Shraddha Chakradhar

It’s Finally Spring! Why Didn’t the Trees Die Over the Long Winter?

It’s been a long winter, but spring is finally in the air! Daffodils and hyacinths are peeping from the ground, the birds are busy in the bushes, and the trees are finally budding leaves! The fresh green shoots are such a welcome change from the drab brown hues we’ve become so accustomed to, although it makes you wonder: how do these trees manage to revive themselves year after year? Winter, with its drastic temperature drops, precipitation and wind, can be be unforgiving, so how do trees manage to survive the harsh environment?

The key to making it through winter is to prevent ice crystals from forming in any of the living parts of a tree— leaves, flowers, fruits, roots, and a small portion of the trunk. Trees have developed a few defense systems to protect against the formation of ice during the frigid winter months, especially in harsher climates. One of these is the shedding of leaves. Leaves are full of water-filled cells. In freezing temperatures, the water in these leaves would turn into ice, which would in turn destroy the cells. Despite millions of years of evolution, most living beings—plants and animals alike—have not developed a defense system against ice; its crystalline structure punctures through cell components. And so, every fall, in most of the northern United States, we see deciduous trees (trees like maple and oak) shed their leaves, thus eliminating one possibility of ice being formed within living cells.

“A ruptured cell is a dead cell,” said John Seiler, tree physiology expert and professor of forestry at Virginia Tech. “So anything trees can do to prevent a cell from rupturing will help ensure its survival into spring.”

Another tactic against ice, most common in conifers such as pines, firs and spruces, is the expulsion of water from living cells. These trees have needle-like leaves that hold less water than the broad, flat leaves that deciduous trees have and so don’t have to worry about shedding leaves. Instead, they begin to push water from the living cells within the trunk and branches to the spaces outside cells and replace the water with starchy sap. This sugary substance acts as a natural antifreeze, allowing for the tree’s protection against temperatures well below freezing. Of course, in northern parts of the country, some deciduous trees combine shedding leaves with this method of forming sugary sap, which is how we can enjoy maple syrup!

But what happens to the trees in parts of Alaska, Canada and the Arctic tundra?Or trees in the lower 48 during a particularly brutal winter? Apart from shedding leaves and stocking up on sap, some trees are capable of triggering a survival mechanism that seems to defy the laws of physics: supercooling. Water that stays in liquid form at temperatures as cool as -40ºC (where the temperature is also -40ºF) is considered supercooled. This is because of something called nucleation. If you’ve ever witnessed the beginning of water freezing over, you know that water begins to freeze at the edges of objects it touches. The perimeter of an ice cube tray, around the rocks in a pond, and so on. That’s because these foreign materials—the plastic of the tray, the rock surface—act like an anchor for the first frozen crystal, which then helps anchor consequent crystals. But without any of these anchors, or nucleating points, the water exists as a homogeneous mixture incapable of freezing exactly at 0ºC. It freezes closer to the -40ºC mark.

Trees like maple and elm are capable of triggering this supercooling effect when faced with particularly brutal temperatures. How they do this is by getting rid of any nucleating points. The shape of the cells are changed to prevent any foreign surfaces that could possibly act as an anchor to begin the freezing process.

While trees do employ these various defense mechanisms to protect against the harmful effects of ice, there are instances in which they succumb. Frost cracks (pictured below) are formed when ice builds up in the plumbing system of trees, known as the xylem. The xylem is largely non-living, so there is no major damage to the life of the tree, but much like freezing pipes bursting due to expanding ice, the trunk or branches which house xylem can burst if the ice outweighs the wood.

“It sounds like a gunshot when it happens,” said Frank Telewski, professor of plant biology at Michigan State University. “The tree cracks open under the pressure and exposes part of the trunk to the outside environment.”

While the frost crack in itself isn’t fatal, the exposure to the environment upon cracking leaves the few living cells within the trunk or branches susceptible to ice. Much like self-healing wounds in humans, trees form a layer of callus layer that often seals the crack and ensures the survival of the rest of the tree.

What’s also interesting about these defenses against ice is that they also prevent the trees from drying out during the winter. But the coast isn’t always clear: some trees have to worry about winter burn.

“It happens during those rare sunny days in winter,” explained Seiler. “The little water left in trees starts to evaporate because of the sun, but more water can’t be pulled up from the roots because the soil is frozen.” The result is a reddish-brown hue in coniferous trees. Entire stretches of trees in the Rockies, for instance, are susceptible to this condition, known as red belt disease.

With the exception of frost cracks and red belt disease, winters are usually fairly stable. An ice storm may cause damage due to the sheer weight of the ice that forms around trees, but snow serves to insulate the roots and cause little damage.

All these defense mechanisms are especially amazing considering that trees, and most plants in general, have no way of regulating temperature. So the temperature of the air around them is often the temperature within them. But thousands of years of evolution has allowed all kinds of trees to weather the harshest of winters. Dendrologists (scientists who study trees) have conducted experiments transplanting trees native to warm climates to colder climates. The health of these trees is often compromised, since they rely on defense mechanisms that they have long discarded. But when the days start to get longer and the ground thaws to let water flow through the trees, even these immigrant trees, along with their native cousins, finally come out of dormancy and signal to the world that they are ready for yet another season.

Originally published April 2014 on Beacon Reader.


The Ouch-Factor Behind a Flu Shot: Explained

Flu season is here! Here in the United States at least, flu season has been in full swing since October, with pharmacies and doctors’ offices posting signs encouraging people to get their flu shot. If you’ve ever had a flu shot (or any shot, for that matter), you know to expect some pain to go with that healthy dose of immunity. But what you may not know is why. And what’s even more surprising: the pain is good.

Despite the latest needles being significantly small, the size of a needle does make a difference when it comes to the amount of pain you may experience when getting a shot. Needle sizes, specifically how thick they are, are measured in gauges, with a lower gauge representing a thicker diameter and a higher gauge representing a thinner diameter. With flu shots, the most commonly used needles are between 20-gauge and 24-gauge, which represent diameters between 0.023 inches and 0.012 inches. The length of a needle, however, is much higher, ranging from between 1 inch to 1.5 inches.

One major difference between a shot and a pinprick is that a pin would not normally penetrate your skin as deeply as a shot does. Flu shots are typically administered intramuscularly, meaning that it penetrates past your skin layers, a subcutaneous layer of fat, and then reaches the muscle layer. Beginning in 2011, however, a subcutaneous method of administration has also been made available in which the needle is not inserted as deeply. There is no major difference in the two types, however, as far as immunity and side effects.

Needle gauges, lengths and penetration depth aside, the most important reason why a flu shot hurts is that your body is responding to the many agents within a flu shot. And many doctors believe that pain after getting a shot is actually a good sign. It’s a sign that everything is functioning like it should, with the body launching the expected attack against the inactivated viruses, or antigens, that are found within seasonal flu shots (the nasal spray version of the vaccination has live attenuated viruses, which are weakened but live forms of the microorganisms). When the body recognizes these antigens, the immune system launches a coordinated effort against them, by-products of which are the symptoms of an infection—soreness, inflammation, and in some cases, fever. These symptoms serve as signs not only to us at the organismal level that something is going on within the body, but also sound the alarm of an invasion to cells in the immune system.

A specialized type of white blood cells, T-cells, are regularly on patrol-duty, monitoring the blood stream for foreign particles and alerting the rest of the system if antigens are recognized. When antigens present themselves in the blood, antibodies are made by another type of specialized white blood cells known as B-cells. These antibodies are then dispatched to attack and destroy the antigens presented by the vaccine. The antibodies stick to the surface of the antigens, in a mechanism akin to solving a jigsaw puzzle. B-cells create an antibody that fits in specifically with the molecules in an antigen. By being locked into the antigen, the antibody is able to manipulate the antigen in such a way as to stop it from moving further within the body. In the process, the antibodies also alert other cells in the immune system to the presence of antigens.

In most cases, your body is successfully able to overcome the foreign agents, both at the time of vaccination and in case of a future infection. Only some of the B-cells that were involved in antibody production actually respond to the antigen. While many antibody-presenting B-cells are dispatched, some never get involved in the attack and instead become memory cells, ready to launch an attack the next time they spot the same enemy. Memory T-cells also function similarly, remembering the antigen from a previous encounter and launching an attack as soon as the antigen presents itself again in the future.

But that’s the thing: the flu shot changes from year to year, which explains why, even if you’ve been diligent in getting a shot every year, you still experience pain. Every year’s flu shot is a mix of what scientists have deemed that year’s mix of flu viruses to beware. They spend months preparing the solution, adding between three and four types of viruses to the vaccine. The vaccine is tested in cell cultures, animal models and, finally, humans before it is made commercially available. In many cases, these concoctions also have other chemicals, known as adjuvants, mixed in.

“Adjuvants are added to vaccines to make them more potent,” said Dr. Richard Malley, a pediatric infectious disease specialist at Children’s Hospital Boston. “Often, they are aluminum phosphate or aluminum hydroxide additives that drive a better immune response in people.” But a major side effect of adjuvants: pain. These adjuvants trigger inflammation and pain responses within the body but have the added benefit of boosting immunity.

Not surprisingly, the benefits of getting a vaccination far outweigh the costs. But if it helps, despite decades of administering and developing vaccines, pain is still one of the first things vaccine researchers take into consideration, according to Malley. So the next time you get a shot, think of the many fighting components in your body working to protect you, but most importantly, embrace the pain.

Originally published on Beacon Reader in December 2013