, or grow more ice and fall as snow.

For the past 40 years or so, scientists have known that bacteria can be one type of dust that acts as a nucleator; what they didn’t realize until 2008 was how common bacteria are in snow and rain. A study done at Louisiana State University (Louisianans studying snow!) by Brent C. Christner found DNA-containing cells in snow from all of their 19 study sites. Antarctica had fewer cells, while samples from Montana and France had more.

Some of the bacteria that scientists find in snowflakes attack plants. The bacteria use a protein that mimics the structure of an ice crystal so well that it can hold water molecules together and help them to crystallize more easily. They can quickly turn water into ice, even at warmer (near-freezing) temperatures. When these bacteria spit their proteins onto a blade of grass, the sharp edges of the resulting ice crystals slice open the plant’s cells and spill juicy nutrients into the bacteria’s waiting arms.

 A tomato plant leaf infected with bacterial speck, the disease caused by Pseudomonas syringae. 
Photo by Alan Collmer, Cornell University.

On a windy day, though, those bacteria might get scooped up into the upper atmosphere. High above the Earth, the bacteria are cold, dry, and hungry. They need to get back down, but they are too light to fall on their own. Here’s where the ice-nucleating protein comes in handy again. The bacteria galvanize water molecules around them and form snowflakes. Like tiny ballerinas, the flakes float across the sky and dance back down toward the earth. Here they may land on a blade of grass (a new source of food) or melt on my cheeks. Eeww?

Happily, the bacteria that form this “bioprecipitation” are not harmful to humans. In fact, not all of them are even harmful to plants. It seems that some types of bacteria that don’t feed on plants (as well as some fungal spores, pollen and probably other microbes) make the ice-nucleating protein simply because of its usefulness in catalyzing the formation of snowflake taxicabs.

Ski resorts, which also need to form snowflakes, often add dead versions of these microbes to their snowmaking water. The proteins facilitate ice formation at temperatures closer to freezing than just dust, so snow can be made at a wider and warmer range of temperatures.

Of course, this doesn’t just benefit skiers. Scientists hypothesize that by choosing crops that support ice-nucleating bacteria, they could bring more rain to drought-prone areas. The Earth may already have been doing this. Bacteria can be found throughout deep cores of glacial ice, where it was likely deposited with ancient snowflakes. Why wouldn’t plants sustain bacteria that could also bring them rain?

I love the ambiguity that this story embraces. Pathogens that shred living cells are bad, right? But what if those same bacteria facilitate life-giving rain? Nothing in Nature is completely bad or completely good. A wolf-killed deer provides food for ravens and eagles. A beaver’s dam kills flooded trees, but the snags support a rookery of herons. Fungus might hollow out a tree, but it provides raccoons with necessary shelter. During infinite acts of creative destruction our world pulses with energy. It’s a pulse so vibrant that even the snow is alive. 

Special Note: Columnist Emily Stone is publishing a second book of her Natural Connections articles as a fundraiser for youth programming at the Cable Natural History Museum. Since kids in the community are often the inspiration for her articles, the Museum is conducting an art contest for kids to illustrate each chapter with a black-and-white line drawing. Find out more at http://cablemuseum.org/connect/.

For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: "Better Together--Celebrating a Natural Community" is now open!